COURSE TITLE: Alternating Current and Machinery

DUTY TITLE: Operation of A.C. Motors

POS #: 2600

TASK: Operation of A.C. Motors

PURPOSE: The student will be able to understand the operation and components of an A.C. Motor.

TASKS:

2601 Explain the theory of operation of alternating current motors.2602 Calculate the synchronous speed of an alternating current motor.2603 Describe operating characteristics of capacitor-start motors.2604 Connect and operate split-phase and capacitor-start motors.2605 Reverse the rotation of a split phase motor.2606 Describe the force between two magnetic fields.2607 Determine operating characteristics of universal motors.2608 Connect and operate a three-phase, squirrel cage motor.2609 Demonstrate how to reverse the rotation of a three-phase motor.2610 Understand and be able to operate a megger.

2611 Understand and be able to describe the operation of a sleeve bearing motor.

2612 Understand and be able to laser align a three phase motor.

2613 Understand and be able to perform the procedures for proper disassembly, testing, and reassembly of an AC motor.NOTE: This task is not on the current Program of Study Task Listing; however this is an important task the students must learn for the Electrical trade. The P.O.S. numbers shown are from a previous task listing.

REVISION: 2019

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Schuylkill Technology Center-

South Campus15 Maple Avenue

Marlin, Pennsylvania 17951(570) 544-4748

RESIDENTIAL & INDUSTRIAL ELECTRICITY

Level 3Task 2600

Name:

Date:

Learning Guide Due Date:

Pre Test Due Date:

Post Test Due Date:

POS # 2600

Total Hours-64Level(s)-3

Project(s) are ADDITIONAL to this learning guide.

ENGLISH LANGUAGE ARTSCC.1.2.11-12.J Acquire and use accurately general academic and domain-specific words and phrases, sufficient for reading, writing, speaking, and listening at the college and career readiness level; demonstrate independence in gathering vocabulary knowledge when considering a word or phrase important to comprehension or expressionCC.1.3.11-12.I Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade level reading and content, choosing flexibly from a range of strategies and tools.

MATHCC.2.1.HS.F.4 Use units as a way to understand problems and to guide the solution of multi-step problems.

READING IN SCIENCE & TECHNOLOGYCC.3.5.11-12.B. Determine the central ideas or conclusions of a text; summarize complex concepts, processes, or information presented in a text by paraphrasing them in simpler but still accurate terms.CC.3.5.11-12.C. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.

WRITING IN SCIENCE & TECHNOLOGYCC.3.6.11-12.E. Use technology, including the Internet, to produce, publish, and update individual or shared writing products in response to ongoing feedback, including new arguments or information.

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*CORE CURRICULUM STANDARDS*

*ACADEMIC STANDARDS * READING, WRITING, SPEAKING & LISTENING

1.1.11.A Locate various texts, assigned for independent projects before reading.1.1.11.D Identify strategies that were most effective in learning1.1.11.E Establish a reading vocabulary by using new words1.1.11.F Understanding the meaning of, and apply key vocabulary across the various subject areas1.4.11.D Maintain a written record of activities1.6.11.A Listen to others, ask questions, and take notes

MATH2.2.11.A Develop and use computation concepts2.2.11.B Use estimation for problems that don’t need exact answers2.2.11.C Constructing and applying mathematical models2.2.11.D Describe and explain errors that may occur in estimates 2.2.11.E Recognize that the degree of precision need in calculating2.3.11.A Selecting and using the right units and tools to measure precise measurements2.5.11.A Using appropriate mathematical concepts for multi-step problems2.5.11.B Use symbols, terminology, mathematical rules, Etc.2.5.11.C Presenting mathematical procedures and results

SCIENCE3.1.12.A Apply concepts of systems, subsystems feedback and control to solve complex technological problems3.1.12.B Apply concepts of models as a method predict and understand science and technology3.1.12.C Assess and apply patterns in science and technology3.1.12D Analyze scale as a way of relating concepts and ideas to one another by some measure3.1.12.E Evaluate change in nature, physical systems and man-made systems3.2.12.A Evaluate the nature of scientific and technological knowledge3.2.12.B Evaluate experimental information for appropriateness3.2.12.C Apply elements of scientific inquiry to solve multi – step problems3.2.12.D Analyze the technological design process to solve problems3.4.12.A Apply concepts about the structure and properties of matter3.4.12.B Apply energy sources and conversions and their relationship to heat and temperature3.4.12.C Apply the principles of motion and force3.8.12.A Synthesize the interactions and constraints of science3.8.12.B Use of ingenuity and technological resources to solve specific societal needs and improve the quality of life3.8.12.C Evaluate the consequences and impacts of scientific and technological solutions

ECOLOGY STANDARDS4.2.10.A Explain that renewable and non-renewable resources supply energy and material.4.2.10.B Evaluate factors affecting availability of natural resources.4.2.10.C Analyze the use of renewable and non-renewable resources.4.2.12.B Analyze factors affecting the availability of renewable and non-renewable resources.4.3.10.A Describe environmental health issues.4.3.10.B Explain how multiple variables determine the effects of pollution on environmental health, natural processes and human practices.4.3.12.C Analyze the need for a healthy environment.4.8.12.A Explain how technology has influenced the sustainability of natural resources over time.

CAREER & EDUCATION13.1.11.A Relate careers to individual interest, abilities, and aptitudes13.2.11.E Demonstrate in the career acquisition process the essential knowledge needed13.3.11.A Evaluate personal attitudes that support career advancement

ASSESSMENT ANCHORSM11.A.3.1.1 Simplify expressions using the order of operationsM11.A.2.1.3 Use proportional relationships in problem solving settingsM11.A.1.2 Apply any number theory concepts to show relationships between real numbers in problem solving

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*ACADEMIC STANDARDS*

STUDENTThe student will be able to understand the operation and components of an A.C. Motor.

TERMINAL PERFORMANCE OBJECTIVEGiven all the electrical tools and materials required, the student will be able to understand the operation and components of an A.C. Motor.

SAFETY Always wear safety glasses when working in the shop. Always check with the instructor before turning power on. Always use tools in the correct manner. Keep work area clean and free of debris. Never wire a project without the correct wiring diagram. Make sure the tip of the soldering iron/gun is directed towards a safe area.

RELATED INFORMATION1. Attend lecture by instructor.2. Obtain handout.3. Review chapters in textbook.4. Define vocabulary words.5. Complete all questions in this packet.6. Complete all projects in this packet.7. Complete K-W-L Literacy Assignment by Picking an Article From the

“Electrical Contractor” Magazine Located in the Theory Room. You can pick any article you feel is important to the electrical trade.

EQUIPMENT & SUPPLIES

1. Safety glasses 11. 85-MT2 Electric Machine Base Unit

2. Allen Wrench Set 12. Split Phase A.C. Motor

3. Screw driver 13. A.C. Synchronous Motor

4. Multi-meter 14. 3 Phase Motor & Split Phase Connection Plates

5. Wire strippers 15. Love Joy Coupling & Key

6. Side cutters 16. Nylon Coupler Spider

7. Cable rippers 17. Synchronous Motor Connection Plate

8. Lineman pliers 18. Dead Blow Hammer

9. Needle nose pliers 19. Straight Edge for Alignment of Coupling

10. Project Packet 20. Small Black tool box W/wires & termination plates

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PROCEDURE

CC.2.1.HS.F.4 Use units as a way to understand problems and to guide the solution of multi-step problems.CC.3.5.11-12.C. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.1.6.11A Listen to others, ask questions, and take notes3.4.12.B Apply energy sources and conversions and their relationship to heat and temperature

UNIT 32Three-Phase Motors

OUTLINE Three-Phase Motors The Rotating Magnetic Field Connecting Dual-Voltage Three-Phase Motors Squirrel-Cage Induction Motors Wound-Rotor Induction Motors Synchronous Motors Selsyn Motors

KEY TERMSAmortisseur winding Wound rotorCode letter TorqueConsequent pole Synchronous speedDifferential selsyn Synchronous condenser Direction of rotation SynchronousDual-voltage motors Squirrel cagePercent slip Single-phasingPhase rotation meter Selsyn motorsRotating magnetic field Rotor frequencyRotating transformers

Anticipatory SetGo over the unit outline and all key terms. Let students know that everything they have learned thus far will be used in these last two units. They should be familiar with terms like single-phase, three-phase, synchronous, frequency, torque, windings, poles, and motors. Display various types of three-phase motors. Have a phase rotation meter available for student use during that section of the unit.

Three-Phase MotorsBriefly preview the three types of three-phase motors, distinguishing them from one another by their rotors. Have students note the stator winding as the motor primary, and the rotor as the motor secondary.

The Rotating Magnetic Field

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Discuss the three factors that determine the rotating magnetic field. Refer to Figure 32-1 through Figure 32-8 as you elaborate on the polarity of the voltages and the significance of the arrangement of the stator windings inside the motor. Ask questions about the rotation progress being made inside the motor, and how that is affecting polarity.

Synchronous SpeedDiscuss the factors that determine synchronous speed and go over the formula used to calculate synchronous speed.

Determining the Direction of Rotation for Three-Phase MotorsExplain that the direction of rotation of a three-phase motor can be changed by reversing two of its stator leads, and that the direction of rotation can be determined without actually allowing the motor to rotate. Demonstrate how to use a phase rotation meter for this purpose, along with a motor that you can turn the shaft of manually. Be sure students understand how to respond to what registers on the meter.

Connecting Dual-Voltage Three-Phase MotorsExplain what is meant by a dual-voltage motor, and under what circumstances this type of motor may be required.Then, referring to Figure 32-13, explain the lead numbering process for both wye and delta connections.

High-Voltage ConnectionsExplain why a high-voltage connection may be necessary, and draw both the wye and delta numbering processes for high voltage on the board as you explain them. Be sure students realize that the high-voltage connections must be in series.

Low-Voltage ConnectionsDraw these on the board as well, explaining where low-voltage connections would be required. Make sure students realize that these connections are in parallel. Also, discuss the adjustments that must be made when a dual-voltage motor has 12 T leads instead of nine.Voltage and Current Relationships for Dual-Voltage MotorsDiscuss the difference that occurs in current flow when a motor is connected to the higher voltage instead of the lower. Review the formulas to solve for phase voltage, line current, total impedance, and phase current.

Squirrel-Cage Induction MotorsDescribe this type of rotor, and display one. Discuss the advantages of this type of rotor.

Principles of OperationExplain how the squirrel-cage motor works, beginning with the creation of the induced voltage. Discuss how the amount of this induced voltage is determined.

Torque

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Make sure students copy the necessary symbols into their notes. Also, be sure students understand why an induction motor can never reach synchronous speed. Have students explain this concept back to you. Also, discuss the significance of the stator and rotor flux being in phase with each other.

Starting CharacteristicsExplain what the factors are that will determine the starting current. Display the nameplate of a squirrel- cage motor, and make sure students can find the needed information on it. Take students through the solution process.

Percent SlipExplain what percent slip is and how to calculate it. Also explain why squirrel-cage motors are considered constant speed motors with minimal percent slip.

Rotor FrequencyExplain what rotor frequency is, how it is calculated, and how it affects rotor bar inductive reactance.

Reduced Voltage StartingExplain why a high starting current is sometimes required with squirrel-cage motors, and why this makes it necessary to reduce the voltage during start-up. Discuss the effect this has on magnetic fields, as well as on the torque.

Code LettersExplain that the code letters indicate the type of bars used in a given squirrel-cage motor. Discuss what the various letters mean as they pertain to resistance levels. Point out the ripple effect of a particular rating on torque, phase, current, induced voltage, and the rotor magnetic field.

The Double Squirrel-Cage RotorDescribe and display this type of rotor. Explain the benefits of having the low-resistance bars deep in the core, and high-resistance bars closer to the surface. Have students explain why this type of motor has good running torque and excellent speed regulation.

Power Factor of a Squirrel-Cage Induction MotorExplain what determines the power factor and how it changes from no load to adding a load.

Single-PhasingExplain what single-phasing is and how it occurs. Then, discuss what can happen to the motor as a result of single- phasing.

The Nameplate

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Display some electric motors, and allow students the opportunity to read the nameplates, and explain what they can do with the information on a nameplate.

Consequent-Pole-Squirrel-Cage MotorsExplain the difference between these motors and regular squirrel-cage motors. Discuss the advantages of being able to change the synchronous speed, and where these motors would be used. Go through the various connections, ensuring student understanding along the way.

Motor CalculationsUsing a wattmeter, demonstrate how to calculate horsepower and motor efficiency. Take students through the solution processes for both.

Wound-Rotor Induction MotorsDescribe and display this motor, explaining why its high starting torque and low starting current make it so widely used in industry. Compare the rotors of this motor and the squirrel-cage motor.

Principles of OperationExplain how this motor functions, beginning with the power applied to the stator winding, and following the process through to the point where all external resistance has been removed from the rotor circuit. Point out that once maximum operating speed is reached, the wound rotor induction motor functions like the squirrel-cage motor.

Starting Characteristics of a Wound-Rotor MotorCompare the starting characteristics of this motor to those of the squirrel-cage motor. Emphasize the difference in starting torque for the two motors, and why it is usually higher in the wound-rotor motor.

Speed ControlExplain how speed control takes place and what effect it has on the system.

Synchronous MotorsDistinguish this motor from other three-phase motors by pointing out its different characteristics.

Rotor ConstructionDescribe and display the rotor, pointing out how similar it is to the rotor of an alternator.

Starting a Synchronous Motor

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Referring to Figure 32-47, explain what the amortisseur winding is, and how this winding is used to start the motor.Also discuss how the windings become electromagnets, allowing the rotor to lock in step with the stator. Make sure that students note, however, that a synchronous motor should never be started with DC connected to the rotor.Explain what can happen if this occurs.

The Field-Discharge ResistorExplain this resistor’s location, and its function in protecting the rotor.

Constant-Speed OperationExplain how this motor starts out as an induction motor, yet doesn’t operate as one. Also, explain what pullout torque is, and what happens when pullout torque is reached.

The Power SupplyDiscuss the methods that can be used to supply power to a synchronous motor.

Power Factor CorrectionExplain how this correction process takes place as the amount of excitation current in the rotor changes.

Interaction of the DC and AC FieldsExplain the relationship of the AC to the DC in making up for weak DC, and in producing the torque necessary to operate the load. Also, discuss how the AC reacts when the DC is overexcited.

Synchronous Motor ApplicationsExplain why these motors are widely used in industry, and what types of functions they have.

Advantages of the Synchronous CondenserExplain the role of the condenser as a power factor correction tool. Discuss why it is better than a bank of capacitors for the same purpose.

Selsyn MotorsExplain what a selsyn motor is and what it is used for. Display one, if possible. Point out that when these motors are used, at least two are used together. Discuss the reasoning behind this.

Selsyn Motor Operation

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Compare the selsyn motor operating process to that of a transformer, as students should already be familiar with transformers. Discuss the rotor position of the transmitter, and how it affects the rotor of the receiver.

The Differential SelsynExplain the construction of the differential selsyn, and discuss how it is used.

Unit Round UpGo through the summary together; having students lead the discussion on each summarized point. Check for any confusion during this time. Have students do the review questions on their own, and then, as a class, discuss them at your next class session. Review any key terms that were not covered in the summary.

UNIT 33Single-Phase Motors

OUTLINESingle-Phase Motors Universal MotorsSplit-Phase Motors Stepping Motors Resistance-Start Induction-Run Motors Single-Phase Synchronous Motors Capacitor-Start Induction-Run Motors Repulsion-Induction MotorsDual-Voltage Split-Phase Motors Repulsion-Start Induction-Run MotorsDetermining the Direction of Rotation Construction of Repulsion Motorsfor Split-Phase Motors Capacitor-Start Capacitor-Run Motors Repulsion-Type Motors Shaded-Pole Induction Motors Multispeed Motors

KEY TERMSCentrifugal switch Warren motorCompensating winding Universal motorConductive compensation Two-phaseConsequent pole-motor Synchronous speed Holtz motor Synchronous motorsInductive compensation Stepping motorsMultispeed motors Start windingNeutral plane Split-phase motorsRepulsion motor Shading coilRun winding Shaded-pole induction motor

Anticipatory Set

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Tell students that this final unit will require them to put everything they have learned to actual use in understanding how the various motors work. Almost all of the terms in this unit will be familiar to them. Display as many of the motors covered in this unit as is possible. Refer to them often as you explain, and demonstrate how they work and where they are used.

Single-Phase MotorsStress the fact that single-phase motors have a variety of operating principles, unlike the three-phase motors just studied.

Split-Phase MotorsDescribe the three categories that split-phase motors fall into. Explain how split-phase motors simulate a two phase power system.

The Two-Phase SystemExplain how a two-phase system is created and why it is created.

Stator WindingsDistinguish between the start winding and the run winding. Discuss the purpose of each of these windings and how they function in creating synchronous speed. Refer to Figures 33-2A and 33-2B as you have this discussion.

Resistance-Start Induction-Run MotorsDiscuss torque in this motor, and the factors involved in determining torque. Also, discuss the difference in inductance between the start winding and the run winding, resulting in phase angle differences.

Disconnecting the Start WindingExplain why the start winding needs to be disconnected. Explain that this is not done by centrifugal force in a hermetically sealed unit.

Starting RelaysExplain why starting relays are used on hermetically sealed motors. Describe and display the three types of starting relays and describe how each works and is used. Refer to Figure 33-8 through Figure 33-12 as you explain the function and operation of each of these relays. Compare the three, and explain which circumstances require which relays.

Relationship of Stator and Rotor FieldsDescribe the rotor of the split-phase motor and its function. Explain what takes place after the centrifugal switch opens.

Direction of Rotation

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Describe the relationship of the rotation of the rotating magnetic field to the direction of rotation for the motor.

Capacitor-Start Induction-Run MotorsCompare this motor to the resistance-start induction-run motor. Distinguish the starting characteristics of the two motors. Have students note that the motor should not be started more than approximately eight times per hour, and explain why this is so.

Dual-Voltage Split-Phase MotorsDescribe and display a dual-voltage split-phase motor. Explain how it functions for 120 Volts and how it functions for 240 Volts. Refer to Figure 33-19 to familiarize students with the schematic for this motor.

Determining the Direction of Rotation for Split-Phase MotorsExplain the connection pattern for both clockwise and counterclockwise rotation.

Capacitor-Start Capacitor-Run MotorsExplain the differences between these motors and the resistance-start induction-run motors. Point out why a centrifugal switch is not required to disconnect the capacitor from the start winding. Discuss the use of the second capacitor, and describe how it is used in a central air-conditioning unit.Explain what the potential starting relay is and how it is used. Refer to Figure 33-30 as you give this explanation.

Shaded-Pole Induction MotorsDescribe and display this motor.

The Shading CoilExplain what the shading coil is and how the short circuit effect of the large loop allows a large amount of current to flow. Give examples of motors that use a shading coil. Discuss what happens to the magnetic .ux as the AC reaches its peak and then drops back down to zero.

SpeedDiscuss the factors that determine the speed of the shaded-pole induction motor.

General Operating CharacteristicsDisplay a shaded-pole motor, and describe how it functions. Point out that the direction of rotation is determined by the direction of the magnetic field moving across the pole face.

Multispeed Motors

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Describe and display the consequent pole single-phase motor. Also, make sure students realize that the shaded pole induction motor is also a multispeed motor. Give examples of where these are used.

Multispeed Fan MotorsReferring to Figure 33-37, explain how the run winding has been set up and tapped. Be sure students note that the start winding is set up in parallel with the run winding. Use a ceiling fan as an example, as you describe what happens at each setting.

Repulsion-Type MotorsGive a brief explanation of each type of repulsion motor.

Construction of Repulsion MotorsDisplay and describe how a repulsion motor is built. Emphasize the difference in the concept of like magnetic poles repelling each other, versus creating a magnetic field.

OperationExplain how the magnetic field created in the poles is repelled by the magnetic field created in the armature. Have students note that repulsion motors will have the same number of brushes as there are stator poles.

Brush PositionDisplay this, if possible, as you explain it. Describe what happens, or doesn’t happen, when the brushes are at 90° to the poles, and when the brushes are lined up with the poles. Then, explain the effect of moving the brushes either clockwise or counterclockwise 15°.

Repulsion-Start Induction-Run MotorsDisplay and describe the two types of repulsion-start induction-run motors. Describe how the brush-riding type functions, and compare it to a squirrel-cage motor. Explain how the brush-lifting type of motor operates, focusing on the role of the weights in the operating process

Repulsion-Induction MotorsDescribe and display this motor, comparing its workings to those of the repulsion motor and the DC compound motor. Guide students through the schematic in Figure 33-47.

Single-Phase Synchronous MotorsExplain how these motors work and where they are used. Be sure students realize that constant speed is required, and no DC excitation current is needed.

Warren Motors

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Display, if available, and describe the Warren motor. If one is not available, refer to Figure 33-48. Compare this motor to the Holtz motor, noting the difference in the rotor.

Holtz MotorDisplay, or refer to Figure 33-49, and describe the construction and operation of this motor.

Stepping MotorsDisplay and describe how different these motors are from the other motors studied, especially the Warren and Holtz motors.

Theory of OperationReferring to Figures 33-50 through 33-53, describe how the stepping motor functions. Discuss the variability of the number of stator poles these motors can have. Have students explain why most stepping motors have eight stator poles.

WindingsDescribe the three-lead motor and compare this type of stepping motor to the bifilar stepping motor. Explain the advantage of the two windings over the three-lead type.

Four-Step Switching (Full-Stepping)Explain this process and discuss where students will encounter this in the field.

Eight-Step Switching (Half-Stepping)Explain this process and compare it to the four-step switching process. Discuss the advantages and disadvantages of each process, in relation to job needs.

AC OperationExplain why stepping motors operating on AC become permanent magnet induction motors. Display, or refer to Figure 33-60, as you describe the construction and functioning of the AC operation of this motor.

Stepping Motor CharacteristicsExplain the versatility of these motors, and have students discuss why their characteristics are advantageous in a work setting.

Universal MotorsCompare the universal motor to the DC series motor, but emphasize the major difference of the compensating winding. Describe how the compensating winding winds around the stator, and how it counteracts the inductive reactance in the armature. Have students explain the advantages of having a compensating winding.

Connecting the Compensating Winding for AC

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Discuss both the conductive compensation connection and the inductive compensation connection. Refer toFigures 33-65 and 33-66 to show how this looks.

The Neutral PlaneExplain how to set the brushes at the neutral plane, using the voltmeter connected to either the series winding or the compensating winding.

Speed RegulationExplain why the speed regulation of this motor is so poor, and give examples of its use.

Changing the Direction of RotationExplain how to change the armature leads with respect to the field leads.

Unit Round UpGo over the summary together and any key terms not covered in the summary. Have students do review questions on their own, lead the discussions covering these questions at your next class session. Listen carefully, to check for any area that appears to be unclear, and go back over those areas if there are any.

4 Pole Induction Motor

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VOCABULARY

CC.1.3.11-12.I Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade level reading and content, choosing flexibly from a range of strategies and toolCC.3.5.11-12.D. Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 11–12 texts and topics.

Consequent Poles:

Slip:

Single Phasing:

Squirrel Cage:

Torque:

Wound Rotor:

Frequency:

Selsyn Motor:

Phase Rotation Meter:

Code Letter:

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Wound Rotor:

Amortisseur Winding:

Centrifugal Switch:

Universal Motor:

Stepping Motor:

Repulsion Motor:

Holtz Motor:

Run Winding:

Shading Coil:

Warren Motor:

Inductive Compensation:

Start Winding:

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Split Phase Motor:

Compensating Winding:

Neutral Plane:

Two Phase:

Rotating Magnetic Field:

Shaded Pole Induction Motor:

REFERENCE PAGES

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Single Phase, AC Motor Breakdown #2

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3 Phase AC Motor Breakdown

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AC Motor Breakdown #2

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Hysteresis and Eddy Current

Early designers of AC motors encountered problems traced to losses unique to alternating current magnetics. These problems were encountered when adapting DC motors to AC operation. Though few AC motors today bear any resemblance to DC motors, these

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problems had to be solved before AC motors of any type could be properly designed before they were built.

Both rotor and stator cores of AC motors are composed of a stack of insulated laminations. The laminations are coated with insulating varnish before stacking and bolting into the final form. Eddy currents are minimized by breaking the potential conductive loop into smaller less lossy segments. (Figure below) The current loops look like shorted transformer secondary turns. The thin isolated laminations break these loops. Also, the silicon (a semiconductor) added to the alloy used in the laminations increases electrical resistance which decreases the magnitude of eddy currents.

Eddy currents in iron cores.

If the laminations are made of silicon alloy grain oriented steel, hysteresis losses are minimized. Magnetic hysteresis is a lagging behind of magnetic field strength as compared to magnetizing force. If a soft iron nail is temporarily magnetized by a solenoid, one would expect the nail to lose the magnetic field once the solenoid is de-energized. However, a small amount of residual magnetization, Br due to hysteresis remains. (Figure below) An alternating current has to expend energy, -Hc the coercive force, in overcoming this residual magnetization before it can magnetize the core back to zero, let alone in the opposite direction. Hysteresis loss is encountered each time the polarity of the AC reverses. The loss is proportional to the area enclosed by the hysteresis loop on the B-H curve. “Soft” iron alloys have lower losses than “hard” high carbon steel alloys. Silicon grain oriented steel, 4% silicon, rolled to preferentially orient the grain or crystalline structure, has still lower losses.

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Hysteresis curves for low and high loss alloys.

Once Steinmetz's Laws of hysteresis could predict iron core losses, it was possible to design AC motors which performed as designed. This was akin to being able to design a bridge ahead of time that would not collapse once it was actually built. This knowledge of eddy current and hysteresis was first applied to building AC commutator motors similar to their DC counterparts. Today this is but a minor category of AC motors. Others invented new types of AC motors bearing little resemblance to their DC kin.

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MOTOR CONNECTION’S

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26

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The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the ac case. Three electrical phases are introduced through terminals, each phase energizing an individual field pole. When each phase reaches its maximum current, the magnetic field at that pole reaches a maximum value. As the current decreases, so does the magnetic field. Since each phase reaches its maximum at a different time within a cycle of the current, that field pole whose magnetic field is largest is constantly changing between the three poles, with the effect that the magnetic field seen by the rotor is rotating. The speed of rotation of the magnetic field, known as the synchronous speed, depends on the frequency of the power supply and the number of poles produced by the stator winding. For a standard 60 Hz supply, as used in the United States, the maximum synchronous speed is 3,600 rpm.

In the three phase induction motor, the windings on the rotor are not connected to a power supply, but are essentially short circuits. The most common type of rotor winding, the squirrel cage winding, bears a strong resemblance to the running wheel used in cages for pet gerbils. When the motor is initially switched on and the rotor is stationary, the rotor conductors experience a changing magnetic field sweeping by at the synchronous speed. From Faraday's law, this situation results in the induction of currents round the rotor windings; the magnitude of this current depends on the impedance of the rotor windings. Since the conditions for motor action are now fulfilled, that is, current carrying conductors are found in a magnetic field, the rotor experiences a torque and starts to turn. The rotor can never rotate at the synchronous speed because there would be no relative motion between the magnetic field and the rotor windings and no current could be induced. The induction motor has a high starting torque.

In squirrel cage motors, the motor speed is determined by the load it drives and by the number of poles generating a magnetic field in the stator. If some poles are switched in or out, the motor speed can be controlled by incremental amounts. In wound-rotor motors, the impedance of the rotor windings can be altered externally, which changes the current in the windings and thus affords continuous speed control.

Three-phase synchronous motors are quite different from induction motors. In the synchronous motor, the rotor uses a DC energized coil to generate a constant magnetic field. After the rotor is brought close to the synchronous speed of the motor, the north (south) pole of the rotor magnet locks to the south (north) pole of the rotating stator field and the rotor rotates at the synchronous speed. The rotor of a synchronous motor will usually include a squirrel cage winding which is used to start the motor rotation before the DC coil is energized. The squirrel cage has no effect at synchronous speeds for the reason explained above.

Single phase induction and synchronous motors, used in most domestic situations; operate on principles similar to those explained for three phase motors. However, various modifications have to be made in order to generate starting torques, since the single phase will not generate a rotating magnetic field alone. Consequently, split phase, capacitor start, or shaded pole designs are used in induction motors. Synchronous single phase motors, used for timers, clocks, tape recorders etc., rely on the reluctance or hysteresis designs.

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The first real electric motorsEuropean writers assert that in 1827, Hungarian Ányos Jedlik started experimenting with electromagnetic rotating devices which he called "electromagnetic self-rotors", he used them as illustrative instruments in the universities, and he demonstrated the first real electric motor using electromagnets for both stationary and rotating parts in Hungary in 1828. He built an electric motor-propelled vehicle that same year. There is no evidence

that this experimentation was communicated to the wider scientific world at that time, or that it influenced the development of electric motors in the following decades.

The first commutator-type direct-current electric motor capable of turning machinery was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837. His motors ran at up to 600 revolutions per minute and powered machine tools and a printing press. Due to the high cost of the zinc electrodes required by primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.

In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse Company.

The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of a relatively-small air gap between rotor and stator. Early motors, for some rotor positions, had comparatively huge air gaps which constituted a very-high-reluctance magnetic circuit. They produced far-lower torque than an equivalent amount of power would produce with efficient designs. The cause of the lack of understanding seems to be that early designs were based on familiarity of distant attraction between a magnet and a piece of ferromagnetic material, or between two electromagnets. Efficient designs, as this article describes, are based on a rotor with a comparatively-small air gap, and flux patterns that create torque.

Note that the armature bars are at some distance (unknown) from the field pole pieces when power is fed to one of the field magnets; the air gap is likely to be considerable. The text tells of the inefficiency of the design. (Electricity was created, as a practical matter, by consuming zinc in wet primary cells!)

In his workshops Froment had an electromotive engine of one-horse power. But, though an interesting application of the transformation of energy, these machines will never be practically applied on the large scale in manufactures, for the expense of the acids and the zinc which they use very far exceeds that of the coal in steam-engines of the same force.

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[...] motors worked by electricity, independently of any question as to the cost of construction, or of the cost of the acids, are at least sixty times as dear to work as steam-engines.

Although Gramme's design was comparatively much more efficient, apparently the Froment motor was still considered illustrative, years later. It is of some interest that the St. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor. Photo of a traditional form of the motor: Note the prominent bar magnets, and the huge air gap at the ends opposite the rotor. Even modern versions still have big air gaps if the rotor poles are not aligned.

Application of electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using shaft, belts, compressed air or hydraulic pressure. Instead every machine could be equipped with its own electric motor, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors consume more than half of all electric energy produced.

Categorization of electric motorsThe classic division of electric motors has been that of Alternating Current (AC) types vs Direct Current (DC) types. This is more a de facto convention, rather than a rigid distinction. For example, many classic DC motors run on AC power, these motors being referred to as universal motors.Rated output power is also used to categorize motors, those of less than 746 Watts, for example, are often referred to as fractional horsepower motors (FHP) in reference to the old imperial measurement.The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation of. The two best examples are: the brushless DC motor and the stepping motor, both being poly-phase AC motors requiring external electronic control, although historically, stepping motors (such as for maritime and naval gyrocompass repeaters) were driven from DC switched by contacts.Considering all rotating (or linear) electric motors require synchronism between a moving magnetic field and a moving current sheet for average torque production, there is a clearer distinction between an asynchronous motor and synchronous types. An asynchronous motor requires slip between the moving magnetic field and a winding set to induce current in the winding set by mutual inductance; the most ubiquitous example being the common AC induction motor which must slip in order to generate torque. In the synchronous types, induction (or slip) is not a requisite for magnetic field or current production (eg. permanent magnet motors, synchronous brush-less wound-rotor doubly-fed electric machine).

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Comparison of motor types

Type Advantages Disadvantages Typical Application

Typical Drive

AC Induction(Shaded Pole)

Least expensiveLong lifehigh power

Rotation slips from frequencyLow starting torque

Fans Uni/Poly-phase AC

AC Induction(split-phase capacitor)

High powerhigh starting torque

Rotation slips from frequency Appliances Uni/Poly-

phase AC

AC Synchronous

Rotation in-sync with freqlong-life (alternator)

More expensiveClocksAudio turntablestape drives

Uni/Poly-phase AC

Stepper DC

Precision positioningHigh holding torque

Requires a controller

Positioning in printers and floppy drives

Multiphase DC

Brushless DC electric motor

Long lifespanlow maintenanceHigh efficiency

High initial costRequires a controller

Hard drivesCD/DVD playerselectric vehicles

Multiphase DC

Brushed DC electric motor

Low initial costSimple speed control (Dynamo)

High maintenance (brushes)Low lifespan

Treadmill exercisersautomotive starters

Direct (PWM)

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Servo motorA servomechanism or servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.

Synchronous electric motor A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque. Synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to rotor. The rotor poles connect to each other and move at the same speed hence the name synchronous motor.

Inducti o n motor An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction. Another commonly used name is squirrel cage motor because the rotor bars with short circuit rings resemble a squirrel cage (hamster wheel). An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an induction motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.

Electrostatic motor(capacitor motor) An electrostatic motor or capacitor motor is a type of electric motor based on the attraction and repulsion of electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically require a high voltage power supply, although very small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages. In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon. Today the electrostatic motor finds frequent use in micro-mechanical (MEMS) systems where their drive voltages are below 100 volts, and where moving charged plates are far easier to fabricate than coils and iron cores. Also, the molecular machinery which runs living cells is often based on linear and rotary electrostatic motors.

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Torque capability of motor typesWhen optimally designed for a given active current (i.e., torque current), voltage, pole-pair number, excitation frequency (i.e., synchronous speed), and core flux density, all categories of electric motors or generators will exhibit virtually the same maximum continuous shaft torque (i.e., operating torque) within a given physical size of electromagnetic core. Some applications require bursts of torque beyond the maximum operating torque, such as short bursts of torque to accelerate an electric vehicle from standstill. Always limited by magnetic core saturation or safe operating temperature rise and voltage, the capacity for torque bursts beyond the maximum operating torque differs significantly between categories of electric motors or generators.

Note: Capacity for bursts of torque should not be confused with Field Weakening capability inherent in fully electromagnetic electric machines (Permanent Magnet (PM) electric machine are excluded). Field Weakening, which is not readily available with PM electric machines, allows an electric machine to operate beyond the designed frequency of excitation without electrical damage.

Electric machines without a transformer circuit topology, such as Field-Wound (i.e., electromagnet) or Permanent Magnet (PM) Synchronous electric machines cannot realize bursts of torque higher than the maximum designed torque without saturating the magnetic core and rendering any increase in current as useless. Furthermore, the permanent magnet assembly of PM synchronous electric machines can be irreparably damaged, if bursts of torque exceeding the maximum operating torque rating are attempted.

Electric machines with a transformer circuit topology, such as Induction (i.e., asynchronous) electric machines, Induction Doubly-Fed electric machines, and Induction or Synchronous Wound-Rotor Doubly-Fed (WRDF) electric machines, exhibit very high bursts of torque because the active current (i.e., Magneto-Motive-Force or the product of current and winding-turns) induced on either side of the transformer oppose each other and as a result, the active current contributes nothing to the transformer coupled magnetic core flux density, which would otherwise lead to core saturation.

Electric machines that rely on Induction or Asynchronous principles short-circuit one port of the transformer circuit and as a result, the reactive impedance of the transformer circuit becomes dominant as slip increases, which limits the magnitude of active (i.e., real) current. Still, bursts of torque that are two to three times higher than the maximum design torque are realizable.

The Synchronous WRDF electric machine is the only electric machine with a truly dual ported transformer circuit topology (i.e., both ports independently excited with no short-circuited port). The dual ported transformer circuit topology is known to be unstable and requires a multiphase slip-ring-brush assembly to propagate limited power to the rotor winding set. If a precision means were available to instantaneously control torque angle and slip for synchronous operation during motoring or generating while simultaneously providing brushless power to the rotor winding set (see Brushless wound-rotor doubly-fed electric machine), the active current of the Synchronous WRDF electric machine

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would be independent of the reactive impedance of the transformer circuit and bursts of torque significantly higher than the maximum operating torque and far beyond the practical capability of any other type of electric machine would be realizable. Torque bursts greater than eight times operating torque have been calculated.

Universal motorsA variant of the wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current, although in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the field and the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) at the same time, and hence the mechanical force generated is always in the same direction. In practice, the motor must be specially designed to cope with the AC (impedance must be taken into account, as must the pulsating force), and the resultant motor is generally less efficient than an equivalent pure DC motor.

Operating at normal power line frequencies, the maximum output of universal motors is limited and motors exceeding one kilowatt (about 1.3 horsepower) are rare. But universal motors also form the basis of the traditional railway traction motor in electric railways. In this application, to keep their electrical efficiency high, they were operated from very low frequency AC supplies, with 25 and 16.7 hertz (Hz) operation being common. Because they are universal motors, locomotives using this design were also commonly capable of operating from a third rail powered by DC.

The advantage of the universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator. As a result such motors are usually used in AC devices such as food mixers and power tools which are used only intermittently. Continuous speed control of a universal motor running on AC is easily obtained by use of a thyristor circuit, while stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC).

Universal motors generally run at high speeds, making them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high RPM operation is desirable. They are also commonly used in portable power tools, such as drills, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed 10,000 RPM, while Dremel and other similar miniature grinders will often exceed 30,000 RPM.

Motor damage may occur due to over speeding (running at an RPM in excess of design limits) if the unit is operated with no significant load. On larger motors, sudden loss of load is to be avoided, and the possibility of such an occurrence is incorporated into the motor's protection and control schemes. In smaller applications, a fan blade attached to the shaft often acts as an artificial load to limit the motor speed to a safe value, as well as a means to circulate cooling airflow over the armature and field windings.

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With the very low cost of semiconductor rectifiers, some applications that would have previously used a universal motor now use a pure DC motor, sometimes with a permanent magnet field.

AC motorsIn 1882, Nikola Tesla invented the rotating magnetic field, and pioneered the use of a rotary field of force to operate machines. He exploited the principle to design a unique two-phase induction motor in 1883. In 1885, Galileo Ferraris independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Tesla had suggested that the commutators from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine.[11] Tesla would later attain U.S. Patent 0,416,194, Electric Motor (December 1889), which resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic motor was an induction motor.

Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. This type of motor is now used for the vast majority of commercial applications.

ComponentsA typical AC motor consists of two parts:

An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;

An inside rotor attached to the output shaft that is given a torque by the rotating field.

Torque motorsA torque motor (also known as a limited torque motor) is a specialized form of induction motor which is capable of operating indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, the motor will apply a steady torque to the load (hence the name).

A common application of a torque motor would be the supply- and take-up reel motors in a tape drive. In this application, driven from a low voltage, the characteristics of these motors allow a relatively-constant light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage, (and so delivering a higher torque), the torque motors can also achieve fast-forward and rewind operation without requiring any additional mechanics such as gears or clutches. In the computer gaming world, torque motors are used in force feedback steering wheels.

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Another common application is the control of the throttle of an internal combustion engine in conjunction with an electronic governor. In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor. The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and, depending on the speed, makes small adjustments to the amount of current applied to the motor. If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more torque, pulling against the return spring and opening the throttle. Should the engine run too fast, the governor will reduce the current being applied to the motor, causing the return spring to pull back and close the throttle.

Slip ringThe slip ring or wound rotor motor is an induction machine where the rotor comprises a set of coils that are terminated in slip rings. These are metal rings rigidly mounted on the rotor, and combined with brushes (as used with commutators), provide continuous unswitched connection to the rotor windings.

In the case of the wound-rotor induction motor, external impedances can be connected to the brushes. The stator is the same as is used with a standard squirrel cage motor. By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered.

(Slip rings are also often used in alternators as well as in synchro angular data-transmission devices, among other applications.)

The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low supply current from zero speed to full speed. This type of motor also offers controllable speed.

Motor speed can be changed because the torque curve of the motor is effectively modified by the amount of resistance connected to the rotor circuit. Increasing the value of resistance will move the speed of maximum torque down. If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced.

When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation is also very poor.

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Stepper motorsClosely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a magnetically-soft rotor with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a rotary solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the stepper motor may not rotate continuously; instead, it "steps" — starts and then quickly stops again — from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards and it may change direction, stop, speed up or slow down arbitrarily at any time.

Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions; more sophisticated drivers can proportionally control the power to the field windings, allowing the rotors to position between the cog points and thereby rotate extremely smoothly. This mode of operation is often called micro stepping. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.

Stepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors are used for read/write head positioning in computer floppy diskette drives. They were used for the same purpose in pre-gigabyte era computer disk drives, where the precision and speed they offered was adequate for the correct positioning of the read/write head of a hard disk drive. As drive density increased, the precision and speed limitations of stepper motors made them obsolete for hard drives—the precision limitation made them unusable, and the speed limitation made them uncompetitive—thus newer hard disk drives use voice coil-based head actuator systems. (The term "voice coil" in this connection is historic; it refers to the structure in a typical (cone type) loudspeaker. This structure was used for a while to position the heads. Modern drives have a pivoted coil mount; the coil swings back and forth, something like a blade of a rotating fan. Nevertheless, like a voice coil, modern actuator coil conductors (the magnet wire) move perpendicular to the magnetic lines of force.)

Stepper motors were and still are often used in computer printers, optical scanners, and digital photocopiers to move the optical scanning element, the print head carriage (of dot matrix and inkjet printers), and the platen. Likewise, many computer plotters (which since the early 1990s have been replaced with large-format inkjet and laser printers) used rotary stepper motors for pen and platen movement; the typical alternatives here were either linear stepper motors or servomotors with complex closed-loop control systems.

So-called quartz analog wristwatches contain the smallest commonplace stepping motors; they have one coil, draw very little power, and have a permanent-magnet rotor. The same kind of motor drives battery-powered quartz clocks. Some of these watches, such as chronographs, contain more than one stepping motor.

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Stepper motors were up scaled to be used in electric vehicles under the term SRM (switched reluctance machine).

Linear motorsA linear motor is essentially an electric motor that has been "unrolled" so that, instead of producing a torque (rotation), it produces a straight-line force along its length by setting up a traveling electromagnetic field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground, and in many roller-coasters where the rapid motion of the motor less railcar is controlled by the rail. On a smaller scale, at least one letter-size (8.5" x 11") computer graphics X-Y pen plotter made by Hewlett-Packard (in the late 1970s to mid 1980's) used two linear stepper motors to move the pen along the two orthogonal axes.

Doubly-fed electric motorDoubly-fed electric motors have two independent multiphase windings that actively participate in the energy conversion process with at least one of the winding sets electronically controlled for variable speed operation. Two is the most active multiphase winding sets possible without duplicating singly-fed or doubly-fed categories in the same package. As a result, doubly-fed electric motors are machines with an effective constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is twice the constant torque speed range as singly-fed electric machines, which have only one active winding set.

A doubly-fed motor allows for a smaller electronic converter but the cost of the rotor winding and slip rings may offset the saving in the power electronics components. Difficulties with controlling speed near synchronous speed limit applications.[12]

Singly-fed electric motorSingly-fed electric machines incorporate a single multiphase winding set that is connected to a power supply. Singly-fed electric machines may be either induction or synchronous. The active winding set can be electronically controlled. Induction machines develop starting torque at zero speed and can operate as standalone machines. Synchronous machines must have auxiliary means for startup, such as a starting induction squirrel-cage winding or an electronic controller. Singly-fed electric machines have an effective constant torque speed range up to synchronous speed for a given excitation frequency.

The induction (asynchronous) motors (i.e., squirrel cage rotor or wound rotor), synchronous motors (i.e., field-excited, permanent magnet or brushless DC motors, reluctance motors, etc.), which are discussed on this page, are examples of singly-fed motors. By far, singly-fed motors are the predominantly installed type of motors.

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Motor standardsThe following are major design and manufacturing standards covering electric motors:

International Electro technical Commission: IEC 60034 Rotating Electrical Machines

National Electrical Manufacturers Association (USA): NEMA MG 1 Motors and Generators

Underwriters Laboratories (USA): UL 1004 - Standard for Electric Motors

UsesElectric motors are used in many, if not most, modern machines. Obvious uses would be in rotating machines such as fans, turbines, drills, the wheels on electric cars, and conveyor belts. Also, in many vibrating or oscillating machines, an electric motor spins an irregular figure with more area on one side of the axle than the other, causing it to appear to be moving up and down.

Electric motors are also popular in robotics. They are used to turn the wheels of vehicular robots, and servo motors are used to turn arms and legs in humanoid robots. In flying robots, along with helicopters, a motor causes a propeller or wide, flat blades to spin and create drag force, allowing vertical motion.

In industrial and manufacturing businesses, electric motors are used to turn saws and blades in cutting and slicing processes, and to spin gears and mixers (the latter very common in food manufacturing). Linear motors are often used to push products into containers horizontally.

Many kitchen appliances also use electric motors to accomplish various jobs. Food processors and grinders spin blades to chop and break up foods. Blenders use electric motors to mix liquids, and microwave ovens use motors to turn the tray food sits on. Toaster ovens also use electric motors to turn a conveyor in order to move food over heating elements.

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Single Phase Motors

Many motor applications use single phase power; especially for smaller horsepower motors. Most of these applications are residential and light commercial, where three phase power is generally unavailable. Single phase AC motors differ from three phase motors. In a three phase motor, the incoming power produces a rotating magnetic current on its own. This allows the three phase motor to be self starting. Single phase motors require additional power in order to produce a rotating magnetic field. Once started, the motor has a changing magnetic field at each pole, allowing the motor to continue running.

The most common method of starting a single phase motor combines a capacitor and auxiliary winding or start circuit. A schematic view shows an auxiliary starting winding, a capacitor, and a centrifugal switch. The auxiliary winding is actually a second winding in the motor.

When current is applied to the motor, both the run winding and the start winding produce magnetic fields. Because the start winding has a lower resistance, a stronger magnetic field is created which causes the motor to begin rotation. Once the motor reaches about 80 percent of its rated speed, a centrifugal switch disconnects the start winding. From this point on, the single phase motor can maintain enough rotating magnetic field to operate on its own. The graph shows a typical torque/speed curve for auxiliary starting on single phase motors.

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There are a variety of starting methods used in the different single phase motor types. These are covered in more detail in the CONTROLS drawer under "Starting." What these starting methods all have in common is the ability to produce a rotating magnetic field using the input power that is applied to the motor.

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Single-phase induction motors

A three phase motor may be run from a single phase power source. (Figure below) However, it will not self-start. It may be hand started in either direction, coming up to speed in a few seconds. It will only develop 2/3 of the 3-φ power rating because one winding is not used.

The single coil of a single phase induction motor does not produce a rotating magnetic field, but a pulsating field reaching maximum intensity at 0o and 180o electrical. (Figure below)

Another view is that the single coil excited by a single phase current produces two counter rotating magnetic field phasors, coinciding twice per revolution at 0o (Figure above-a) and 180o (figure e). When the phasors rotate to 90o and -90o they cancel in figure b. At 45o and -45o (figure c) they are partially additive along the +x axis and cancel along the y axis. An analogous situation exists in figure d. The sum of these two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed.

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However, if the rotor is rotated forward at a bit less than the synchronous speed, It will develop maximum torque at 10% slip with respect to the forward rotating phasor. Less torque will be developed above or below 10% slip. The rotor will see 200% - 10% slip with respect to the counter rotating magnetic field phasor. Little torque (see torque vs slip curve) other than a double freqency ripple is developed from the counter rotating phasor. Thus, the single phase coil will develop torque, once the rotor is started. If the rotor is started in the reverse direction, it will develop a similar large torque as it nears the speed of the backward rotating phasor.

Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of steel laminations, typical of poly-phase induction motors.

Permanent-split capacitor motor

One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent-split capacitor motor in Figure below.

This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor. This motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere in this chapter.

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Single phase induction motors may have coils embedded into the stator as shown in Figure above for larger size motors. Though, the smaller sizes use less complex to build concentrated windings with salient poles.

Capacitor-start induction motor

In Figure below a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes.

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Capacitor-run motor induction motor

A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.

A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

Resistance split-phase motor induction motor

If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. (Figure below) With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.

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This motor has more starting torque than a shaded pole motor (next section), but not as much as a two phase motor built from the same parts. The current density in the auxiliary winding is so high during starting that the consequent rapid temperature rise precludes frequent restarting or slow starting loads.

Summary: Single-phase induction motors

Single-phase induction motors are not self-starting without an auxiliary stator winding driven by an out of phase current of near 90o. Once started the auxiliary winding is optional.

The auxiliary winding of a permanent-split capacitor motor has a capacitor in series with it during starting and running.

A capacitor-start induction motor only has a capacitor in series with the auxiliary winding during starting.

A capacitor-run motor typically has a large non-polarized electrolytic capacitor in series with the auxiliary winding for starting, then a smaller non-electrolytic capacitor during running.

The auxiliary winding of a resistance split-phase motor develops a phase difference versus the main winding during starting by virtue of the difference in resistance.

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Shaded pole induction motor

An easy way to provide starting torque to a single phase motor is to embed a shorted turn in each pole at 30o to 60o to the main winding. (Figure below) Typically 1/3 of the pole is enclosed by a bare copper strap. These shading coils produce a time lagging damped flux spaced 30o to 60o from the main field. This lagging flux with the undamped main component, produces a rotating field with a small torque to start the rotor.

Starting torque is so low that shaded pole motors are only manufactured in smaller sizes, below 50 watts. Low cost and simplicity suit this motor to small fans, air circulators, and other low torque applications. Motor speed can be lowered by switching reactance in series to limit current and torque, or by switching motor coil taps as in Figure below.

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2-pha s e servo motor

A servo motor is typically part of a feedback loop containing electronic, mechanical, and electrical components. The servo loop is a means of controlling the motion of an object via the motor. A requirement of many such systems is fast response. To reduce acceleration robbing inertial, the iron core is removed from the rotor leaving only a shaft mounted aluminum cup to rotate. (Figure below) The iron core is reinserted within the cup as a static (non-rotating) component to complete the magnetic circuit. Otherwise, the construction is typical of a two phase motor. The low mass rotor can accelerate more rapidly than a squirrel cage rotor.

One phase is connected to the single phase line; the other is driven by an amplifier. One of the windings is driven by a 90o phase shifted waveform. In the above figure, this is accomplished by a series capacitor in the power line winding. The other winding is driven by a variable amplitude sine wave to control motor speed. The phase of the waveform may invert (180o phase shift) to reverse the direction of the motor. This variable sine wave is the output of an error amplifier. See synchro CT section for example. Aircraft control surfaces may be positioned by 400 Hz 2-φ servo motors.

  Hysteresis motor

If the low hysteresis Si-steel laminated rotor of an induction motor is replaced by a slotless winding less cylinder of hardened magnet steel, hysteresis, or lagging behind of rotor magnetization, is greatly accentuated. The resulting low torque synchronous motor develops constant torque from stall to synchronous speed. Because of the low torque, the hysteresis motor is only available in very small sizes, and is only used for constant speed applications like clock drives, and formerly, phonograph turntables.

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  Eddy current clutch

If the stator of an induction motor or a synchronous motor is mounted to rotate independently of the rotor, an eddy current clutch results. The coils are excited with DC and attached to the mechanical load. The squirrel cage rotor is attached to the driving motor. The drive motor is started with no DC excitation to the clutch. The DC excitation is adjusted from zero to the desired final value providing a continuously and smoothly variable torque. The operation of the eddy current clutch is similar to an analog eddy current automotive speedometer.

Summary: Other specialized motors The shaded pole induction motor, used in under 50 watt low torque

applications, develops a second phase from shorted turns in the stator.

Hysteresis motors are a small low torque synchronous motor once used in clocks and phonographs.

The eddy current clutch provides an adjustable torque.

Selsyn (synchro) motors

Normally, the rotor windings of a wound rotor induction motor are shorted out after starting. During starting, resistance may be placed in series with the rotor windings to limit starting current. If these windings are connected to a common starting resistance, the two rotors will remain synchronized during starting. (Figure below) This is usefull for printing presses and draw bridges, where two motors need to be synchronized during starting. Once started, and the rotors are shorted, the synchronizing torque is absent. The higher the resistance during starting, the higher the synchronizing torque for a pair of motors. If the starting resistors are removed, but the rotors still paralleled, there is no starting torque. However there is a substantial synchronizing torque. This is a selsyn, which is an abbreviation for “self synchronous”.

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The rotors may be stationary. If one rotor is moved through an angle θ, the other selsyn shaft will move through an angle θ. If drag is applied to one selsyn, this will be felt when attempting to rotate the other shaft. While multi-horsepower (multi-kilowatt) selsyns exist, the main application is small units of a few watts for instrumentation applications– remote position indication.

Instrumentation selsyns have no use for starting resistors. (Figure above) They are not intended to be self rotating. Since the rotors are not shorted out nor resistor loaded, no starting torque is developed. However, manual rotation of one shaft will produce an unbalance in the rotor currents until the parallel unit's shaft follows. Note that a common source of three phase power is applied to both stators. Though we show three phase rotors above, a single phase powered rotor is sufficient as shown in Figure below.

Wound rotor induction motors

A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting. (Figure below) This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart.

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Wound rotor induction motor.

Why put resistance in series with the rotor? Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting. While this is not a severe problem for small motors, it is for large (10's of kW) motors. Placing resistance in series with the rotor windings not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT). Figure below shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting.

Breakdown torque peak is shifted to zero speed by increasing rotor resistance.

The resistance decreases the torque available at full running speed. But that resistance is shorted out by the time the rotor is started. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor.

This motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its' synchronous speed.

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Speed control

Motor speed may be varied by putting variable resistance back into the rotor circuit. This reduces rotor current and speed. The high starting torque available at zero speed, the down shifted break down torque, is not available at high speed. See R2 curve at 90% Ns, Figure below. Resistors R0R1R2R3 increase in value from zero. A higher resistance at R3 reduces the speed further. Speed regulation is poor with respect to changing torque loads. This speed control technique is only useful over a range of 50% to 100% of full speed. Speed control works well with variable speed loads like elevators and printing presses.

Rotor resistance controls speed of wound rotor induction motor.

Wound rotor induction motor qualities. Excellent starting torque for high inertia loads.

Low starting current compared to squirrel cage induction motor.

Speed is resistance variable over 50% to 100% full speed.

Higher maintenance of brushes and slip rings compared to squirrel cage motor.

The generator version of the wound rotor machine is known as a doubly-fed induction generator, a variable speed machine.

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Tesla poly-phase induction motors

Most AC motors are induction motors. Induction motors are favored due to their ruggedness and simplicity. In fact, 90% of industrial motors are induction motors.

Nikola Tesla conceived the basic principals of the polyphase induction motor in 1883, and had a half horsepower (400 watt) model by 1888. Tesla sold the manufacturing rights to George Westinghouse for $65,000.

Most large ( > 1 hp or 1 kW) industrial motors are poly-phase induction motors. By poly-phase, we mean that the stator contains multiple distinct windings per motor pole, driven by corresponding time shifted sine waves. In practice, this is two or three phases. Large industrial motors are 3-phase. While we include numerous illustrations of two-phase motors for simplicity, we must emphasize that nearly all poly-phase motors are three-phase. By induction motor, we mean that the stator windings induce a current flow in the rotor conductors, like a transformer, unlike a brushed DC commutator motor.

Construction

An induction motor is composed of a rotor, known as an armature, and a stator containing windings connected to a poly-phase energy source as shown in Figure below. The simple 2-phase induction motor below is similar to the 1/2 horsepower motor which Nikola Tesla introduced in 1888.

Tesla polyphase induction motor.

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The stator in Figure above is wound with pairs of coils corresponding to the phases of electrical energy available. The 2-phase induction motor stator above has 2-pairs of coils, one pair for each of the two phases of AC. The individual coils of a pair are connected in series and correspond to the opposite poles of an electromagnet. That is, one coil corresponds to a N-pole, the other to a S-pole until the phase of AC changes polarity. The other pair of coils is oriented 90o in space to the first pair. This pair of coils is connected to AC shifted in time by 90o in the case of a 2-phase motor. In Tesla's time, the source of the two phases of AC was a 2-phase alternator.

The stator in Figure above has salient, obvious protruding poles, as used on Tesla's early induction motor. This design is used to this day for sub-fractional horsepower motors (<50 watts). However, for larger motors less torque pulsation and higher efficiency results if the coils are embedded into slots cut into the stator laminations. (Figure below)

Stator frame showing slots for windings.

The stator laminations are thin insulated rings with slots punched from sheets of electrical grade steel. A stack of these is secured by end screws, which may also hold the end housings.

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Stator with (a) 2-φ and (b) 3-φ windings.

In Figure above, the windings for both a two-phase motor and a three-phase motor have been installed in the stator slots. The coils are wound on an external fixture, then worked into the slots. Insulation wedged between the coil periphery and the slot protects against abrasion.

Actual stator windings are more complex than the single windings per pole in Figure above. Comparing the 2-φ motor to Tesla's 2-φ motor with salient poles, the number of coils is the same. In actual large motors, a pole winding, is divided into identical coils inserted into many smaller slots than above. This group is called a phase belt. See Figure below. The distributed coils of the phase belt cancel some of the odd harmonics, producing a more sinusoidal magnetic field distribution across the pole. This is shown in the synchronous motor section. The slots at the edge of the pole may have fewer turns than the other slots. Edge slots may contain windings from two phases. That is, the phase belts overlap.

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The key to the popularity of the AC induction motor is simplicity as evidenced by the simple rotor (Figure below). The rotor consists of a shaft, a steel laminated rotor, and an embedded copper or aluminum squirrel cage, shown at (b) removed from the rotor. As compared to a DC motor armature, there is no commutator. This eliminates the brushes, arcing, sparking, graphite dust, brush adjustment and replacement, and re-machining of the commutator.

Laminated rotor with (a) embedded squirrel cage, (b) conductive cage removed from rotor.

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The squirrel cage conductors may be skewed, twisted, with respect to the shaft. The misalignment with the stator slots reduces torque pulsations.

Both rotor and stator cores are composed of a stack of insulated laminations. The laminations are coated with insulating oxide or varnish to minimize eddy current losses. The alloy used in the laminations is selected for low hysteresis losses.

Theory of operation

A short explanation of operation is that the stator creates a rotating magnetic field which drags the rotor around.

The theory of operation of induction motors is based on a rotating magnetic field. One means of creating a rotating magnetic field is to rotate a permanent magnet as shown in Figure below. If the moving magnetic lines of flux cut a conductive disk, it will follow the motion of the magnet. The lines of flux cutting the conductor will induce a voltage, and consequent current flow, in the conductive disk. This current flow creates an electromagnet whose polarity opposes the motion of the permanent magnet– Lenz's Law. The polarity of the electromagnet is such that it pulls against the permanent magnet. The disk follows with a little less speed than the permanent magnet.

Rotating magnetic field produces torque in conductive disk.

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The torque developed by the disk is proportional to the number of flux lines cutting the disk and the rate at which it cuts the disk. If the disk were to spin at the same rate as the permanent magnet, there would be no flux cutting the disk, no induced current flow, no electromagnet field, no torque. Thus, the disk speed will always fall behind that of the rotating permanent magnet, so that lines of flux cut the disk induce a current, create an electromagnetic field in the disk, which follows the permanent magnet. If a load is applied to the disk, slowing it, more torque will be developed as more lines of flux cut the disk. Torque is proportional to slip, the degree to which the disk falls behind the rotating magnet. More slip corresponds to more flux cutting the conductive disk, developing more torque.

An analog automotive eddy current speedometer is based on the principle illustrated above. With the disk restrained by a spring, disk and needle deflection is proportional to magnet rotation rate.

A rotating magnetic field is created by two coils placed at right angles to each other, driven by currents which are 90o out of phase. This should not be surprising if you are familiar with oscilloscope Lissajous patterns.

Out of phase (90o) sine waves produce circular Lissajous pattern.

In Figure above, a circular Lissajous is produced by driving the horizontal and vertical oscilloscope inputs with 90o out of phase sine waves. Starting at (a) with maximum “X” and minimum “Y” deflection, the trace moves up and left toward (b). Between (a) and (b) the two waveforms are equal to 0.707 Vpk at 45o. This point (0.707, 0.707) falls on the radius of the circle between (a) and (b) The trace moves to (b) with minimum “X” and maximum “Y” deflection. With maximum negative “X” and minimum “Y” deflection, the trace moves to (c). Then with minimum “X” and maximum negative “Y”, it moves to (d), and on back to (a), completing one cycle.

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X-axis sine and Y-axis cosine trace circle.

Figure above shows the two 90o phase shifted sine waves applied to oscilloscope deflection plates which are at right angles in space. If this were not the case, a one dimensional line would display. The combination of 90o phased sine waves and right angle deflection, results in a two dimensional pattern– a circle. This circle is traced out by a counterclockwise rotating electron beam.

For reference, Figure below shows why in-phase sine waves will not produce a circular pattern. Equal “X” and “Y” deflection moves the illuminated spot from the origin at (a) up to right (1,1) at (b), back down left to origin at (c),down left to (-1.-1) at (d), and back up right to origin. The line is produced by equal deflections along both axes; y=x is a straight line.

No circular motion from in-phase waveforms.

If a pair of 90o out of phase sine waves produces a circular Lissajous, a similar pair of currents should be able to produce a circular rotating magnetic field. Such is the case for a 2-phase motor. By analogy three windings placed 120o apart in space, and fed with corresponding 120o phased currents will also produce a rotating magnetic field.

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Rotating magnetic field from 90o phased sinewaves.

As the 90o phased sinewaves, Figure above, progress from points (a) through (d), the magnetic field rotates counterclockwise (figures a-d) as follows:

(a) φ-1 maximum, φ-2 zero

(a') φ-1 70%, φ-2 70%

(b) φ-1 zero, φ-2 maximum

(c) φ-1 maximum negative, φ-2 zero

(d) φ-1 zero, φ-2 maximum negative

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Motor speed

The rotation rate of a stator rotating magnetic field is related to the number of pole pairs per stator phase. The “full speed” Figure below has a total of six poles or three pole-pairs and three phases. However,there is but one pole pair per phase– the number we need. The magnetic field will rotate once per sine wave cycle. In the case of 60 Hz power, the field rotates at 60 times per second or 3600 revolutions per minute (rpm). For 50 Hz power, it rotates at 50 rotations per second, or 3000 rpm. The 3600 and 3000 rpm, are the synchronous speed of the motor. Though the rotor of an induction motor never achieves this speed, it certainly is an upper limit. If we double the number of motor poles, the synchronous speed is cut in half because the magnetic field rotates 180o in space for 360o of electrical sine wave.

Doubling the stator poles halves the synchronous speed.

The synchronous speed is given by:

Ns = 120·f/PNs = synchronous speed in rpmf = frequency of applied power, HzP = total number of poles per phase, a multiple of 2Example:

The “half speed” Figure above has four poles per phase (3-phase). The synchronous speed for 50 Hz power is:

S = 120·50/4 = 1500 rpm

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The short explanation of the induction motor is that the rotating magnetic field produced by the stator drags the rotor around with it.

The longer more correct explanation is that the stator's magnetic field induces an alternating current into the rotor squirrel cage conductors which constitutes a transformer secondary. This induced rotor current in turn creates a magnetic field. The rotating stator magnetic field interacts with this rotor field. The rotor field attempts to align with the rotating stator field. The result is rotation of the squirrel cage rotor. If there were no mechanical motor torque load, no bearing, windage, or other losses, the rotor would rotate at the synchronous speed. However, the slip between the rotor and the synchronous speed stator field develops torque. It is the magnetic flux cutting the rotor conductors as it slips which develops torque. Thus, a loaded motor will slip in proportion to the mechanical load. If the rotor were to run at synchronous speed, there would be no stator flux cutting the rotor, no current induced in the rotor, no torque.

Torque When power is first applied to the motor, the rotor is at rest, while the stator magnetic field rotates at the synchronous speed Ns. The stator field is cutting the rotor at the synchronous speed Ns. The current induced in the rotor shorted turns is maximum, as is the frequency of the current, the line frequency. As the rotor speeds up, the rate at which stator flux cuts the rotor is the difference between synchronous speed Ns and actual rotor speed N, or (Ns - N). The ratio of actual flux cutting the rotor to synchronous speed is defined as slip: s = (Ns - N)/Ns where: Ns = synchronous speed,   N = rotor speed The frequency of the current induced into the rotor conductors is only as high as the line frequency at motor start, decreasing as the rotor approaches synchronous speed. Rotor frequency is given by: fr = s·f where: s = slip,   f = stator power line frequency Slip at 100% torque is typically 5% or less in induction motors. Thus for f = 50 Hz line frequency, the frequency of the induced current in the rotor fr = 0.05·50 = 2.5 Hz. Why is it so low? The stator magnetic field rotates at 50 Hz. The rotor speed is 5% less. The rotating magnetic field is only cutting the rotor at 2.5 Hz. The 2.5 Hz is the difference between the synchronous speed and the actual rotor speed. If the rotor spins a little faster, at the synchronous speed, no flux will cut the rotor at all, fr = 0.

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Torque and speed vs %Slip. %Ns=%Synchronous Speed.The Figure above graph shows that starting torque known as locked rotor torque (LRT) is higher than 100% of the full load torque (FLT), the safe continuous torque rating. The locked rotor torque is about 175% of FLT for the example motor graphed above. Starting current known as locked rotor current (LRC) is 500% of full load current (FLC), the safe running current. The current is high because this is analogous to a shorted secondary on a transformer. As the rotor starts to rotate the torque may decrease a bit for certain classes of motors to a value known as the pull up torque. This is the lowest value of torque ever encountered by the starting motor. As the rotor gains 80% of synchronous speed, torque increases from 175% up to 300% of the full load torque. This breakdown torque is due to the larger than normal 20% slip. The current has decreased only slightly at this point, but will decrease rapidly beyond this point. As the rotor accelerates to within a few percent of synchronous speed, both torque and current will decrease substantially. Slip will be only a few percent during normal operation. For a running motor, any portion of the torque curve below 100% rated torque is normal. The motor load determines the operating point on the torque curve. While the motor torque and current may exceed 100% for a few seconds during starting, continuous operation above 100% can damage the motor. Any motor torque load above the breakdown torque will stall the motor. The torque, slip, and current will approach zero for a “no mechanical torque” load condition. This condition is analogous to an open secondary transformer.

There are several basic induction motor designs (Figure below) showing considerable variation from the torque curve above. The different designs are optimized for starting and running different types of loads. The locked rotor torque (LRT) for various motor designs and sizes ranges from 60% to 350% of full load torque (FLT). Starting current or locked rotor current (LRC) can range from 500% to 1400% of full load current (FLC). This current draw can present a starting problem for large induction motors.

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NEMA design classesVarious standard classes (or designs) for motors, corresponding to the torque curves (Figure below) have been developed to better drive various type loads. The National Electrical Manufacturers Association (NEMA) has specified motor classes A, B, C, and D to meet these drive requirements. Similar International Electrotechnical Commission (IEC) classes N and H correspond to NEMA B and C designs respectively.

Characteristics for NEMA designs.

All motors, except class D, operate at %5 slip or less at full load.

Class B (IEC Class N) motors are the default motor to use in most applications. With a starting torque of LRT = 150% to 170% of FLT, it can start most loads, without excessive starting current (LRT). Efficiency and power factor are high. It typically drives pumps, fans, and machine tools.

Class A starting torque is the same as class B. Drop out torque and starting current (LRT) are higher. This motor handles transient overloads as encountered in injection molding machines.

Class C (IEC Class H) has higher starting torque than class A and B at LRT = 200% of FLT. This motor is applied to hard-starting loads which need to be driven at constant speed like conveyors, crushers, and reciprocating pumps and compressors.

Class D motors have the highest starting torque (LRT) coupled with low starting current due to high slip ( 5% to 13% at FLT). The high slip results in

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lower speed. Speed regulation is poor. However, the motor excels at driving highly variable speed loads like those requiring an energy storage flywheel. Applications include punch presses, shears, and elevators.

Class E motors are a higher efficiency version of class B.

Class F motors have much lower LRC, LRT, and break down torque than class B. They drive constant easily started loads.

Power factor Induction motors present a lagging (inductive) power factor to the power line.The power factor in large fully loaded high speed motors can be as favorable as 90% for large high speed motors. At 3/4 full load the largest high speed motor power factor can be 92%. The power factor for small low speed motors can be as low as 50%. At starting, the power factor can be in the range of 10% to 25%, rising as the rotor achieves speed.

Power factor (PF) varies considerably with the motor mechanical load (Figure below). An unloaded motor is analogous to a transformer with no resistive load on the secondary. Little resistance is reflected from the secondary (rotor) to the primary (stator). Thus the power line sees a reactive load, as low as 10% PF. As the rotor is loaded an increasing resistive component is reflected from rotor to stator, increasing the power factor.

Induction motor power factor and efficiency.

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Synchronous Motors

Single phase synchronous motors are available in small sizes for applications requiring precise timing such as time keeping, (clocks) and tape players. Though battery powered quartz regulated clocks are widely available, the AC line operated variety has better long term accuracy-- over a period of months. This is due to power plant operators purposely maintaining the long term accuracy of the frequency of the AC distribution system. If it falls behind by a few cycles, they will make up the lost cycles of AC so that clocks lose no time.

Above 10 Horsepower (10 kW) the higher efficiency and leading powerfactor make large synchronous motors useful in industry. Large synchronous motors are a few percent more efficient than the more common induction motors. Though, the synchronous motor is more complex.

Since motors and generators are similar in construction, it should be possible to use a generator as a motor, conversely, use a motor as a generator. A synchronous motor is similar to an alternator with a rotating field. The figure below shows small alternators with a permanent magnet rotating field. This figure below could either be two paralleled and synchronized alternators driven by a mechanical energy sources, or an alternator driving a synchronous motor. Or, it could be two motors, if an external power source were connected. The point is that in either case the rotors must run at the same nominal frequency, and be in phase with each other. That is, they must be synchronized. The procedure for synchronizing two alternators is to (1) open the switch, (2) drive both alternators at the same rotational rate, (3) advance or retard the phase of one unit until both AC outputs are in phase, (4) close the switch before they drift out of phase. Once synchronized, the alternators will be locked to each other, requiring considerable torque to break one unit loose (out of synchronization) from the other.

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Three Phase Sine Wave & Three Phase Stator

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1:

2:

3:

4:

5:

6:

7:

8:

9a: 9b:

10:

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A.C. Motor LabelingName: Date: Level:

1

2

3

4

5

6

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8

9a

9b 10

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NAME: LEVEL: DATE:

CHECK LIST FOR OPERATION OF A.C. MOTORS PACKET

STEPS/TASKS

1) The student completed all vocabulary associated with this learning guide to 80% accuracy.

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2) The student completed all written work associated with this learning guide to 80% accuracy.

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3) The student completed the post-test associated with this learning guide to 80% accuracy.

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4) The student completed project #1 to industry standards. 505) The student completed project #2 to industry standards. 506) The student completed project #3 to industry standards. 507) The student completed project #4 to industry standards. 508) The student completed project #5 to industry standards. 509) The student completed project #6 to industry standards. 50

Total Points 375

* ALL STEPS/TASKS MUST MEET THE STANDARDS IN ORDER TO ACHIEVE MASTERY.*

COMMENTS:

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1: 2: 3: 4: 5: 6: 7: 8: 9a: 9b: 10: 11:

1: 2: 3: 4: 5: 6: 7: 8: 9a: 9b: 10: 11:

1: 2: 3: 4: 5: 6: 7: 8: 9a: 9b: 10: 11:

PointsAvailable

Correct/Out of Grade Percentage Check One Percentage Task Grade

Below Basic 0%-69% 0-6 Basic 70%-85% 7 Competent 86%-92% 8-9 Advanced 93%-100% 10

Correct/Out of Grade Percentage Check One Percentage Task Grade

Below Basic 0%-69% 0-6 Basic 70%-85% 7 Competent 86%-92% 8-9 Advanced 93%-100% 10

Correct/Out of Grade Percentage Check One Percentage Task Grade

Below Basic 0%-69% 0-6 Basic 70%-85% 7 Competent 86%-92% 8-9 Advanced 93%-100% 10

Correct/Out of Grade Percentage Check One Percentage Task Grade

Below Basic 0%-69% 0-6 Basic 70%-85% 7 Competent 86%-92% 8-9 Advanced 93%-100% 10

Correct/Out of 375 Grade Percentage Check One Percentage Task Grade

Below Basic 0%-69% 0-6 Basic 70%-85% 7 Competent 86%-92% 8-9 Advanced 93%-100% 10

PointsEarned

INSTRUCTOR SIGNATURE: DATE:

******************* PROJECT #1 *******************

“Split Phase Motor Operation ”

1. Obtain Rotating Electric Machines Project Packet(Supplied by Instructor), LAP-5 Split Phase Motors.2. Read and understand pages 15 – 22.3. Perform project steps on pages 23 – 27 (Record required readings as you go along)4. Complete “self review” on page 28.5. Hand packet into instructor for review.

******************** PROJECT #2 ********************

“How To Reverse The Rotation Of a Split Phase Motor ”

1. Obtain Rotating Electric Machines Project Packet(Supplied by Instructor), LAP-5 Split Phase Motors.2. Read and understand page 43.3. Perform project steps on pages 43- 45(Record required readings as you go along)4. Complete “self review” on page 46.5. Hand packet into instructor for review.

******************* PROJECT #3 *******************

“Capacitor Start AC Motors ”

1. Obtain Rotating Electric Machines Project Packet(Supplied by Instructor), LAP-6 Capacitor Start AC Motors.2. Read and understand pages 23 – 24.3. Perform project steps on pages 25 – 29 (Record required readings as you go along)4. Complete “self review” on page 30.5. Hand packet into instructor for review.

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******************* PROJECT #4 *******************

“Capacitor Start AC Motors ”

1. Obtain Rotating Electric Machines Project Packet(Supplied by Instructor), LAP-6 Capacitor Start AC Motors.2. Read and understand page 39.3. Perform project steps on pages 39 – 41 (Record required readings as you go along)4. Complete “self review” on page 42.5. Hand packet into instructor for review.

******************* PROJECT #5 *******************

“Synchronous Motors ”

1. Obtain Rotating Electric Machines Project Packet(Supplied by Instructor), LAP-13 Synchronous Motors.2. Read and understand pages 4 - 14.3. Perform project steps on pages 15 – 20 (Record required readings as you go along)4. Complete “self review” on page 21.5. Make sure you fill out safety checks on pages 16 & 17.6. Hand packet into instructor for review.

******************* PROJECT #6 *******************

“Synchronous Motors ”

1. Obtain Rotating Electric Machines Project Packet(Supplied by Instructor), LAP-13 Synchronous Motors.2. Read and understand page 47.3. Perform project steps on pages 48 - 51 (Record required readings as you go along)4. Complete “self review” on page 52.5. Make sure you fill out safety checks on page 49.6. Hand packet into instructor for review.

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***************100 POINTS EXTRA CREDIT EACH *************** Any student who brings in a picture of a Single Phase A.C. Motor.

Any student who brings in a picture of a Three Phase A.C. Motor.

Any student who brings in a picture of a Synchronous A.C. Motor.

Any student who brings in a picture of a Capacitor Start, Capacitor Run Motor.

REMEMBER SAFETY FIRST… DO NOT ATTEMPT ANY UNSAFE ACT TO AQUIRE

PICTURES!!!

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NAME: DATE:

OPERATION OF AC MOTORS MODULE POST-TEST

Multiple ChoiceIdentify the choice that best completes the statement or answers the question.

____ 1. In a three-phase motor, the speed of rotation of the magnetic field is called the _____ speed.a. magneticb. slipc. synchronousd. rotor

____ 2. To reverse the direction of rotation of a three-phase motor, reversea. any one leadb. any two leadsc. any three leadsd. all leads

____ 3. If a piece of machinery needs a replacement motor and will be damaged if the new motor rotates the wrong way, the test equipment needed to test the rotation direction of the new motor is aa. phase angle voltmeterb. MEGGER®

c. digital multimeterd. phase rotation meter

____ 4. Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the higher of the two voltages, the windings are connected ina. seriesb. parallel

____ 5. Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the lower of the two voltages, the windings are connected ina. seriesb. parallel

____ 6. The squirrel-cage rotor is constructed of _____ and end _____ embedded in steel laminations.a. bars, barsb. rings, ringsc. bars, ringsd. rings, bars

____ 7. The torque of a squirrel-cage induction motor will be maximum when the phase angle between the stator and rotor fields isa. 0°b. 90°c. 180°d. 270°

____ 8. When a squirrel-cage motor is first started, the value of the current draw isa. equal to the running currentb. smaller than the running current

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c. twice the running currentd. several times the running current

____ 9. An induction motor rotor rotates slower than the magnetic field. How much slower is normally stated as slip speed in RPM or as percent slip? For most squirrel-cage induction motors the typical percent slip is about _____ percent.a. 0 to 1b. 2 to 5c. 5 to 10d. 10 to 15

____ 10. When a normal induction motor is running, the rotor does not rotate as fast as the magnetic field rotates. The rotor is being cut by the magnetic field which is inducing a voltage which is at the “rotor frequency.” This frequency is _____ the synchronous frequency.a. higher thanb. the same asc. lower thand. half of

____ 11. A motor has a synchronous speed of 3600 RPM and an actual rotor speed of 3450 RPM. What is the percent slip?a. 4.17%b. 4.35%c. 24%d. 23%

____ 12. The squirrel-cage rotor code letters indicate the shape (type) of rotor bars. Different types of bars have a large effect on the _____ of a motor.a. operating voltageb. overload currentc. starting torqued. synchronous speed

____ 13. If one phase of voltage to a three-phase motor opens, the motor may slow down, overheat, or possibly stop. This condition is known asa. double phasingb. single phasingc. triple phasingd. free running

____ 14. In the consequent pole motor, when the polarity of half of the pole windings is reversed, thereby effectively doubling the number of poles, the motor speed willa. stay the sameb. cut in halfc. doubled. go to zero

____ 15. The wound-rotor induction motor is popular in industry because it has _____ starting torque and _____ starting current.a. low, lowb. low, highc. high, lowd. high, high

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____ 16. A motor is operating at 3450 RPM and is producing a torque of 32 lb-in. What is the output horsepower of the motor?a. 1.75 hpb. 21 hpc. 0.576 hpd. 6.93 hp

____ 17. The set of squirrel-cage bars on the rotor of a synchronous motor is used to start the motor and is known as thea. amortisseur windingb. starting barsc. synchronous starterd. rotor bars

____ 18. A synchronous motor operated with no load and used for power factor correction is sometimes called aa. synchronizerb. capacitorc. selsynd. synchronous condenser

____ 19. Selsyn motors operate on _____ phase voltage.a. twob. singlec. three

____ 20. The differential selsyn has three stator windings connected in a wye and a rotor made ofa. three windings connected in a wyeb. one winding

____ 21. Split-phase motors are so named because they operate on single-phase voltage, but internally they divide the voltage in such a manner as to create a _____ magnetic field.a. fixedb. pulsatingc. vibratingd. rotating

____ 22. A two-phase power system has two voltages which area. in phaseb. 90° out of phasec. 180° out of phased. 360° out of phase

____ 23. The _____ winding is made of many turns of small wire and is located at the top of the stator core.a. startb. runc. synchronousd. disconnect

____ 24. The _____ winding is made of a few turns of large wire and is located at the bottom of the stator core.a. startb. runc. synchronous

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d. disconnect

____ 25. In a motor that has a rotating magnetic field, the speed of that magnetic field is referred to as thea. rotor speedb. stator speedc. synchronous speedd. slip speed

____ 26. In single-phase motors such as the resistance start induction-run motor, the start winding is disconnected by thea. centrifugal switchb. disconnect switchc. operatord. overload switch

____ 27. In the shaded pole induction motor, the direction of rotation of the magnetic field isa. clockwiseb. counterclockwisec. away from the shading coild. toward the shading coil

____ 28. In the shaded pole motor, the shading coil acts like a(n) _____ transformer secondary.a. openb. shorted

____ 29. Multispeed fan motors achieve multiple speeds by inserting additional _____ in series with the run winding.a. windingsb. resistorsc. capacitorsd. poles

____ 30. The consequent pole motor is essentially a _____ speed motor.a. oneb. twoc. threed. four

____ 31. A _____ motor operates on the principle that like magnetic poles repel.a. impedanceb. hysteresisc. repulsiond. inductive

____ 32. The single-phase synchronous motors are based on the stator of the shaded pole motor. They operate at a speed which is synchronous with the applied frequency. These motors are also called _____ motors.a. impedanceb. repulsionc. hysteresisd. inductive

____ 33. The Warren motor is a variation on the shaded pole motor. It operates at _____ RPM.a. 1200b. 1725c. 1800d. 3600

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____ 34. The Holtz motor is a variation on the shaded pole motor. It operates at _____ RPM.a. 1200b. 1725c. 1800d. 3600

____ 35. The output shaft of a stepping motora. moves through a specific angle for each input pulseb. rotates at a constant speed during each input pulsec. steps continuously when power is appliedd. rotates continuously when power is applied

____ 36. The _____ motor can operate on either AC or DC voltage.a. consequent poleb. universalc. shaded poled. split-phase

____ 37. The universal motor has a wound armature and brushes like an AC motor. The universal motor has an added component, thea. inductive windingb. compensating windingc. neutral planed. synchronous winding

____ 38. Conductive compensation is accomplished by connecting the compensating winding in _____ with the armature.a. seriesb. parallel

____ 39. In a universal motor, inductive compensation can be used when the motor is connected toa. AC onlyb. DC onlyc. either AC or DC

____ 40. In a universal motor, setting the brushes to the neutral plane can be done by connecting voltage to the armature, a voltmeter to the series field, and adjusting the brush position for a _____ on the voltmeter.a. minimumb. maximum

____ 41. In a universal motor, setting the brushes to the neutral plane can be done by connecting voltage to the armature, a voltmeter to the compensating winding, and adjusting the brush position for a _____ on the voltmeter.a. minimumb. maximum

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OPERATION OF AC MOTORS MODULE PRE-TEST

Multiple ChoiceIdentify the choice that best completes the statement or answers the question.

____ 1. In a three-phase motor, the speed of rotation of the magnetic field is called the _____ speed.a. magneticb. slipc. synchronousd. rotor

____ 2. To reverse the direction of rotation of a three-phase motor, reversea. any one leadb. any two leadsc. any three leadsd. all leads

____ 3. If a piece of machinery needs a replacement motor and will be damaged if the new motor rotates the wrong way, the test equipment needed to test the rotation direction of the new motor is aa. phase angle voltmeterb. MEGGER®

c. digital multimeterd. phase rotation meter

____ 4. Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the higher of the two voltages, the windings are connected ina. seriesb. parallel

____ 5. Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the lower of the two voltages, the windings are connected ina. seriesb. parallel

____ 6. The squirrel-cage rotor is constructed of _____ and end _____ embedded in steel laminations.a. bars, barsb. rings, ringsc. bars, ringsd. rings, bars

____ 7. The torque of a squirrel-cage induction motor will be maximum when the phase angle between the stator and rotor fields isa. 0°b. 90°c. 180°d. 270°

____ 8. When a squirrel-cage motor is first started, the value of the current draw is

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a. equal to the running currentb. smaller than the running currentc. twice the running currentd. several times the running current

____ 9. An induction motor rotor rotates slower than the magnetic field. How much slower is normally stated as slip speed in RPM or as percent slip? For most squirrel-cage induction motors the typical percent slip is about _____ percent.a. 0 to 1b. 2 to 5c. 5 to 10d. 10 to 15

____ 10. When a normal induction motor is running, the rotor does not rotate as fast as the magnetic field rotates. The rotor is being cut by the magnetic field which is inducing a voltage which is at the “rotor frequency.” This frequency is _____ the synchronous frequency.a. higher thanb. the same asc. lower thand. half of

____ 11. A motor has a synchronous speed of 3600 RPM and an actual rotor speed of 3450 RPM. What is the percent slip?a. 4.17%b. 4.35%c. 24%d. 23%

____ 12. The squirrel-cage rotor code letters indicate the shape (type) of rotor bars. Different types of bars have a large effect on the _____ of a motor.a. operating voltageb. overload currentc. starting torqued. synchronous speed

____ 13. If one phase of voltage to a three-phase motor opens, the motor may slow down, overheat, or possibly stop. This condition is known asa. double phasingb. single phasingc. triple phasingd. free running

____ 14. In the consequent pole motor, when the polarity of half of the pole windings is reversed, thereby effectively doubling the number of poles, the motor speed willa. stay the sameb. cut in halfc. doubled. go to zero

____ 15. The wound-rotor induction motor is popular in industry because it has _____ starting torque and _____ starting current.a. low, lowb. low, highc. high, lowd. high, high

80

____ 16. A motor is operating at 3450 RPM and is producing a torque of 32 lb-in. What is the output horsepower of the motor?a. 1.75 hpb. 21 hpc. 0.576 hpd. 6.93 hp

____ 17. The set of squirrel-cage bars on the rotor of a synchronous motor is used to start the motor and is known as thea. amortisseur windingb. starting barsc. synchronous starterd. rotor bars

____ 18. A synchronous motor operated with no load and used for power factor correction is sometimes called aa. synchronizerb. capacitorc. selsynd. synchronous condenser

____ 19. Selsyn motors operate on _____ phase voltage.a. twob. singlec. three

____ 20. The differential selsyn has three stator windings connected in a wye and a rotor made ofa. three windings connected in a wyeb. one winding

____ 21. Split-phase motors are so named because they operate on single-phase voltage, but internally they divide the voltage in such a manner as to create a _____ magnetic field.a. fixedb. pulsatingc. vibratingd. rotating

____ 22. A two-phase power system has two voltages which area. in phaseb. 90° out of phasec. 180° out of phased. 360° out of phase

____ 23. The _____ winding is made of many turns of small wire and is located at the top of the stator core.a. startb. runc. synchronousd. disconnect

____ 24. The _____ winding is made of a few turns of large wire and is located at the bottom of the stator core.a. start

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b. runc. synchronousd. disconnect

____ 25. In a motor that has a rotating magnetic field, the speed of that magnetic field is referred to as thea. rotor speedb. stator speedc. synchronous speedd. slip speed

____ 26. In single-phase motors such as the resistance start induction-run motor, the start winding is disconnected by thea. centrifugal switchb. disconnect switchc. operatord. overload switch

____ 27. In the shaded pole induction motor, the direction of rotation of the magnetic field isa. clockwiseb. counterclockwisec. away from the shading coild. toward the shading coil

____ 28. In the shaded pole motor, the shading coil acts like a(n) _____ transformer secondary.a. openb. shorted

____ 29. Multispeed fan motors achieve multiple speeds by inserting additional _____ in series with the run winding.a. windingsb. resistorsc. capacitorsd. poles

____ 30. The consequent pole motor is essentially a _____ speed motor.a. oneb. twoc. threed. four

____ 31. A _____ motor operates on the principle that like magnetic poles repel.a. impedanceb. hysteresisc. repulsiond. inductive

____ 32. The single-phase synchronous motors are based on the stator of the shaded pole motor. They operate at a speed which is synchronous with the applied frequency. These motors are also called _____ motors.a. impedanceb. repulsionc. hysteresisd. inductive

____ 33. The Warren motor is a variation on the shaded pole motor. It operates at _____ RPM.a. 1200b. 1725

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c. 1800d. 3600

____ 34. The Holtz motor is a variation on the shaded pole motor. It operates at _____ RPM.a. 1200b. 1725c. 1800d. 3600

____ 35. The output shaft of a stepping motora. moves through a specific angle for each input pulseb. rotates at a constant speed during each input pulsec. steps continuously when power is appliedd. rotates continuously when power is applied

____ 36. The _____ motor can operate on either AC or DC voltage.a. consequent poleb. universalc. shaded poled. split-phase

____ 37. The universal motor has a wound armature and brushes like an AC motor. The universal motor has an added component, thea. inductive windingb. compensating windingc. neutral planed. synchronous winding

____ 38. Conductive compensation is accomplished by connecting the compensating winding in _____ with the armature.a. seriesb. parallel

____ 39. In a universal motor, inductive compensation can be used when the motor is connected toa. AC onlyb. DC onlyc. either AC or DC

____ 40. In a universal motor, setting the brushes to the neutral plane can be done by connecting voltage to the armature, a voltmeter to the series field, and adjusting the brush position for a _____ on the voltmeter.a. minimumb. maximum

____ 41. In a universal motor, setting the brushes to the neutral plane can be done by connecting voltage to the armature, a voltmeter to the compensating winding, and adjusting the brush position for a _____ on the voltmeter.a. minimumb. maximum

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