5. · Web view5.1.1. The Three Phase Squirrel Cage Induction Motor A squirrel-cage rotor is the...

5. Three Phase Motors and Starters By

Ritesh Hariram

5.1. Introduction to Three Phase Motors

5.1.1. The Three Phase Squirrel Cage Induction Motor A squirrel-cage rotor is the rotating part (rotor) used in the most common form of AC induction motor. It consists of a cylinder of steel with aluminium or copper conductors embedded in its surface. An electric motor with a squirrel-cage rotor is termed a squirrel-cage motor. (wiki)

(wiki)

Figure 1: Squirrel cage rotor

5.1.2. Principal of Operation The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and in effect cause a rotational motion on the rotor.

However, for these currents to be induced, the speed of the physical rotor must be less than the speed of the rotating magnetic field in the stator, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unit less and is the ratio between the relative speeds of the magnetic field as seen by the rotor (the slip speed) to the speed of the rotating stator field. Due to this an induction motor is sometimes referred to as an asynchronous machine.

5.1.3. Construction Any Induction Motor has a Stator and a Rotor. The construction of Stator for any induction motor is almost the same but the rotor construction differs with respect to the type of motor used.

The Stator

Figure 2

The stator is the outer most component in the motor which can be seen. It may be constructed for single phase, three phase or even poly phase motors. But basically

only the windings on the stator vary, not the basic layout of the stator. It is almost the same for any given synchronous motor or generator. It is made up of number of stampings, which are slotted to receive the windings. The three phase windings are placed on the slots of laminated core and these windings are electrically spaced 120 degrees apart. These windings are connected as either star or delta depending upon the requirement. The leads are taken out, usually three in number, brought out to the terminal box, mounted on the motor frame. The insulations between the windings are generally varnish or oxide coated.

The Rotor (Squirrel Cage)

This kind of rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors, which are not wires, as we think, but thick, heavy bars of copper or aluminium (aluminium) or its alloys. The conductor bars are inserted from one end of the rotor and as one bar in each slot. There are end rings which are welded or electrically braced or even bolted at both ends of the rotor, thus maintaining electrical continuity. These end rings are short-circuited, after which they give a beautiful look similar to a squirrel thus the name.

One important point to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, thus it is not possible to add any external resistance in series with the rotor circuit for starting purpose. The rotor conducting bars are usually not parallel to the shaft, but are purposely given slight skew. In small motors, the rotor is fabricated in a different way. The entire rotor core is placed in a mould and the rotor bars & end-rings are cast into one piece. The metal commonly used is aluminium alloy. Some very small rotors which operate on the basis of eddy current, have their rotor as solid steel without any conductors.

(thubengineeringhtt2)

Figure 3: Rotor

5.1.4. Advantages of the Squirrel Cage Motor They are easy to manufacture. For the same size they are cheap compared to other types of motors. They are robust and require very little maintenance. They are available in wide range of sizes and power outputs. For the same size three phase motors provide more power and torque than

single phase motors. They can be connected in Star or Delta, provided that the winding insulation is

suitable for the voltage. They can be mounted horizontally or vertically. With suitable inverter power drives, their speed can be varied.

(Swart, 2012)

5.1.5. Applications Squirrel cage induction motors are simple and rugged in construction, are relatively cheap, and require little maintenance. Hence, squirrel cage induction motors are preferred in most of the industrial applications such as in:

1. Lathes

2. Drilling machines

3. Agricultural and industrial pumps

4. Industrial drives.

5.1.6. Calculations on Slip, Power and Efficiency Synchronous Speed and Rotor Speed

The rotor currents will only be induced if there is relative movement between the rotating field and the rotor conductors. Therefore the rotor cannot reach the speed of the rotating magnetic field. The difference between rotor speed and the stator’s rotating magnetic field is called the “slip” of the motor. The speed at which the stator’s magnetic field rotates is called the synchronous speed and it is determined by the number of pole pairs per phase and the supply frequency.

Synchronous Speed ns=60×fp RPM

Since the slip is the difference between the synchronous speed and the rotor speed as a percentage of the synchronous speed, it can be calculated as follows:

Slip s=Synchronous speed−Rotor speedSynchronous speed

=ns−nr

ns

By rearing the formula we can use the synchronous speed of the magnetic field and the slip to calculate the rotor speed of the motor.

Motor speed nr= ns(1−s¿ RPM

From the speed on the name plate of a motor one can determine how many poles the motor has. For a supply frequency of 50Hz the synchronous speed of the magnetic field is reduced with every additional pole pair, for instance 300rpm, 1500rpm ,750 rpm etc. To determine the poles, the synchronous speed that is closest to the motor speed on the name plate is taken.

Power

Three phase induction motors can also be connected in star and delta provided the insulation of the windings is suitable to handle the applied voltage, especially when connected in delta. The star and delta formula also hold true for three phase motors. (Swart, 2012)

Table 1

Star configuration Delta configurationVoltage V L=√3×v p

v V l = V p vCurrent I L = IP A I l = √ 3x I P A

The power formulas are as follows:

Total three phase apparent power: Pa = √3×V L× I L VA

Total three phase active power: P=√3×V L×ILcos θ W

Total three phase apparent power: Pr= √3×V L ×I LsinθVAr

Power Factor PF= PPa = cosθ

Efficiency

Similar to transformers the induction motor is also susceptible to internal losses which can be placed in the following categories:

Copper losses, which are the I 2R heat losses due to the currents in the resistance of the stator and rotor conductors.

Magnetic or Iron Losses, due to hysteresis of the magnetic field and eddy currents in the core material where the magnetic flux passes through.

Mechanical losses due to the friction of the bearings and the wind resistance of the cooling fan.

These losses are quantified in watts and must be subtracted from the real power that the motor delivers to the axis of the rotor. Usually motors are 85% efficient, but this figure varies with the size of the motor, the type of motor and the load of the motor. Larger motors have a higher efficiency than small motors, and motors running at full load also have higher efficiencies than motors running at lower than their rated mechanical loads. Once the real power component of a motor has been determined, the mechanical output power can be calculated by multiplying the real power with the efficiency. The power that is given on the name plate of a motor is the available full load mechanical power. Therefore for a three phase motor:

Mechanical Power Pout = P. η

= Pa. cosθ.η

= √ 3×V L× I L×cos θ.η W

Efficiency η = Pout

P¿ =

P ¿−Plosses

P¿

(Swart, 2012)

5.1.7. Characteristic curve of Speed vs. Torque Error: Reference source not found below shows typical characteristic curves of a squirrel cage induction motor. Graphs A to D show the different graphs and their applications are represented in Table 2

Figure 4: Typical characteristic curves of a squirrel cage induction motor

Table 2

DesignType

StartingTorque

StartingCurrent

BreakdownTorque

Full LoadSlip

TypicalApplications

ASeldom used

normal high high <5% machine tools, fans, pumps

B normal normal normal <5% same as AC high normal low <5% compressors, crushers,

conveyorsD very

highlow n/a >5% punch presses, high

inertial loads elevators

(htt2)

Starting Properties of Induction Motors

Graph 1: Current and Torque versus speed of a Motor

Figure 5: Current and Torque versus speed of a Motor

Induction motors have a speed and torque relationship as displayed in the graph above. Since power is also the product of speed and torque, and torque is also a function of the current, we have an optimal point at which the motor functions. We also know that speed is dependent on frequency. If the speed of the rotor is reduced for some reason, power is lost and consequently the torque and the current rise to make up for the loss in speed and to maintain the power. If the speed is reduced beyond the breakdown torque, the motor stalls and an overload occurs. (Swart, 2012)

Worked Examples 1. A three phase motor with 9 poles is connected to a 400V/50Hz supply.

Calculate the speed of the motor if it has a full load slip of 6%.

Solution:

9 poles mean that the motor actually has 3 poles per phase.

Pole pairs p= pole per phase2

¿ 32

¿1.5

Synchronous speed n=60×fp

=60×501.5

=2000 rpm

Motor speed nr=n (1−s )

¿2000 (1−0.06 )=1880 rpm

2. A three phase Delta connected motor has an output of 100kW when connected to a 400V supply. The full load efficiency is 89% with a power factor of 0.9. Calculate the full load line and phase current of the motor.

Solution:

P=√3×V L×I LcosθWATTS

100×1000=1,732×400×I L×0,9

I L=160.4 A

Since the motor is only 89% efficient, the input should be 11% higher to deliver 100kW.

Therefore

160,4+ 160,4×11100

¿160,4+17,6

¿178 A

In the Delta connection, phase voltage is equal to line voltage.

But phase current= 1√3

× line current

1√3

×I L

1781,732

¿102,8 A

3. A 15kW three phase star connected motor draws a current of 20A, when supplied from a 440V line. The power factor is 0.8. Calculate the efficiency of the motor at full load.

%Efficiency=outputinput

×100%

¿√3×V L×I L× cosθ

15×1000

¿ 1.732×440×20×0.815×1000 ×100%

¿82%

5.2. Synchronous Speed

5.2.1. What is Synchronous Speed?Synchronous speed is the speed of the rotation of the magnetic field in rotary machine like motors & generators, its unit is R.P.M or revolution per minute.

The synchronous speed depends on two factors:

Supply frequency Number of paired poles

Rotary machines are divided into two main categories:

Synchronous machines, where the rotor of the machine rotates at the same speed of the magnetic field.

Asynchronous machines, where the rotor of the machine rotates at a speed lower than speed of the magnetic field.

Synchronous Speed ns=60×FP

RPM

5.2.2. Relation of Synchronous speed to generated power

5.3. Electrical and mechanical aspects of 3-phase (3ϕ) motors

5.3.1. Fault finding / TroubleshootingProblem Possible Cause Tests Solution

Motor will not start. 1. Fault with supply. 2. Motor or load locked

up. 3. Wrong connections in

control circuit.

1. Check for correct voltage at motor terminals.

2. Make sure motor and load are free to turn.

3. Check to ensure contactors operate.

1. Fit new fuses, re-set circuit breakers, etc.

2. Remove clamps, locks, etc.

3. Sort out control circuit.

Supply or Started trips out at start.

1. Wrong or loose connections.

2. Motor overloaded. 3. Inertia of load to high. 4. Low Voltage due to volt

drop in cables 5. Overload or circuit

breaker incorrectly set or sized.

1. Check all lugs are properly crimped or soldered, and connections are tight.

2. Check load performance data against motor performance data.

3. Measure voltage at motor terminals while motor starting.

4. Check settings of overload and circuit breaker and allow for starting current

1. Fix up connections. 2. Change motor for

correct size. 3. Change cables for

correct size. 4. Correct setting of

overload or breaker or change.

Motor starts but has no torque. Motor does not reach full speed or takes a long time to accelerate.

1. Incorrect connection. 2. Delta wound motor

connect in star. 3. Star/Delta starter

staying in Star. 4. Inertia of load to high. 5. Motor overloaded. 6. Low voltage due to volt

drop in cables.

1. Check connection diagram and nameplate data.

2. Check load performance data against motor performance data.

3. Measure voltage at motor terminals while motor starting

1. Sort out and correct connections.

2. Check timer and starter control circuit.

3. Change motor for correct size.

4. Change cables for correct size.

Motor Overheating. 1. Motor overloaded. 2. Ineffective cooling. 3. Excessive ambient. 4. Wrong connection. 5. Delta wound motor in

star. 6. Motor “Single Phasing”. 7. Wrong voltage or

frequency. 8. Supply voltage

unbalanced.

1. Check load performance data.

2. Check fan and air flow and temperature of air. Look for build up of dirt.

3. Check connection diagram and nameplate data.

4. Check volts and amps in all three phases.

5. Check nameplate 6. Measure phase to

phase voltage accurately

1. Fix problem with load or fit larger motor.

2. Clean motor. Sort out cooling of air temp. and flow.

3. Sort out connections.

4. Restore supply to all phases.

5. Correct voltage or frequency.

6. Balance supply or accept unbalance.

No load current in excess of Full load current

1. Incorrect connection 2. Star wound motor

connection in Delta. 3. Voltage in excess of

nameplate. 4. Motor supplied for

different voltage or frequency.

1 & 2. Check connection diagram and nameplate data.

3. Measure voltage at motor terminals.4. Compare supply voltage and frequency to nameplate.

1 & 2. Sort out and correct connections at motor terminals. 3. Correct supply voltage 4. Change motor for correct voltage and frequency

Mechanical Noise or Vibration. Noisy bearings. Bearings overheating.

1. Thrust from load or misalignment.

2. Damaged bearings, too much grease, no grease, or foreign matter in grease.

3. Rotor pulling or foreign matter in air gap.

4. Out of balance load, coupling or pulley.

5. Excessive belt pull. 6. Motor foundations not

rigid.

1. Check gap between coupling halves and alignment.2 & 3. Turn shaft slowly by hand and feel for roughness or stiffness. Check for bent shaft or fan rubbing.4. Run motor disconnected from load and then with pulley or coupling removed.5. Run motor without belts.6. Check design and construction foundations

1. Re-align couplings2 & 3. Clean bearing housing, change bearings and repack with fresh grease.4. Fix up out of balance items5. Loosen belt tension6. Increase strength of foundations

Motor current in excess of nameplate full load current

1. Motor overloaded. 2. Low supply voltage. 3. Wrong voltage or

frequency. 4. Wrong Connections. 5. Motor ‘Single-Phasing’. 6. Supply voltage

unbalanced. 7. Motor Speed not

matched to load.

1. Check load and performance data.2. Measure voltage at motor terminals3. Check nameplate.4. Check nameplate5 & 6. Check volts and amps in all three phases.7. Measure motor speed and check load speed requirements.

1. Fix problem with load or fit larger motor.2. Fix problem, maybe with larger cables.3. Correct voltage or frequency.4. Sort out and correct.5 & 6. Restore balanced supply to all three phases.7. Change motor for correct motor speed.

Excessive electric noise

1. Wrong connections. 2. Wrong voltage. 3. Motor ‘Single-Phasing’.

1. Check connections2. Check voltage with nameplate3. Check volts with amps in all three phases.

1. Fix up connections2. Correct voltage.3. Restore supply to all three phases.

Unbalanced current in different phases when motor loaded

1. Unbalanced supply voltage

1. Measure phase to phase voltage accurately

1. Balance supply or accept unbalance

Motor runs in wrong direction

1. Wrong connections. 1. Watch shaft rotation 1. Swap any two phases of supply.

5.3.2. Motor Testing Checking the outside of the Motor

Figure 6

(Wiki)

Check the outside of the motor. If the motor has any of the following issues on the outside, they may be problems that can shorten the life of the motor because of previous overloading, wrong application, or both. Look for:

Broken mounting holes or feet.

Darkened paint in the middle of the motor (indicating excessive heat). Evidence of dirt and other foreign matter having been pulled into the motor

windings through openings in the housing.

Check the Nameplate on the Motor

The nameplate is a metal or other durable tag or label that is riveted or otherwise affixed to the outside of motor housing called the '"stator" or "frame". Important information about the motor is on the label; without it, it will be difficult to determine its suitability to a task. Typical information found on most motors includes (but not limited to):

•Manufacturer's Name — the name of the company the made the motor

•Model and Serial Number — information that identifies your particular motor

•RPM — the number of revolutions the rotor makes in one minute

•Horsepower — how much work it can perform

•Wiring diagram — how to connect for different voltages, speeds and direction of rotation

•Voltage — voltage and phase requirements

•Current — amperage requirements

•Frame Style — physical dimensions and mounting pattern

•Type — describes if frame is open, drip proof, total enclosed fan cooled, etc.

Figure 7

Check the Bearing

Figure 8

Many electric motor failures are caused by

bearing failures. The bearings allow the shaft or rotor assembly to turn freely and smoothly in the frame. Bearings are located at both ends of the motor which are sometimes called "bell housings" or "end bells".

There are several types of bearings used. Two popular types are brass sleeve bearings and steel ball bearings. Many have fittings for lubrication while others are permanently lubricated or "maintenance free".

To perform a cursory check of the bearings, place the motor on a solid surface and place one hand on the top of the motor, spin the shaft/rotor with the other hand. Closely watch, feel, and listen for any indication of rubbing, scraping, or unevenness of the spinning rotor. The rotor should spin quietly, freely and evenly.

Push and pull the shaft in and out of the frame. A small amount of movement in and out is permitted, but the closer to "none" the better. A motor that has bearing-related issues when run will be loud, overheat the bearings, and potentially fail catastrophically.

Check the Windings

Figure 9

With an ohmmeter set to the Resistance or Ohms test setting, place test probes into the appropriate jacks, usually the "Common" and "Ohms" jacks. (Check the meter's operation manual if necessary) Choose the highest scale (R X 1000 or similar) and zero the meter by touching both probes against each other. Adjust the needle to 0 if possible. Locate a ground screw (often a green, hex head type) or any metal part of the frame (scrape away paint if needed to make good contact with metal) and press a test probe to this spot and the other test probe to each of the motor leads, one at a time. Ideally, the meter should barely move off the highest resistance indication. Make sure your hands are not touching the metal probe tips, as doing so will cause the reading to be inaccurate.

It may move a fair amount, but the meter should always indicate a resistance value in the millions of ohms (or "megohms"). Occasionally, values as low as several hundred thousand ohms (500,000 or so), *may* be acceptable, but a higher number is more desirable.

Many digital meters do not offer the ability to zero, so skip the "zeroing" information above if yours is a digital meter.

Many simple "across the line" single-phase and 3-phase motors (used in household appliances and industry respectively) can be checked simply by changing the range of the ohm meter to the lowest offered (R X 1), zeroing the meter again, and measuring the resistance between the leads of the motor. In this case, consult the wiring diagram of the motor to be sure that the meter is measuring across each winding.

Expect to see a very low value of resistance in ohms. Low, single digit resistance values are expected. Make sure your hands are not touching the metal probe tips, as

doing so will cause the reading to be inaccurate. Values greater than this indicate a potential problem and values significantly greater than this indicate the winding has failed opened. A motor with high resistance will not run - or not run with speed control (as is the case when a 3-phase motor winding opens while running).

5.3.3. Commissioning. The process involved in preparing the motor and starter to be used by the operator.

This is the process involved in preparing the motor and starter to be used by the operator. The following steps must be followed when commissioning a motor:

1. All work is to be carried out only when there is no voltage on the motor. The installation must be carried out according to the valid regulations by qualified skilled personnel.

2. The mains conditions (voltage and frequency) must be compared with the data on the rating plate of the motor.

3. The dimensions of the connecting cables must be adjusted in line with the rated currents of the motor.

4. Always start the motors with an over-current protection device that is set appropriately.

5. When the motor is connected for the first time it is recommended to check the insulation resistances between winding and earth and between phases.

6. After prolonged storage it is absolutely essential that the insulation resistance is measured.

7. Before coupling the motor to the driven machine, check the direction of rotation of the motor to prevent possible damage being caused to the driven machine.

8. If the power lines are connected with the phase sequence L1, L2, L3 to U, V, W, the direction of rotation is clockwise.

9. If two terminals are changed, the direction of rotation is counter clockwise (i.e. L1, L2, and L3 to V, U, and W).

10.For machines with only one direction of rotation the required sense of rotation is marked by an arrow on the machine.

11.Before closing the terminal box the following should be checked: - All connections must be made in accordance with the wiring diagram. - All connections are tightened. - The interior of the terminal box is clean and free from dirt and foreign

objects. - All unused cable entries are blanked off and threaded plugs with seals

are tightened. - The seal on the terminal box is clean and glued on all surfaces.

12.Before starting up the motor check that all safety regulations are strictly adhered to, that the machine is correctly installed and aligned, that all fixing parts and earthing connections are tightened, that the auxiliary and additional devices are functionally and correctly connected and if a second shaft end is fitted that the key is secured against being thrown aside.

13. If possible the motor is to be connected without load. If the motor is running smoothly and without any abnormal noises, the load of the driven machine is to be applied onto the motor.

14. When the motor is started up it is recommended to monitor the current consumption if the motor is loaded with its driven machine so that any possible overloads and asymmetries occurring in the mains can be recognised immediately.

15.The starter must always be in the starting position during starting. 16.With slip ring motors the correct running of the brushes must be monitored.

They must be absolutely spark-free.

5.4. Practical: Conduct trouble shooting on a faulty motor and rectify the problem.Time: 45 Min Total: 30Aim: To trouble shoot and repair a faulty 3 phase motor

Apparatus:

3 phase motor

Procedure:

Your teacher has presented you with a faulty 3 phase AC Motor. The motor has the following problems:

1. Upon connection to the starter the motor runs in the incorrect direction.

2. Once started the motor does not develop sufficient torque. 3. The motor, when started is noisy.

Compile a report on each problem outlined above. Your teacher will assess you as you complete each problem. Be sure to adhere to all safety precautions and use the correct tool for each task.

1. The motor runs in the incorrect direction.

1.1. State the possible cause of this problem. (2)

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

1.2. How would you rectify the problem? (3)

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. The motor does not develop sufficient torque.

2.1. State the possible causes. (3)

_________________________________________________________________________________________________________________________________________________________________________________________________________


____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. The motor is noisy when started.

3.1. What is the possible cause? (4)

_________________________________________________________________________________________________________________________________________________________________________________________________________


_________________________________________________________________________________________________________________________________________________________________________________________________________

4. Observations

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Rubric

TASK Description

Mark Allocation (Tick the appropriate level next to the Task Indicated)

Not Achieved

Not yet competent Competent Highly

Competent Outstanding

Problem Rectification

The learner was unable to rectify any of the problems

0

The learner was able to

conduct rectification, but

made more than 8 errors

2


conduct rectification and made 7 or less

errors

4


conduct rectification and made 4 or less

errors

6

The learner correctly rectified all problems and was able to troubleshoot

accurately

8

Housekeeping

The learner did no

housekeeping.

0

The learner did

housekeeping after she/he

was reminded by the teacher.

1

The learner was able to do housekeeping without

supervision or being reminded by the teacher.

2

Rubric (Maximum of 10)

Questions 20

5.5. Practical: Conduct a Motor test on a MotorTime: 45 Min Total= 50

Aim: When conducting an inspection and test of an AC motor it is advised to make use of a checklist or report as is shown below. Make use of the list below to conduct an inspection and test on an electrical motor. Your teacher will supply you with a motor to test.

Details of the motor under test: (3)

Phase: Supply voltage:

Pole pares: Speed:

Efficiency: Current:

lucky, 2014-11-11,

No marks?

DESCRIPTION VISUAL INSPECTION &READINGS TAKEN (Megger)

MARKS ALLOCATED

Condition of windings:Measurements Taken

Test 1: Continuity of the windings (3 marks)A1 – A2

B1 – B2

C1 – C2

Test 2: Insulation resistance between windings (3 marks)

A1 – B1

A1 – C1

B1 – C1

Test 3 – Insulation resistance to earth (3 marks)

A1 – Earth

B1 – Earth

C1 – Earth

Test 4 – Mechanical inspection

Note all errors (9 marks)

Condition of rotor and shaft

Key/Key way

Front bearing

Back bearing

Condition of motor frame

Condition of termination box

Flange/Foot mount

Front/Back-end shield

Stator/Field housing

Mounting bolts and nuts/ Screws

Condition of cooling fan, fan cover and cooling fins

Draw and label the correct connection of internal wiring on the provided drawing below:(3=Coils)

(2=Labels)

Test Finding (3 marks)

Is motor operational?

Earth resistance

Insulation resistance

List the recommended repairs that should be affected on the electrical motor under test.

(1)

TOTAL: 30

Task Description

Mark Allocation (Tick the appropriate level next to the Task Indicated)1

Not Achieved

2Not yet

competent

3Competent

4Highly

Competent

5Outstanding

Inspection points

The learner did not identify any testing points.

The learner was unable to identify more than two testing points.

The learner was able to identify more than two testing points but could not motivate why these are used.

The learner was able to identify testing points on the motor and inside the motor. The learner was also able to motivate why these points have to be tested.

The learner was able to successfully indicate all testing points in and on the motor. The learner was also to motivate why these points should be tested and was able to list symptoms that indicated certain errors.

Test continuity The learner was unable to test continuity

The learner was able to test continuity, but did not know why this was done.

The learner was able to correctly test continuity and had a basic idea of the reason for this.

The learner was able to correctly test continuity and had a solid knowledge of the meters and the reasons for their use.

Test earth resistance

The learner was unable to test insulation resistance.

. The learner was able to test insulation resistance, but did not know why this was done.

The learner was able to correctly test insulation resistance and had a basic idea of the reason for this.

The learner was able to correctly insulation resistance and had a solid knowledge of the meters and the reasons for their use.

Task Description


Not Achieved

2Not yet

competent

3Competent

4Highly

Competent

5Outstanding

Housekeeping The learner did no housekeeping.

The learner did housekeeping under duress.

The learner did housekeeping under the supervision of the teacher

The learner did housekeeping after she/he was reminded by the teacher.

The learner was able to do housekeeping without supervision or being reminded by the teacher. Housekeeping was done excellently.

Total of the Rubric (Maximum of 20)

Written Task (Maximum of 30)

Total (Maximum of 50)

5.6. Practical: Commissioning of a new motor with starter.Time: 45 Min Total: 40

Aim: To commission a new motor and starter.

You are required to commission a new motor and starter. Complete the checklist below and state your observations.

Commissioning Checklist (2 marks for each test/inspection)

MOTORItem Checked Type of inspection/test

conductedNotes/Measurements

End ShieldBearings Name PlateTerminal BoxMountingsStator windingsInsulation resistance to earthInsulation resistance between windingsCondition of terminal box wiring Internal check of terminal

boxStarter

Wiring of control circuitWiring of main circuitTest on normally open and normally closed contactsOverload relay check No Volt coil checkStart, stop and emergency stop switch check

NotesIs the motor operational?Is the starter safe to use?What is the minimum accepted value of the insulation tests?What is the overall condition of the motor and starter?

5.7. 3Φ Direct On Line Starter with overloadDirect on line starters are normally used for smaller types of motors since the starting current of an induction motor is always high and this type of starter does not cater for control of starting current. An overload mechanism and no-volt release coil form an inherent part of the contactor. In the case of the overload protection, it is desirable that some form of hand reset device be incorporated with the overload release to prevent the starter from automatically reclosing after tripping on overload. For safety reasons all motor starters should automatically return to the “off” position in the event of a supply failure; for this purpose a no-volt release must be fitted. The no-volt release coil, which also serves as the holding in coil of the contacts, is electro magnetically operated by the main supply. Any interruption of the main supply will automatically disconnect the supply to the motor. This type of starter is suitable for remote control purposes.

Principal of Operation

To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.

As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed.

The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.

Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.

The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.

This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much. (htt4)

Wiring Diagram of the Main and Control Circuit

Figure 10

(htt3)

Components of the Direct on Line Starter (htt3)

Contactors and Coil

Figure 11

Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.

Magnetic motor controllers use electromagnetic energy for closing switches. The electromagnet consists of a coil of wire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized, attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap in the magnetic circuit.

Line-voltage magnetic motor starters are electromechanical devices that provide a safe, convenient, and economical means of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor controllers sold are of this type.

Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to a voltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils are used. The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the control circuit. A contact is conducting metal parts which completes or interrupt an electrical circuit.

NO-normally open NC-normally closed

Overload Protection

Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.

Under any condition of overload, a motor draws excessive current that causes overheating. Since motor winding insulation deteriorates due to overheating, there are established limits on motor operating temperatures to protect a motor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.

Figure 12

The overload relay does not provide short circuit protection. This is the function of over current protective equipment like fuses and circuit breakers, generally located in the disconnecting switch enclosure.

The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similar to the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. The operation of the protective device should be such that the motor is allowed to carry harmless over-loads but is quickly removed from the line when an overload has persisted too long.

Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and a damaging overload. Selection of Fuse is depend on motor full-load current, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.

The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in during the accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-load current when the motor is running. Overload relays are renewable and can withstand repeated trip and reset cycles without need of replacement. Overload relays cannot, however, take the place of over current protection equipment.

The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit.

Overload relays can be classified as being thermal, magnetic, or electronic:

1. Thermal Relay: As the name implies, thermal overload relays rely on the rising temperatures caused by the overload current to trip the overload mechanism. Thermal overload relays can be further subdivided into two types: melting alloy and bimetallic.

2. Magnetic Relay: Magnetic overload relays react only to current excesses and are not affected by temperature.

3. Electronic Relay: Electronic or solid-state overload relays, provide the combination of high-speed trip, adjustability, and ease of installation. They can be ideal in many precise applications.

Setting the Overload Relay

The Overload relay is one of the most important devices for motor control. It can prevent the motor from overheating or windings burning due current overload.

The setting of the overload relay must be done accurately. This setting is dependent on the motor application and the motor full load current (FLC). Should the setting be well below FLC then the motor will continuously trip. A setting much higher than FLC will be dangerous as this will prevent the motor from tripping and could cause damage to the motor.

Taking the above two scenarios into account it is imperative that this setting is calculated accurately.

Normally the overload relay settings depend on the FLC (Full Load Current) of the motor. The nameplate of the motor normally gives an indication of the motors FLC. The overload relay is usually set between 5 to 10% of the motors FLC.

5.8. Practical: Connect a DoL Starter to a motor, set the overload. Start & Stop the motor.Time: 1 Hour Total = 60

Aim: To connect a Direct on Line Starter to a three phase motor.

Equipment:

Coloured pencils Three phase motor 3 fuses for short circuit protection A contactor with auxiliary contacts and overload relay Stop/start station Connecting cables

Requirements

1. In the space below draw the complete control and main circuit of a three phase Direct on Line starter. Use colour pencils to enhance your answer. (10)

2. Wire your motor according to the diagram in 1 above.

Wiring Diagram

Task Description


Not Achieved

1Not yet

competent

2Competent

4Highly Competent

5Outstanding

Identification and purpose

The learner was unable to identify any

The learner was able to identify less than

The learner was able to identify all parts,

The learner was able to successfully identify all parts and knew the

The learner was able to successfully identify all parts and knew the purpose of all

Task Description


Not Achieved

1Not yet

competent

2Competent

4Highly Competent

5Outstanding

of parts parts. three parts. but did not know the function thereof.

purpose of most of the parts.

the parts.

Control circuit wiring

The learner was unable to wire the control circuit.

The learner was able to wire part of the control circuit.

The learner was able to wire the control circuit, but could not establish retention at start.

The learner was able to successfully wire the control circuit.

The learner was able to successfully wire the control circuit. The learner followed a step by step approach, testing along the way and included pilot lights

Troubleshooting: Control Circuit

The learner's circuit was not complete and she/he was unable to conduct trouble-shooting.

The circuit was complete, but was not functional. The learner was unable to identify the problem.

The circuit was complete, but not functional and the learner was able to identify and rectify one mistake.

The circuit was complete and the learner was able to identify and rectify two mistakes. The circuit is functional.

The circuit was complete and the learner was able to identify and rectify all mistakes.

Control circuit working

The circuit did not work.

The circuit worked. This must correlate with the circuit wiring marks.

Main circuit wiring

The learner was unable wire the main circuit.

The learner was able to wire the main circuit partly correct, but did not use overload protection.

The learner was able to wire the main circuit including overload protection but did not know why it was used.

The learner was able to wire the main circuit and test the overload protection and had a working knowledge of the circuit.

The learner was able to correctly test the main circuit after assembly and had a well founded knowledge of all the working parts. The learner was able to quickly re-assemble the circuit accurately without the aid of the circuit diagram

Troubleshooting: Main Circuit






Main circuit working


The circuit worked. This must correlate with the main circuit wiring marks.

Tools selection and use

The learner was unable to identify and select any tools.

The learner identified and selected the incorrect tools.

The learner was able to select the correct tools, but used them incorrectly/unsafely.

The learner was able to identify and select all tools correctly and used them correctly.

The learner identified and selected tools quickly and without the help of the teacher. The learner was also able to use tools correctly in a safe ergonomic manner.

Troubleshooting






Safety The learner did not work safely.

The learner worked safely after being reprimanded.

The learner worked safely under supervision of the teacher.

The learner worked safely without being reminded by the teacher.

The learner was able to do work safely without supervision or being reminded by the teacher. Safety was excellent.

Housekeeping

The learner did no housekeeping.


The learner did housekeeping under the supervision of the teacher.




5.9. 3Φ Forward and Reverse Starter with overloadThere are certain applications that require forward and reverse direction of motor rotation, such as a conveyor system, or perhaps the opening and closing of valves. Another example is with overhead cranes which would also require a hoist up and down function, traverse forward and backward, and a left and right travelling function. Such application would most certainly make use of a forward and reverse motor control which is intended to achieve clockwise and counter clockwise motor rotational direction.

The electrical schematic diagram below provides an illustration of the power circuit of a forward reverse motor controller used in every electrical industry involving industrial process automation control technology. The diagram appears similar to the DOL motor controller except for an additional reverse contactor connected in parallel across the forward contactor.

Figure 13The main circuit breaker is the main power supply switch that provides power to the line side terminals L1, L2, L3 of the two contactors, waiting for any of these two contactors to close to deliver the voltage to the terminal of the motor U1, V1, and W1 in order to run the motor.

The forward contactor is configured in such a way as to run the motor to its normal forward rotation with L1 connected to U1, L2 to V1, and L3 to W1. Whereas the configuration of the reverse contactor is wired in contrast to the configuration of the forward contactor so as to provide a reverse rotation of the motor, with L1 to W1 instead U1, then L3 to U1 instead of W1, while maintaining L2 connected to V1.

The thermal overload relay provides motor overload protection which detects motor overload current to shut down the control system of the forward reverse motor controller. The forward reverse motor control circuit controls the forward reverse power circuit.

The forward reverse motor control circuit consists of two DOL motor controller circuit which are connected side by side to accommodate the required forward and reverse function of the system.

Note that the control circuit incorporates an interlocking contact inserted between the push button and the contactor coils of each circuit to prevent the simultaneous activation of both forward and reverse contactor coils at the same time.

Pressing the forward push button will activate the forward contactor coil which will close a corresponding contact connected across the forward push button switch which is intended to latch the circuit to allow the operator to release his finger from the switch while maintaining the forward contactor circuit locked, with the motor continuously running uninterrupted in the forward rotation.

Consequently, the activation of the forward contactor coil would also open the interlock contact connected before the reverse contactor coil, which provides a safety measure that prevents electricity to pass through to the reverse contactor coil in case of an unintentional switching of the reverse push button switch while the forward contactor is energized.

Figure 14

In a similar manner, when the reverse contactor coil is energized in the first place with the pressing of the reverse push button switch, its associated contact placed as

an interlock before the coil of the forward contactor will also be opened, hence preventing any incidental power flow to reach the forward contactor coil, thus, avoiding any simultaneous activation of both forward and reverse coil at the same time.

Changing the rotation of the motor regardless of whether it is actively running in the forward or the reverse direction is made possible by firstly pushing the STOP push button switch to release the locked status of any energized coil, the OFF push button refreshes both forward and reverse circuit to OFF state which would return all contacts to their initial ready state, awaiting command from the forward or reverse push button switch.

The thermal overload contact serves as a safety disconnect switch that disengages any energized coil in order to shut down the motor upon detection of overload current driven by the motor. (htt3)

5.10. Practical: Connect a 3Φ Forward and Reverse Starter to a 3-phase motor. Set the overload. Start & StopTime: 1 Hour Total = 60

Aim: To connect a three phase motor to a forward and reverse starter.

Equipment:

1. Set of coloured pencils 2. 3 fuses for short circuit protection. 3. 2 contactors with auxiliary contacts. 4. Overload relay5. A forward- reverse-stop push button station. 6. Three phase motor.

Requirements

1. In the space below draw the complete control and main circuit of a three phase Forward-Reverse starter. Use colour pencils to enhance your answer. (10)

2. Wire your motor according to the diagram in 1 above.

Wiring Diagram

Task Description


Not Achieved

1Not yet

competent

2Competent

4Highly Competent

5Outstanding

Identification and purpose of parts

The learner was unable to identify any parts.

The learner was able to identify less than three parts.

The learner was able to identify all parts, but did not know the function thereof.

The learner was able to successfully identify all parts and knew the purpose of most of the parts.

The learner was able to successfully identify all parts and knew the purpose of all the parts.
















Main circuit wiring















Tools selection and

The learner was unable to identify and

The learner identified and selected the

The learner was able to select the correct

The learner was able to identify and select all tools correctly and used

The learner identified and selected tools quickly and without the help of the

Task Description


Not Achieved

1Not yet

competent

2Competent

4Highly Competent

5Outstanding

use select any tools.

incorrect tools. tools, but used them incorrectly/unsafely.

them correctly. teacher. The learner was also able to use tools correctly in a safe ergonomic manner.

Troubleshooting











Housekeeping







5.11. 3Φ Sequence Motor Control Starter with overload (Without Timer)

Figure 15

Sequence motor control starters are used in conjunction with Direct on Line Starters. They are often employed with or without timers. We will be discussing the type without timers. The diagram above shows the main circuit of the Sequence starter. The motor starter consists of two circuits that are separated by the contactors.

Operation of the Main Circuit

In this circuit we have two Direct on Line starters which switch the three phases directly to the motors when the contactors are closed. L1 goes through contacts 1 and 2 and through the element of the overload relay to U. L2 goes through contacts 3 and 4 and through the element of the overload relay to V. L3 goes through contacts 5 and 6 and through the element of the overload relay W.

When contactor 2 is closed, exactly the same will happen. L1 goes through contacts 1 and 2 and through the element of the overload relay to U. L2 goes through contacts 3 and 4 and through the element of the overload relay to V. L3 goes through contacts 5 and 6 and through the element of the overload relay to W.

Figure 16

Operation of the Control Circuit

When the start push button No1 is pressed, it closes contacts 13 and 14. The current will now flow from L through the fuse, through the overload contacts 95 and 96, through the contacts of the stop button 15 and 16, through the start button and through the No-Volt coil MCC1 to N. With the circuit completed, the no-volt coil will be energised and the holding contacts 13 and 14 on. Motor contactor MC will close and latch the circuit. The auxiliary contacts 43 and 44 on MC1 will close in the control circuit of motor No 2. If we want to open the circuit, the stop button or the overload unit must be activated.

When the start push button No 2 is pressed, it closes contacts 13 and 14. The current will flow from L through the fuse, through the overload contacts 95 and 96, through the contacts of the stop button 15 and 16, through the auxiliary contacts 43 and 44 on MC2 (provided that motor contactor N01 is closed) through the start button 2, through the no volt coil MCC2 to N. With the circuit completed, the no volt will be energised and the holding contacts 13 and 14 on MC2 (motor contactor) will close and latch the circuit. If we want to open the circuit, the stop button or the overload unit must be activated. (Swart, 2012)

5.11.1. Function of components on diagrams

5.11.2. Principle of Operation

5.11.3. Diagram

5.11.4. Wiring on a panel

5.12. Practical: Connect a 3Φ Sequence motor starter to a squirrel cage motor. Set the overload. Start & StopTime: 1 Hour Total = 70

Aim: To connect a 3 Phase sequence motor starter to a squirrel cage motor.

Equipment:

1. A set of coloured pencils. 2. 3 fuses for short circuit protection. 3. 2 contactors with auxiliary contacts. 4. 2 overload relays 5. Two stop - start push button station. 6. 2 three phase squirrel cage motors

Requirements:

1. Using the colour pencils draw in the space provided the main and control circuits of the starter. (10)

2. Wire the starter as per your diagram. (50)

Wiring Diagram

lucky, 2014-11-11,

Put content!

Task Description


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competent

2Competent

4Highly Competent

5Outstanding






















Main circuit wiring





















Troubleshooting











Housekeeping







5.13. 3Φ Sequence Motor Control Starter with overload (With Timer)

Figure 17

The motor starter consists of two circuits that are separated by the contactors:

The main circuit and The control circuit.

The circuit above shows the main circuit of the sequence starter.

In this circuit we have two Direct on Line starters which switch the three phases directly to the motors when the contactors are closed. L1 goes through contacts 1 and 2 and through the element of the overload relay to U. L2 goes through contacts 3 and 4 and through the element of the overload relay to V. L3 goes through contacts 5 and 6 and through the element of the overload relay W.

When contactor 2 is closed, exactly the same will happen. L1 goes through contacts 1 and 2 and through the element of the overload relay to U. L2 goes through contacts 3 and 4 and through the element of the overload relay to V. L3 goes through contacts 5 and 6 and through the element of the overload relay to W.

Figure 18

When the start pushbutton No 1 is pressed, it closes contacts 13 and 14. The current will now flow from L through the fuse, through the overload contacts 95 and 96, through the contacts of the stop button 15 and 16, through the start pushbutton 1 and through the no-volt coil MCC1 to N. With the circuit completed, the no-volt coil will be energised and the contacts 13 and 14 on MC (motor contactor) will close and latch the circuit. The timer coil will also be energised as it is connected in parallel with MCC1. After the pre-set time has elapsed the normally open contacts 15 and 18 on the timer will also close in the control circuit of motor 2. If we want to open the circuit, the stop pushbutton or the overload unit must be activated.

When the start pushbutton No 2 is pressed, it closes contacts 13 and 14. The current will now flow from L through the fuse, through the overload contacts 95 and 96, through the contacts of the stop button 15 and 16, through the timer contacts 15 and 18 (provided that the pre-set timer has elapsed) through the start button 2 and through the no-volt coil MCC2 to N. With the circuit completed, the no-volt will be energised and the holding contacts 13 and 14 on MC2 will close and latch the circuit. If we want to open the circuit, the stop button or the overload unit must be activated.

(Swart, 2012)



5.13.3. Diagram

5.13.4. Wiring on a panel

5.14. Practical: Connect a Sequence Motor starter. Set the overload and timer. Start & StopTime: 1 Hour Total: 60

Aim: To connect a sequence Motor Starter with Timer.

Equipment:

1. A set of coloured pencils.2. 3 fuses for short circuit protection.3. 2 contactors with auxiliary contacts. 4. 2 overload relays. 5. 2 stop-start pushbutton station. 6. On-delay timer. 7. 2 3 phase motors 8. Connecting leads.

Method:

1. In the space provided draw the main and control circuits for the Sequence Start Motor Starter with timer. (10)

2. Connect the starter to the motor and set the timer. (50)

Wiring Diagrams

lucky, 2014-11-11,

Content?

Task Description


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1Not yet

competent

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4Highly Competent

5Outstanding






















Main circuit wiring





















Troubleshooting

The learner's circuit was not complete and she/he was unable to

The circuit was complete, but was not functional. The learner was

The circuit was complete, but not functional and the learner was able to

The circuit was complete and the learner was able to identify and rectify two mistakes. The circuit is


Task Description


Not Achieved

1Not yet

competent

2Competent

4Highly Competent

5Outstanding

conduct trouble-shooting.

unable to identify the problem.

identify and rectify one mistake.

functional.






Housekeeping







5.15. 3Φ Automatic Star Delta Starter with overload The Star Delta starter is the most commonly used connection configuration for starting three phase motors and has the following features:

1. Uses a six core connecting cable. 2. Can be used for low and high power three phase AC Motors 3. Reduced starting current. 4. Reduced starting torque.

Figure 17A Figure 17B

Figure 19

Figure 19 A is connected in Star thus only 57% of the line voltage is across each motor winding.

lucky, 2014-11-11,

Are you referring to the figure below?

Figure 19B is connected in Delta thus the line voltage is directly across each motor winding.

The value of I phase in the star connection is 1√3 lower than in the delta connection.

The value of I line in the star connection is 1√3 lower than in delta connection.

Thus as the phase current is 1√3 lower in the Delta connection the line current will

also be 1√3 lower and then the total reduction in current is:

¿ 1√3

× 1√3

¿ 13

The motor starter consists of the following components:

1. Three contactors 2. Overload relay 3. No-volt coil (inside the contactor) 4. Red stop pushbutton 5. Green start pushbutton 6. Delay on timer

Figure below shows the main circuit of the starter.

Figure 20

The main circuit switching of the Star Delta starter is performed by three contactors, of which two operate exclusively, either star or delta. The control circuit causes the star contactor to close first to create the star point by connecting V2, U2 and W2 together. Now the main contactor closes to connect L1 to U1, L2 to V1 and L3 to W1. When the motor has picked up speed, the star contactor falls out and the delta contactor closes to connect V2 to L1 and U2, W2 to L2 and V1 and U2 to L3 AND W1, to form the delta connection.

Figure 21 below shows the control circuit for the Automatic Star Delta Starter.

Figure 21

When the start pushbutton is pressed, it closes contacts 13 and 14. The current will now flow from L through the fuse, through the overload contacts 95 and 96, through the contacts of the stop button 15 and 16, through the start button, through the normally closed contacts 15 and 16 of the timer, through the normally closed auxiliary contacts 21 and 22 of the Delta contactor to the no-volt coil of the Star contactor (MCCSr) to N. With the circuit completed, the no-volt coil of the Star contactor (MCCSr) will be energised and the contacts 13 and 14 on MCSr will close. The circuit of the main contactor and the timer will now also close. The holding in contacts 13 and 14 as well as a set of auxiliary contacts 43 and 44 on the MCM will close, latching the main contactor. After the elapse of a pre-set time, the timer’s normally closed contacts 15 and 16 will open, and the normally open contacts 15 and 18 will close. The Star contactor drops out and the Delta contactor is now closed. (Swart, 2012)



5.15.3. Diagram

5.15.4. Wiring on a panel (Practical) & Calculation of the overload value and setting of the overload.

5.16. Practical: Connect a Star Delta starter to a squirrel cage motor. Set the overload and timer. Start & StopTime: 1 Hour Total: 50

Aim: To connect a Star-Delta starter to a squirrel cage motor

Equipment:

1. 3 fuses for short circuit protection2. Three contactors with auxiliary contacts 3. An overload relay. 4. On-delay timer5. A stop-start pushbutton station. 6. A three phase induction motor.

Requirements

1. using the diagrams below connect the Star Delta starter to the squirrel cage motor.

Wiring Diagrams

lucky, 2014-11-11,

Content?

Figure 22

Figure 23

Observations

__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Task Description


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competent

2Competent

4Highly Competent

5Outstanding






















Main circuit wiring







The learner's circuit was not

The circuit was complete, but

The circuit was complete,

The circuit was complete and the

The circuit was complete and the learner was able to

Task Description


Not Achieved

1Not yet

competent

2Competent

4Highly Competent

5Outstanding

complete and she/he was unable to conduct trouble-shooting.

was not functional. The learner was unable to identify the problem.

but not functional and the learner was able to identify and rectify one mistake.

learner was able to identify and rectify two mistakes. The circuit is functional.

identify and rectify all mistakes.










Troubleshooting The learner's circuit was not complete and she/he was unable to conduct trouble-shooting.
















5.17. PAT Simulations 3 & 4Simulation 3 Time: 3 hours

Learner Name:

School:

Examination Number:

Three-Phase-Direct-On-line-Starter

1. Purpose:

Practical simulation of a three-phase-direct-on-line starter.

NOTE: Teachers may use alternative DoL circuits.

2. What you are going to do

Build (Assemble) the power and control circuits of a three-phase-direct-on-line starter. You will also set the overloads and use the correct wire size or plug in leads. The circuit will be checked, tested and the motor must be started.

3. What you will need

1. One, three-phase contactor with auxiliary contacts2. One three-phase overload relay3. One stop button, (press-button type)4. One start button (press-button)5. One three-phase circuit-breaker6. One fuse for the control circuit7. One 380 V delta induction motor (squirrel-cage)8. Correct wire size or plug in leads9. Multi-meter or continuity tester10. Power supply – three-phase

4. What you must do

1. Consult the control and power circuit.2. Construct/Wire the power and control circuit on the given panel.

3. Connect the motor to the power circuit and set the overload. 4. Now ask the teacher to check the circuits. If they are incorrect repair the fault. 5. When the circuits are correct switch the supply on and start the motor.6. Stop the motor and switch the supply off.7. On completion of the task switch the supply off and strip the circuits.

5. Conclusion:

In which type of industrial application would DoL starters be used? Motivate your answer.

__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Rubric Simulation 3: Three-Phase-Direct-On-line-Starter

Task Description


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competent

2Competent

4Highly Competent

5Outstanding



















Control circuit


The circuit worked. This must correlate with the circuit

Task Description


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competent

2Competent

4Highly Competent

5Outstanding

working wiring marks.

Main circuit wiring





















Troubleshooting











Housekeeping







School:

Simulation 4 Time: 3 hours

Learner Name: Examination Number:

Three-Phase Forward and Reverse Starter

1. Purpose

Practical simulation of a three-phase forward reverse starter.

NOTE: The teacher can use an alternative forward reverse starter circuit.

2. What you are going to do?

Build (Assemble) the power and control circuits of a three-phase forward and reverse starter. You will also set the overloads and use the correct wire size or plug in leads. The circuit will be checked, tested and the motor must be started.

3. What you will need?

1. Two, three-phase contactors with auxiliary contacts2. One timer with normally open and closed contacts3. Two stops, one for the emergency stop (press button type)4. One start (press button)5. One three-phase circuit-breaker6. One overload relay7. Two fuses for the control circuit8. One 380 V delta induction motor (squirrel-cage)9. Correct wire size or plug in leads10. Multi-meter or continuity tester11. Power supply

4. What you must do?

1. Consult the control and power circuit.2. Construct/Wire the power and control circuit on the given panel.3. Connect the motor to the power circuit and set the overload. 4. Now ask the teacher to check the circuits. If they are incorrect repair the fault. 5. When the circuits are correct, switch the supply on and start the motor.6. Stop the motor and switch the supply off.7. On completion of the task switch the supply off and strip the circuits.

5. Conclusion

Give TWO examples where this circuit can be used effectively.

_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Rubric Simulation 4: Forward Reverse Motor Starter

Task Description


Not Achieved

2Not yet

competent

3Competent

4Highly

Competent

5Outstanding









The learner was able to wire the forward part of the circuit only.

The learner was able to wire both the forward and the reverse, but did not utilise interlocking.

The learner was able to successfully wire the control circuit for forward and reverse utilising interlocking.

The learner was able to successfully wire the control circuit for forward and reverse utilising interlocking. The learner followed a step by step approach, testing along the way and included pilot lights







The circuit was complete and the learner was able to identify and rectify one mistake.

The circuit was complete and the learner was able to identify and rectify two mistakes.


Main circuit wiring The learner The learner The learner was The learner was The learner was able to

Task Description


Not Achieved

2Not yet

competent

3Competent

4Highly

Competent

5Outstanding

was unable to wire the main circuit.

was able to wire the main circuit partly correct, but did not use overload protection.

able to wire the main circuit including overload protection, but did not know why it was used.

able to wire the main circuit and test the overload protection and has a working knowledge of the circuit.

correctly test the main circuit after assembly and had a well founded knowledge of all the working parts. The learner was able to quickly re-assemble the circuit accurately without the aid of the circuit diagram.







The circuit was complete and the learner was able to identify and rectify one mistake.

The circuit was complete and the learner was able to identify and rectify two mistakes.










The learner did housekeeping under the supervision of the teacher





The learner worked safely under supervision of the teacher




5.18. Homework Exercises1. List 2 advantages of a Squirrel Cage Induction motor.

_________________________________________________________________________________________________________________________________________________________________________________________________________

2. Define the term “Slip”.

_________________________________________________________________________________________________________________________________________________________________________________________________________

3. Define the term “Synchronous Speed” with respect to AC Motors.

_________________________________________________________________________________________________________________________________________________________________________________________________________

4. Provide two reasons why motors require starters.

_________________________________________________________________________________________________________________________________________________________________________________________________________

5. Consider the circuit below and answer the questions that follow.

5.1.1. Provide a name for the circuit.

______________________________________________________________________________________________________________________________________

5.1.2. Provide labels for parts labelled A-D.

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5.1.3. Draw the control circuit for the circuit above

6. The input power of a 380 V, three phase, delta connected induction motor is 91 kW. The power factor of the motor is 0,9 lagging.

GIVEN: VL = 380 V P = 91 kW PF = 0,9

Calculate:

a) The line current

b) The phase current

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7. A three-phase delta connected motor draws 25A from a 380 V supply at a power factor of 0,8 lagging GIVEN: VL = 380 V IL = 25 A PF = 0,8

Calculate: a) The input power

b) The apparent power

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8. What is the purpose of the no volt coil in motor control circuits?

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9. Describe the term N/O with reference to electromagnetic relays.

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10.Mention THREE examples of what could be included in the safety circuit of a motor starter.

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MEMORANDUM1.

For the same size they are cheap compared to other types of motors. They are robust and require very little maintenance. They are available in wide range of sizes and power outputs. For the same size three phase motors provide more power and torque than

single phase motors. They can be connected in Star or Delta, provided that the winding insulation is

suitable for the voltage. They can be mounted horizontally or vertically. With suitable inverter power drives, their speed can be varied.

Any two acceptable

2. The difference between rotor speed and the stator’s rotating magnetic field is called the “slip” of the motor.

3. Synchronous speed is the speed of the rotation of the magnetic field in rotary machine like motors & generators, its unit is R.P.M or revolution per minute.

4.

So that the operator/user has control of the motor. Helps to reduce the starting current.

5.

5.1.1. Main circuit diagram of Sequence control of two motors.

5.1.2.

A- Contactor 1

B- Contactor 2

C- Thermal overload Relay 1

D- Thermal overload relay 2

5.1.3.

6.

a)

Ps=√3Vl . Il . cosθ

Il= P√3 xVl x cosθ

Il=153,62 A

b)

Il=√3 x IpIp= Il

√3Ip=153,62 A

√3Ip=88,69 A

7.

a)

Ps=√3Vl . Il . cosθPs=√3 x 380V x25 A x 0,8

Ps=13,16kW

b)

P=√3Vl . IlP=√3 x380V x25 A

P=16,45kVA

8. To prevent the circuit from switching on after a power failure etc.

9. Normally open contacts are contacts open in the de-energised state and close in the energised state.

10. No volt coil Isolator switch Overload protection fuses

Bibliography(n.d.). Retrieved from http://en.wikipedia.org/wiki/Squirrel-cage_rotor

(n.d.). Retrieved from http://www.brighthubengineering.com/diy-electronics-devices/43723-how-are-squirrel-cage-induction-motors-constructed/#imgn_3

(n.d.). Retrieved from http://www.nrcan.gc.ca/energy/products/reference/15176

(n.d.). Retrieved from http://www.scslow.co.uk/WEG%20MANUAL%20STARTERS.htm

(n.d.). Retrieved from http://electrical-engineering-portal.com/direct-on-line-dol-motor-starter

Swart, R. a. (2012). Electrical Technology Grade 12 CAPS.

Wiki. (n.d.). Retrieved from http://www.wikihow.com/Check-an-Electric-Motor

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