DC Machine Lab manuals 11 ES 80.docx

Lab Experiment ReportsOf DC Machines (EE-223)

Instructor: Sir Engr Gulam SarwarPerformed By: Hassan Ali11EL80Electrical Engineering (B) 4th SemesterUniversity College of Engineering & TechnologyThe Islamia University of Bahawalpur

Index of Performed Experiments:Sr #Experiments TitlePage #

0.Safety precaution3

1. To identify and study main parts of a DC machine4

2. Different Types of Connections in Dc Generators8

3. O.C.C of Separately Excited Dc Generator10

4. External characteristics of Separately Excited Dc Generator12

5. TA / IA Characteristics of DC shunt motor14

6. N / IA Characteristics of DC shunt motor16

7. N / TA Characteristics of DC shunt motor18

8. STANDARDS FOR TESTING DC MOTORS20

9. LOAD CHARACTERISTICS OF SEPARATELY EXCITED GENERATORS22

10. MAGNETIZATION CHARACTERISTICS OF DC GENERATOR24

11. LOAD CHARACTERISTICS OF DC SHUNT GENERATOR26

12. LOAD CHARACTERISTICS OF A SERIES GENERATOR28

13. LOAD CHARACTERISTICS OF A DC COMPOUND GENERATOR30

14. VARIATION OF SPEED OF A DC SHUNT MOTOR32

15.REVERSAL OF DC SHUNT MOTOR

SAFETY PRECAUTIONSNine rules for safe practice and to avoid electric shocks:1. Be sure of the conditions of the equipment and the dangers present BEFORE working on a piece of equipment. Many sportsmen are killed by supposedly unloaded guns; many technicians are killed by supposedly dead. Circuits,2. NEVER rely on safety devices such as fuses, relays and interlock systems to protect you. They may not be working and may fail to protect when most needed.3. NEVER remove the grounding prong of a three wire input plug .this eliminates the grounding feature of the equipment making it a potential shock hazard. 4. Disorganized mess of connecting leads, components and tools only leads to careless thinking circuits, shocks and accidents.5. DO NOT WORK ON WET FLOOR Do not work on wet floor or bare footed. Always work on a rubber mate or an insulated floor.6. DO NOT WORK ALONE.Its just good to have someone around to shot off the power give artificial respiration and to call a doctor.7. WORK WITH ONE HAND WHILE WORKING WITH ELCTRIC CIRCUITSA current in between a hands crosses your heart and can be more let than a current from hand to foot .a wise technician always work with one hand .watch your service man 8. NEVER TALK TO ANYONE WHILE WORKING Do not let yourself distracted. Also do not talk to anyone, if he is working on dangerous equipment. Do not be the cause of an accident.9. ALWAYS MOVE SLOWLYWhen work around electrical circuits. Violent and rapid movements lead to accidental shock and short circuits.

The Islamia University of BahawalpurUniversity College of Engineering & TechnologyElectrical Engineering (4th Semester)Direct Current Machines EE-223LAB EXPERIMENT:01

Name: Roll No: Lab Instructor Signature: Date:

Identify and Study Main Parts of DC Generator

Objective: To study main parts of a DC generator.Apparatus: DC generator Yoke, Poles, Armature, Commutator, Brushes.Theory: A DC generator is comprised of following main parts1. Field system2. Armature Core3. Armature Winding4. Commutator5. Carbon Brushes

1. Field System:The function of the field system is to produce uniform magnetic field within which the armature rotates. It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are mounted on the poles and carry the d.c exciting current. The field coils are connected in such a way that adjacent pole shave opposite polarity.

The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature and the frame Practical d.c. machines have air gaps ranging from 0.5 mm to 1.5 mm. Since armature and field systems are composed of materials that have high permeability, most of the m.m.f. of field coils is required to set up flux in the air gap. By reducing the length of air gap, we can reduce the size of field coils (i.e. number of turns). 2. Armature Core:The armature core is keyed to the machine shaft and rotates between the field poles. It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in Fig. The laminations

are individually coated with a thin insulating film so that they do not come in electrical contact with each other. The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate and provide mechanical security to the armature winding and to give shorter air gap for the flux to cross between the pole face and the armature teeth. Armature Winding:The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding. This is the winding in which working e.m.f. is induced. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current. The armature winding of a d.c. machine is a closed-circuit winding; the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops. 3. Commutator:A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into direct voltage across the brushes. The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine (See Fig). The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding. Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking. The sparks may bum the brushes and overheat and carbonise the commutator.4. Carbon Brushes:The purpose of brushes is to ensure electrical connections between the rotating commutator and stationary external load circuit. The brushes are made of carbonand rest on the commutator. The brush pressure is adjusted by means of adjustable springs (See Fig). If the brush pressure is very large, the friction produces heating of the commutator and the brushes. On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking.

Multipole machines have as many brushes as they have poles. For example, a 4-pole machine has 4 brushes. As we go round the commutator, the successive brushes have positive and negative polarities. Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and the -ve terminal.



Different Types of Connections in Dc GeneratorsObjectives: To understand different types of DC Machines1. Separately Excited DC Generator2. Shunt Excited DC Generator3. Series Excited DC GeneratorApparatus:1. DC Generator SM 26412. DC Power Supply3. Connecting Leads4. VoltmeterCircuit Diagram:

(Separately excited dc generator) (Shunt dc-generator)(Series dc generator)

Theory:DC Machines are classified according to manner in which armature circuit is connected to the field circuit. So there are following main types1. Separate Excited DC Generator2. Shunt Excited DC Generator3. Series Excited DC GeneratorIn a separate excited DC Generator the armature and field circuits are supplied by separate voltage sources. In a shunt excited DC Generator both circuits are connected in parallel to each other.In a series excited DC Machine both the field and armature circuits are connected in series to each other. These connections are shown in the circuit diagram.In this lab exercise our aim is to achieve above stated connections.Procedure:Make connections according to the given circuit diagram for each type of machine separately. After that you will see that we got different values for different connections.



O.C.C of Separately Excited Dc GeneratorObjective: To determine open circuit characteristics of a Separate Excited DC Generator.Apparatus:1. Power Supply Unit(SM 2635)2. Torque Measuring Unit (MV 1052)3. Drive machine, DC Machine (SM 2641 )4. Test Generator, DC Machine (SM 2641)5. Voltmeter6. AmmeterCircuit Diagram:

Theory:Open circuit characteristics curve also sometimes called no-load characteristic, is a graph showing the relation between induced e.m.f of a generator on no-load and the field current. The e.m.f of the generator at no-load is given by:Eo NIf the speed be kept constant while this characteristic is being drawn in that case Eo becomes proportional to flux , but flux is proportional to field current If. The curve between E0 and If is known as open circuit characteristic.Procedure:1. Achieve the connections as shown in circuit diagram.2. Turn on the DC Power Supply and increase the excitation voltage gradually from zero to full value while keeping speed of prime mover to be constant (prime mover not shown in the diagram).3. Record the value of output voltage against each value of field current.4. Turn off power to the machine after accomplishing the task.5. Draw the graph between E0 and If .Observations:S. No.IfEo

01

02

03

04

05

06

Graph between E0 and If:



External characteristics of Separately Excited Dc GeneratorObjective: To determine external characteristics of a Separately Excited DC Generator.Apparatus:1. Power Supply Unit (SM 2635)2. Torque Measuring Unit (MV 1052)3. Drive Machine, DC Machine SM 26414. Test Generator, DC Machine SM 26415. voltmeter 6. Ammeter7. Resistive load (SM 2676)Circuit Diagram:

Theory:In External characteristics curve showing the relation between terminal voltage of a generator and load current. The terminal voltage will be less then E due to voltage drop in the armature circuit .Therefore, this curve will lie below the internal characteristic. The formula of terminal voltage for external characteristic is V = E- IaRa armature reaction drop As the load increase, the terminal voltage falls due to armature reaction drop and voltage drop across armature resistance. Here load current and armature current is same because both are in series connection.

Procedure:1. Achieve the connections as shown in circuit diagram.2. Turn on the DC Power Supply and increase the excitation voltage gradually from zero to full value while keeping speed of prime mover to be constant .3. Record the value of output voltage against each value of load current.4. Turn off power to the machine after accomplishing the task.5. Draw the graph between V and IL.Observations:S. No.ILVT

01

02

03

04

05

Graph between V and IL:



Ta /Ia Characteristics of DC shunt motorObjectives:To find the Ta /Ia Characteristics of self excited DC shunt motor

Apparatus:1. Power Supply Unit (SM 2635)2. Torque Measuring Unit (MV 1052)3. DC Machine (SM 2641)4. Voltmeter and Ammeters5. Resistive load (SM 2676)

Circuit Diagram:

Theory: It is the curve between armature torque Ta and armature current Ia of DC motor. It is also known as electrical characteristics of the motors .In shunt motors the field current Ish is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant. In a dc motors,

Ta IaAs flux is constant in shunt motor .So, Ta Ia As both the armature current and the torque are directly proportional so their characteristic curve is straight line. And therefore large current is required to start a heavy load .

Procedure:1. Achieve the connections as shown in circuit diagram.2. Turn on the DC Power Supply and increase the load gradually from minimum to maximum value.3. Record the value of torque Ta and armature current Ia.4. Turn off power to the machine after accomplishing the task.5. Draw the graph between Ta & Ia

OBSERVATIONS:Sr. No.IA (Amperes)Ta (N-m)

1

2

3

4

5

Graph Between Torque (Ta) and speed (Ia):



N /Ia Characteristics of DC shunt motor

Objectives:To find the N /Ia Characteristics of self excited DC shunt motor

Apparatus:1. Power Supply Unit (SM 2635)2. Torque Measuring Unit (MV 1052)3. DC Machine (SM 2641)4. Voltmeter and Ammeters5. Resistive laod (SM 2676)

Circuit Diagram:

Theory: It is the curve between speed N and armature current Ia of DC motor. .In shunt motors the field current Ish is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant.

N Eb/Since flux is constant so we can say that speed is effected when Eb changes. And very small variation occurs in speed.

Procedure:1. Achieve the connections as shown in circuit diagram.2. Turn on the DC Power Supply and increase the load gradually from minimum to maximum value.3. Record the value of torque N and armature current Ia.4. Turn off power to the machine after accomplishing the task.5. Draw the graph between N & Ia

OBSERVATIONS:Sr. No.IA(rpm)N (r.p.m)

1

2

3

4

5

Graph Between Torque (Ta) and speed (Ia):



N/Ta Characteristics of DC shunt motor

Objectives:To find the N/Ta Characteristics of self excited DC shunt motor

Apparatus: Power Supply Unit SM 2631 Terminal Board SM 2635 Torque Measuring Unit MV 1052 DC Machine SM 2641 Volt and AmmetersCircuit Diagram:

Theory:It is the curve between speed N and the armature torque Ta of DC motor. N/Ta Characteristics is also known as mechanical characteristics. In a shunt motor the torque of an electric motor is not necessarily dependent on its speed but also on armature current. In this curve speed falls somewhat as the load torque increase. Increasing the load decreases the speed linearly. If the field current is varied within an appropriate range, constant speed can be maintained from no load to rated load. The Rotational Losses of a DC machine includes all speed dependent losses, such as bearings and brushes friction losses, windage losses, and eddy current and hysteresis losses in the armature core. These losses are independent of the load (ignoring the armature reaction effect). The other losses are due to the resistance of the windings. Some depend on the load (copper losses in the armature and series field windings), others on the applied voltage (copper losses in the shunt field winding).

Procedure:1. Achieve the connections as shown in circuit diagram.2. Turn on the DC Power Supply and increase the load gradually from minimum to maximum value.3. Record the value of torque Ta, Speed N and armature current Ia.4. Turn off power to the machine after accomplishing the task.5. Draw the graph between N & Ta

Observations:Sr. No.T (N-m)N (r.p.m)

1

2

3

4

5

Graph Between Torque (Ta) and speed (N) :



STANDARDS FOR TESTING DC MOTORSIndian standards IS 4722 deals with dc motors in a general way, but does not give details. IEEE test codes are available for dc machines.The tests are grouped into four categories:1. Preliminary tests2. Performance tests,3. Heat run tests, and4. Miscellaneous tests.Preliminary tests(i) Measurement of airgap under main poles and under interpoles from the commutator and the other end of the machine, with and without liners,(ii) Potential drop and polarity tests for field coils as well as for interpole connections, (iii) Insulation tests. (iv) High-voltage test: The high-voltage test for the armature should be done at a voltage of 1000 + twice the rated armature voltage, (v) Winding resistance measurements: Measure resistance of armature, field windings, interpole windings, adjust them to 75C.(vi) Vibration lest: The vibration test is done at no load and rated speed, (vii) Brush setting.Performance tests(i) No load saturation test(ii) Commutation test (iii} Load test(iv) Speed regulation testHeat run testsFor large machines, it is difficult to measure direct input-output accurately and it is recommended that the efficiency be worked out by segregation of various losses. The various losses are:Armature I2ra loss, series field copper loss I2rse, stray load loss, brush contact loss, shunt field copper loss I2frf, rheostat loss, brush friction loss, friction and windage loss, core loss, and ventilation loss.The above losses can either be calculated after measurement of resistance or can be measured.In the following sections, only the laboratory tests normally done on laboratory machines are explained in detail, with reference to performance point of view. The theory has been discussed in the previous chapters.

Name: Roll No: Lab Instructor Signature: Date: The Islamia University of BahawalpurUniversity College of Engineering & TechnologyElectrical Engineering (4th Semester)Direct Current Machines EE-223LAB EXPERIMENT:09

LOAD CHARACTERISTICS OF SEPARATELY EXCITED GENERATORSObjectives To determine the load characteristics of a separately excited generator, viz. the relation between terminal voltage of the generator and the load current by actually loading the machine.Apparatus A motor generator set, having a shunt motor with a starter and regulator driving the separately excited generator, one ammeter to measure field current, one voltmeter to measure terminal voltage of the generator and one ammeter to measure the load current, loading resistance, and tachcometer to measure speed.Diagram of connections DC supply

Diagram of connections for Load test on separately excited generator.Procedures Start the dc shunt motor (this is shown as drive in this experiment) driving the generator. Bring it up to the rated speed of the set by adjusting the shunt field regulator. Measure the speed by a tacheometer. Supply generator field by dc source. Adjust the field current of the generator by adjusting the resistance in the field circuit to a value which will give rated voltage of the generator. Keep the current constant during experiment. Measure the voltage of the generator. Then switch on the load (this may be lamp load or loading resistance which can be varied and adjusted to the required loads).Table of Observations Speed : rpmArmature Resistance ra ohms

Generator field currentLoad CurrentGenerator terminal

Results: Plot the graph of the generator terminal voltage against the load current. These arc the external characteristics or load characteristics of the separately excited generator.Calculate Iara drop. See if the voltage on the open circuit minus drop of volts Iara at various loads is equal to the terminal voltage. If there is difference, how much, if any, and why?

The Islamia University of BahawalpurUniversity College of Engineering & TechnologyElectrical Engineering (4th Semester)Direct Current Machines EE-223LAB EXPERIMENT:10Name: Roll No: Lab Instructor Signature: Date:

MAGNETIZATION CHARACTERISTICS OF DC GENERATORObjective To determine the magnetization characteristics of dc generator. Apparatus required The same as in the last experiment for motor drive, and for separate excitation. The generator should be separately excited for this test. Ammeter for measuring field current, voltmeter to measure generator voltage, tacheometer to measure speed.Circuit diagram

Procedure Start the motor driving the generator and adjust the speed to the rated speed. During the experiment, maintain the speed to the rated speed. Adjust the excitation from zero to maximum (to a value to give generator voltage about 20 per cent more than the rated voltage on open circuit). Read the open circuit voltage at the terminals of the generator. If the behaviour of the machine regarding voltage induced on open circuit by increasing or decreasing the excitation values are required so as to obtain hysteresis loop, the arrangement should be made to reverse the field connection through a double-pole double-throw switch. For only open circuit characteristics, read only emf at the terminals on open circuit vs the field current from zero to maximum value in positive direction only. Keep the speed constant.

Table of observations Speed constant.Field CurrentOpen circuit voltage


LOAD CHARACTERISTICS OF DC SHUNT GENERATORObjectives To determine the external characteristics of a dc shunt generator. To find the relation between the terminal voltage of the generator vs the load current.Diagram of connections Apparatus required A dc shunt motor drive with a starter and field regulator to maintain the speed of the set constant during test; shunt generator driven by motor; shunt field rheostat, ammeter, voltmeter, to read load current, loading resistance or lamp load (for smaller loads), and tacheometer.Procedure Start the motor so that the set is started, adjust the field of the motor so that the speed is equal to the rated speed. Adjust the excitation of the generator by changing the resistance in the shunt field circuit of the generator to the rated voltage on open circuit. Make sure that the generator builds up in the right direction. Switch on double pole switch. Load the generator by adjusting the load current to 100 per cent, 75 per cent, 50 per cent, 25 per cent, 10 per cent, 110 per cent of the rated load on the machine. Read the load current and the terminal voltage of the generator keeping the speed constant.

Table of Observations: Keep speed constant. Speed: rpm Field CurrentLoad CurrentTerminal Voltage


LOAD CHARACTERISTICS OF A SERIES GENERATORObjectives To determine the load characteristics of a dc series generator, and to find the relation between the terminal voltage vs the load current when the generator is loaded.Diagram of connections

Apparatus required Motor generator set; dc shunt motor with a starter field regulator, etc. (keep the speed constant during experiment); a de scries generator coupled to motor drive; voltmeter; ammeter; and loading resistance (adjustable).Procedure Start the motor and run it at the rated speed. Initially the generator will have small voltage due to residual magnetism (negligible). Load the generator in steps preferably for loads 10 %, 25 %, 50 %, 75 %, 100 %, 110 % approximately, by adjusting the loading resistance. Read the current and the terminal voltage. This being scries machine, the Held current and load current is the same and as load current rises, the field current also increases and the terminal voltage increases, unlike other types of generators.Table of observations Keep the speed of the set constant. Speed: rpmLoad CurrentTerminal Voltage

AV

Results and discussionsPlot the curve of terminal voltage vs load current.



LOAD CHARACTERISTICS OF A DC COMPOUND GENERATORObjectives To determine the load characteristics of a dc compound wound generator, and to find the relation between the terminal voltage and the load current.Diagram of connections

.Apparatus required M.G. set, with motor starter, regulator, generator, shunt field regulator for generator, ammeter, voltmeter, loading resistance or lamp load, double pole switch, and tacheomctcr.Procedure Start the motor (driving). Adjust its speed to the rated speed of the setgenerator. Keep the speed constant during the test. Excite the shunt field till the emf on open circuit or on no load is the rated voltage. Put on the double-pole switch. Load the generator in steps preferably 10 %, 25 %, 50 %, 75 %, 100 % and 110 % load. Read the load current (A) and the terminal voltage of the generator.Table of observations Speed constant during test: rpmLoad CurrentTerminal Voltage

AV

It may be worthwhile to measure the voltage across the series field and across the shunt field or the armature separately. Measure the armature resistance, the shunt field resistance and the series field resistance.Results and discussionsPlot the curve of terminal voltage vs load current.


VARIATION OF SPEED OF A DC SHUNT MOTORObjectives To find out the methods of varying speed of a dc shunt motor (a) by varying the resistance in the shunt filed circuit, and (b) by varying the resistance in armature circuit; and to study the effects.Apparatus required A dc shunt motor, starter, field regulator or resistance to be inserted in shunt field circuit, ammeter to measure 1^ voltmeter to measure terminal voltage of the machine, tacheometer to measure speed, and ammeter to measure armature current.Circuit diagram

Procedure Start the dc motor with the starter and bring it to the rated speed by adjusting the resistance in the shunt field circuit. Read armature current, voltage across the terminals of the armature, field current If. Speed, then vary the resistance in the shunt field by adjusting the rheostat connected in the series with the shunt field. Note down the shunt field current, armature current and speed. Increase the speed in steps till it is 130 % of the rated speed also decrease it to 100 % in steps. Measure the resistance of the shunt field, and the resistance of the armature.Table of observations Armature resistance Ra Shunt field resistance RfSpeed N rmpShunt FieldArmature currentTerminal Voltage

(a) Variation of speed of a dc shunt motor by resistance in shunt field circuit and (b) Variation of speed of dc shunt motor by resistance in armature circuit :Calculations Terminal voltage across the armature is V. Armature current Ia resistance ra. The back emf in the armature

Check the expressions for various values of speed Ia, If, V.

Results and discussionsPlot the curve of speed vs shunt field current. Discuss the nature of the curve, explaining why it is so.



REVERSAL OF DC SHUNT MOTORObjectiveTo find out how the direction of the rotation of the dc motor can be reversed.

Apparatus RequiredA dc shunt motor and starter

Diagrams of connections

Connect the motor windings as shown

ProcedureConnect the motor with the starter. Observe the direction of the rotation.See from the commutator side.Now connect with different patterns and observe the rotation as given in the diagrams.

Table of Observations

Field connectionsArmature connectinsDirection of rotation

DC Machine Lab manuals 11 ES 80.docx

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