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DC MACHINES LAB-I

Dept of E.E.E 1

OBJECTIVES: The student has to learn about:

No load and load characteristics of DC generators

Various tests on DC motors

The speed control techniques of DC motors

The following experiments are required to be conducted as compulsory experiments: 1. Magnetization characteristics of DC shunt generator. Determination of critical field resistance

and critical speed.

2. Load test on DC shunt generator. Determination of characteristics.

3. Brake test on DC shunt motor. Determination of performance curves.

4. Load test on DC compound generator. Determination of characteristics.

5. Hopkinson’s tests on DC shunt machines. Predetermination of efficiency.

6. Fields test on DC series machines. Determination of efficiency.

7. Swinburne’s test and speed control of DC shunt motor. Predetermination of efficiencies.

8. Brake test on DC compound motor. Determination of performance curves.

In addition to the above eight experiments, atleast any two of the experiments from the following list are required to be conducted. 9. Load test on DC series generator. Determination of characteristics.

10. Retardation test on DC shunt motor. Determination of losses at rated speed.

11. Separation of losses in DC shunt motor.

OUTCOMES: The student should be able to do the following:

Conduct experiments to obtain the no-load and load characteristics of D.C. Generators

Conduct tests on D.C. motors for predetermination of efficiency

Conduct tests on D.C. motors for determination of efficiency

Control the speed of D.C. motor in a given range using appropriate method

Identify the reason as to why D.C. Generator is not building up voltage

DC MACHINES LAB-I

Dept of E.E.E 2

BRAKE TEST ON D.C COMPOUND MOTOR Aim: To obtain the performance characteristics of D.C Compound Motor by direct loading. Apparatus:

S. No. Name of the Equipment Range Type Quantity

1 Voltmeter (0-300)V MC 1

2 Ammeter (0-20)A MC 1

3 Rheostat 400Ω/1.7A Wire Wound 1

4 Tachometer (0-9999)rpm Digital 1

5 Connecting Wires - - Required Some

Circuit Diagram:

Precautions:

1. Field rheostat of the motor must be kept in minimum resistance position. 2. Starter arm must be in OFF position.

Procedure:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Start the motor with the help of the starter. 4. By using motor field rheostat bring the motor to its rated speed and load the motor in steps to

its rated full load current and note down all the meter readings. 5. If speed gradually decreases then the motor is said to be cumulatively compounded, if not

motor is said to be differentially compounded. 6. Observing the precautions switch OFF the supply. 7. Now reverse series field winding connections i.e., Y & YY and then repeat step 4. 8. Observing the precautions switch OFF the supply.

DC MACHINES LAB-I

Dept of E.E.E 3

Tabular Columns: Cumulative Compounding:

S. No. VL (V) IL

(A) N

(rpm)

Spring Balance Reading

Torque (N-m)

Input (kW)

Output (kW)

Ŋ

(%) S1 S2 S1-S2

Differential Compounding:

S. No. VL (V) IL

(A) N

(rpm)


Torque (N-m)

Input (kW)

Output (kW)

Ŋ

(%) S1 S2 S1-S2

Formulae: Torque = 9.81 X (S1-S2) X R N-m Input = VL IL kW Output = (2∏N)τ/60 kW Efficiency = ŋ % = (Output/Input) X 100 Model Graph:

DC MACHINES LAB-I

Dept of E.E.E 4

Result:

DC MACHINES LAB-I

Dept of E.E.E 5

VIVA VOICE QUESTIONS:

1) What are the different types of compound motors?

2) What is the different between cumulative and differential compound motors?

3) How the two fluxes are aided or opposed in a compound motor?

4) What are number of series field turns required for compound motors?

5) Why two windings are used in dc compound motor?

6) What are the applications of cumulative compound motors?

7) What is speed equation of DC compound motor?

8) What is the difference between long shunt and short compound motor?

9) What is the difference between series field and shunt field winding compound motor?

10) Which type of compound motor is having high speed?

DC MACHINES LAB-I

Dept of E.E.E 6

BRAKE TEST ON D.C SHUNT MOTOR Aim: To obtain the performance characteristics of D.C Shunt Motor by direct loading. Apparatus:







Circuit Diagram:

Precautions:

1. Motor field rheostat must be kept in minimum resistance position. 2. Starter arm must be in OFF position.

Procedure:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions switch ON 220V D.C supply. 3. Start the motor with the help of the starter. 4. By adjusting the motor field rheostat bring the motor to its rated speed. 5. Now load the motor in steps to its full load and note down all the meter readings. 6. Observing the precautions switch OFF the supply.

DC MACHINES LAB-I

Dept of E.E.E 7

Tabular Column:

S. No.

VL

(V)

IL

(A) N

(rpm)


Torque (N-m)

Input (kW)

Output (kW)

Ŋ (%)

S1 S2 S1-S2

1

2

3

4

5

6

Formulae: Torque = 9.81 X (S1-S2) X R N-m Input = VL IL kW Output = (2∏N)τ/60 kW Efficiency = ŋ % = (Output/Input) X 100 Model Graph:

Result:

DC MACHINES LAB-I

Dept of E.E.E 8


1) What is the principle operation of DC motor?

2) What is the Range of Shunt field Resistance?

3) What is no load current of Dc shunt motor?

4) How the Direction of a motor can be reversed?

5) What is Back emf or counter EMF ?

6) Why the Shunt motor is called a constant speed motor?

7) What are the applications of DC shunt motor?

8) What is purpose of starter?

9) What meant by the DPDTS?

10)What is the output power the dc motor?

11) What is speed regulation of DC motor?

12) Explain principle of operation of DC motor?

13) Why shunt motor field winding has more no of turns?

14) What happens when DC motor is connected across AC supply ?

DC MACHINES LAB-I

Dept of E.E.E 9

FIELD’S TEST

Aim: To determine the performance characteristics of a D.C Series motor using Field Test. Apparatus:






Circuit Diagram:

Precautions:

1. Make the connections tightly. 2. Starter arm must be in OFF position. 3. Initial Machine – II i.e., Generator is loaded with 7A of load. 4. Readings should not be taken when there is any fluctuation.

Procedure: 1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C Supply. 3. Machine – I is started as motor by keeping the load on Machine – II (i.e., generator) to such a

value that Machine – II is kept in safe speed limit. 4. By gradually loading in Machine – II in steps till maximum rated current of any of the machine,

note down the meter readings and speed at each step at each. 5. Observing the precautions switch OFF the supply.

DC MACHINES LAB-I

Dept of E.E.E 10

Tabular Column:

V1

(V) V2

(V) I1

(A) I2

(A) N

(rpm)

Motor Generator

Input (W)

Total Losses

(W)

Ŋ (%)

Output (W)

Losses (W)

Ŋ (%)

Formulae: Motor: Motor Input = V1I1 Motor Losses = Armature + Field Losses + Stray Losses Wm = (Ra + Rse) I1

2 + Ws

% ŋm = (V1I1 – Wm) / (V1I1) Generator: Generator Output = V2I2 Field Losses = I1

2Rse Armature Cu Losses = I2

2Ra

Stray Losses = Ws=2

ci WW

Total Losses = I12Rse + I2

2Ra + Ws = Wg

Total Losses = Wt = V1I1 – V2I2

Wc = (Ra + 2 RSe ) I12 + I2

2 Ra % ŋg = (V2I2) / (V2I2 + Wg) Model Graph:

DC MACHINES LAB-I

Dept of E.E.E 11

Result:

DC MACHINES LAB-I

Dept of E.E.E 12


1) What is the necessity of field test on series machines?

2) What is the no load speed of dc series motor?

3) What is the no load current of dc series generator?

4) What are the advantages of compensating windings for series motor?

5) What are the applications of dc series motor?

6) Why the series motor can’t operate under no load condition?

7) Why it is not a regenerative test?

8) What is rated currant of series generator?

9) Explain Principle operation of motor

10)Explain Faradays laws of Electromagnetic Induction

11).Why a DC series motor should never be stared without load?

12). Why a DC series motor has a high starting torque?

13).Compare the resistances of the field windings of DC shunt and series motor?

14). Comment on the Speed – Torque characteristics of a DC series motor.

DC MACHINES LAB-I

Dept of E.E.E 13

HOPKINSONS TEST

Aim: To conduct a regenerative test on two identical mechanically coupled D.C shunt machines to

determine the efficiency of each machine. Apparatus:


1 Voltmeter

(0-600)V MC 1

(0-300)V MC 1

(0-30)V MC 1

2 Ammeter

(0-20)A MC 3

(0-5)A MC 1

(0-2)A MC 2

3 Rheostat

400Ω/1.7A Wire Wound 1

290Ω/2.8A Wire Wound 1

100Ω/5A Wire Wound 1



Circuit Diagram:

DC MACHINES LAB-I

Dept of E.E.E 14

To Find Armature Resistance:

Precautions:

1. Field rheostat of the motor must be kept in minimum resistance position. 2. Field rheostat of the generator must be kept in maximum resistance position. 3. Armature rheostat of the generator must be kept in maximum resistance position. 4. Starter arm must be in OFF position. 5. SPST Switch must be kept open.

Procedure: To Conduct Load Test:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Start the Motor Generator set with the help of starter. 4. Adjust the field rheostat of the motor and bring the Motor Generator set to its rated speed. 5. Adjust the field rheostat of the generator and bring the voltage across the SPST Switch to zero. 6. By using the field rheostat of the generator in steps note down all the meter readings till

maximum rated current of any of the machine i.e., Motor or Generator. 7. Observing the precautions switch OFF the supply.

To find Armature Resistance:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST switch and switch ON 220V D.C supply. 3. By adjusting the rheostat for different values note down all the meter readings. 4. Observing the precautions switch OFF the supply.

DC MACHINES LAB-I

Dept of E.E.E 15

Tabular Columns: For Motor:

VL

(V) Ifm

(A) IL

(A) Iam

(A) Iag

(A) Ifg

(A)

Losses WTL

(kW)

Output (kW)

Input (kW)

Ŋ (%)

For Generator:

VL

(V) I1

(A) I2

(A) I3

(A) I4

(A) I5

(A)

Losses WTL

(kW)

Output (kW)

Input (kW)

Ŋ (%)


S. No. Va (V) Ia (A) Ra (Ω)

1

2

3

Formula: Generator: Armature Copper Losses = Iag

2 Ra Stray Losses = Ws = VIL - (Iam

2 Ra + Iag2 Ra )

Output = Iag V Input = Output + Losses

Total Losses = 2

SW + Iag

2 Ra + Ifg vL

Efficiency = ŋ % = (Output/Input) X 100

DC MACHINES LAB-I

Dept of E.E.E 16

Motor:

Total Losses = 2

SW + Iam

2 Ra + Iaf v

Input = V Iam Output = Input – Losses Efficiency = ŋ % = (Output/Input) X 100 Model Graph:

Result:

DC MACHINES LAB-I

Dept of E.E.E 17


1) What are the other names of Hopkinson’s test?

2) Why this test is conducted on two identical machines?

3) Why the energy is not sufficient to drive the motor from the generator?

4) What are the advantages and disadvantages of the test?

5) Why the test is called as regenerative test?

6) Why field rheostat of motor kept minimum position at starting condition?

7) Why field rheostat of generator kept maximum position at starting condition?

8)What are different types of testing of DC machines?

9) Difference between direct and indirect testing?

10)Explain difference between DC and AC supply systems

DC MACHINES LAB-I

Dept of E.E.E 18

LOAD TEST ON D.C COMPOUND GENERATOR Aim: To perform load test on D.C Compound Generator and plot its load characteristics. Apparatus:



(0-30)V MC 1

2 Ammeter

(0-20)A MC 1

(0-2)A MC 1

(0-5)A MC 1





Circuit Diagram:

To Find Armature and Series Field Resistance of the Generator:

DC MACHINES LAB-I

Dept of E.E.E 19

Precautions: 1. Field rheostat of the motor must be kept in minimum resistance position. 2. Field rheostat of the generator must be kept in maximum resistance position. 3. Armature rheostat of the generator must be kept in maximum resistance position. 4. Initially the load must be in OFF position. 5. Starter arm must be in OFF position.

Procedure: To Conduct Load Test:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Start the Motor Generator set with the help of starter. 4. Adjust the motor field rheostat and bring the motor speed to its rated value. 5. Adjust the field rheostat of the generator so that generator gives rated terminal voltage of the

load. 6. Now load the generator in steps and note down all the meter readings. If the voltmeter shows

an increase in voltage that indicates the compounding is cumulative otherwise differential. 7. Repeat the procedure for another compounding by reversing the series field terminals of the

generator. 8. Observing the precautions switch OFF the supply.

To find Armature & Series Field Resistance of the Generator:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST switch and switch ON 220V D.C Supply. 3. By adjusting the rheostat for different values note down the meter readings. 4. Observing the precautions switch OFF the supply.

Tabular Columns:

Differential Compounding Generator Cumulative Compounding Generator

IL

(A) If

(A) VL

(V) Ia

(A) Eg

(V) IL

(A) If

(A) VL

(V) Ia

(A) Eg

(V)

To find Armature & Series Field Resistance of the Generator:

S. No. Va (V) Ia (A) Ra + RSe (Ω)

1

2

Avg=

DC MACHINES LAB-I

Dept of E.E.E 20

Formulae: For Cumulative Compound: IL = Ia + If Eg = VL + IL (Ra + RSe ) For Differential Compound: IL = Ia + If Eg = V + Ia (Ra + RSe )

Model Graph:

Result:

DC MACHINES LAB-I

Dept of E.E.E 21


1) What are the different types of compound generators?

2) What are the types of load characteristics of a dc compound generator?

3) What is meant by flat compound Generator?

4) What is no load voltage of Dc compound Generator?

5) What is the voltage regulation of a dc compound generator?

6) What are the applications of dc differential compound generator?

7) What are the applications of dc cumulative compound generator?

8) What are the differences between under, flat and over compound generators?

9) What is formula for induced voltage of compound generator?

10) Up to where we can load the dc compound generator?

DC MACHINES LAB-I

Dept of E.E.E 22

LOAD TEST ON D.C SERIES GENERATOR Aim: To obtain the load characteristics of D.C Series Generator. Apparatus:



(0-30)V MC 1


(0-5)A MC 1



4 SPST Switch - - 1



Circuit Diagram:

To find Armature and Series Field Resistance of the Generator:

DC MACHINES LAB-I

Dept of E.E.E 23

Precautions: 1. Field rheostat of the motor must be kept in minimum resistance position. 2. Armature rheostat of the generator must be kept in maximum resistance position. 3. Starter arm must be in OFF position. 4. Load must be in OFF position.

Procedure: 1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Start the motor-generator set with the help of starter. 4. Adjust the motor field rheostat and bring the motor speed to its rated value and maintain the

speed constant throughout the experiment. 5. Now load the generator in steps till generator’s maximum rated current and note down all the

meter readings. 6. Observing the precautions switch OFF the supply.

To find Armature and Series Field Resistance of the Generator:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST switch and switch ON 220V D.C Supply. 3. By adjusting the rheostat for different values of current note down all the meter readings. 4. Observing the precautions switch OFF the supply.

Tabular Columns:

S. No. IL (A) VL (V) IL (Ra + RSe)

Eg (V)

1 0 10 0 10

2 0.2 14 0.624 14.624

3 0.44 19 1.3728 20.3728

4 7.18 170 22.4016 192.4016

5 10.2 200 31.824 231.824

6 12.8 215 39.936 254.936

To find Armature Resistance and Series Field Resistance of the Generator:

S.No. Ia (A) Va (V) Ra + RSe (Ω)

1 2.25 5 2.22

2 3.6 8 2.22

3 Avg=2.22

Formulae: Eg = VL + IL (Ra + RSe) (V)

DC MACHINES LAB-I

Dept of E.E.E 24

Model Graph:

Result:

DC MACHINES LAB-I

Dept of E.E.E 25

VIVA VOICE QUESTIONS: 1) What is the no load voltage of dc series generator?

2) What is armature reaction?

3) Why the load test on dc series generator is conducted?

4) What is the relation of load voltage with the load current in a dc series generator?

5) Why the series generators are used as boosters?

6) What happens when the armature circuit is open circuited during running conditions?

7) What is input for series generator?

8) Why the Dc series Generator having rising nature of characteristics?

9) What is the difference between internal and external characteristics of series generator?

10) Why series field winding having less number of turns with thick wire?

11) Explain importance of LEZ’s Law in Electrical Machinery

12) Define Flemings Right hand Rule

DC MACHINES LAB-I

Dept of E.E.E 26

LOAD TEST ON D.C SHUNT GENERATOR Aim:

To conduct load test on a D.C Shunt Generator and to determine the internal and external characteristics. Apparatus:



(0-30)V MC 1


(0-5)A MC 1





Circuit Diagram:

To find Armature Resistance of the Generator:

DC MACHINES LAB-I

Dept of E.E.E 27

Precautions: 1. Field rheostat of the motor must be kept in minimum resistance position. 2. Field rheostat of the generator must be kept in maximum resistance position. 3. Armature rheostat of the generator must be kept in maximum resistance position. 4. DPST Switch on the generator side must be kept open. 5. Initially load must be in OFF position. 6. Starter arm must be in OFF position.

Procedure:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Start the motor-generator set with the help of starter. 4. Adjust the motor field rheostat and bring the motor to its rated speed and by varying the field

rheostat of the generator apply the rated voltage of the load and close the DPSTS2 switch. 5. Now load the generator in steps till maximum rated current of the generator and note down all

the meter readings. 6. Observing the precautions switch OFF the supply.


1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST switch and switch ON 220V D.C Supply. 3. By adjusting the rheostat note down all the meter readings. 4. Observing the precautions switch OFF the supply.

Tabular Columns:

S. No. V (V) IL (A) If (A) Ia = IL + If (A) Eg = V + IaRa

(V)

1

2

3


S.No. Ia (A) Va (V) Ra (Ω)

1

2

Model Graph:

DC MACHINES LAB-I

Dept of E.E.E 28

Result:

DC MACHINES LAB-I

Dept of E.E.E 29


1) What is no load voltage of Dc shunt Generator?

2) What is meant DC shunt generator?

3) What is rated current of Dc shunt Generator?

4) What is the field current of DC shunt generator?

5) What versus we can draw internal characteristics of Dc shunt Generator?

6) What are the applications of DC shunt Generator?

7) What versus we can draw internal characteristics of Dc shunt Generator?

8) Why the Dc shunt Generator having drooping nature of characteristics?

9) What is armature reaction of Dc shunt Generator?

10) Why the armature reaction drop is very low?

DC MACHINES LAB-I

Dept of E.E.E 30

OPEN CIRCUIT CHARACTERISTICS OF A D.C GENERATOR Aim:

To find critical field resistance of a separately excited D.C generator from its open circuit characteristics. Apparatus:








Circuit Diagram:

Precautions:

1. Motor field rheostat must be kept in minimum resistance position. 2. Potential Divider must be kept in maximum resistance position. 3. Starter arm must be in OFF position.

Procedure:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Start the Motor Generator set with the help of starter. 4. Adjust the speed of the Motor Generator Set to rated speed value by adjusting motor field

rheostat.

DC MACHINES LAB-I

Dept of E.E.E 31

5. Increase the excitation of the generator in steps by adjusting the potential divider and note down the corresponding voltmeter and ammeter readings.

6. Take the readings up to a value little higher than the rated voltage of the generator. 7. Again decrease the excitation in the same steps till field current is zero by adjusting the

potential divider noting down the corresponding voltmeter and ammeter readings. 8. Observing the precautions switch OFF the supply.

Tabular Column:

S. No. If (A) Eg (V)

Increasing Eg (V)

Decreasing

1 0 10 16

2 0.1 50 58

3 0.2 88 100

4 0.3 122 136

5 0.4 154 168

6 0.5 186 196

7 0.6 208 218

8 0.8 246 246

Model Graph:

Result:

DC MACHINES LAB-I

Dept of E.E.E 32

VIVA VOICE QUESTIONS: 1) Why the magnetization curve is a non linear curve?

2) What is critical Speed and Critical Resistance?

3) What are conditions to failure the self excitation?

4) What are the different methods of excitations?

5) Magnetization curves are also known as?

6) What are the characteristics of a dc generator?

7) What is Residual magnetism?

8) What is meant by magnetic saturation?

9) What is meant by the field flashing method?

10) What is meant by the residual voltage?

11) Why saturation curve for DC generator does not start with zero?

12) What is Open Circuit Characteristics of DC generator?

13) What are the different types of DC generators?

14) What are the characteristics of DC generators?

DC MACHINES LAB-I

Dept of E.E.E 33

SEPARATION OF LOSSES IN A D.C SHUNT MOTOR Aim:

To separate hysteresis, eddy current and mechanical losses of a D.C Shunt Machine. Apparatus:



(0-30)V MC 1


(0-2)A MC 1


100 Ω/5A Wire Wound 1



Circuit Diagram:


DC MACHINES LAB-I

Dept of E.E.E 34

Precautions: 1. Field rheostat must be kept in minimum resistance position. 2. Armature rheostat must be kept in maximum resistance position.

Procedure: 1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST switch and switch ON 220V D.C supply. 3. Start the motor by adjusting the armature rheostat and obtain the rated voltage across the

terminals of the armature. Now adjust the field rheostat and bring the speed of motor to its rated value.

4. Now vary the armature rheostat in steps and for each step note down the speed and all the meter readings.

5. Repeat the procedure for another excitation (3/4 or ½ the rated value) suitably adjusting the field rheostat.

6. Observing the precautions switch OFF the supply. To find Armature Resistance:

1. Connect the circuit as shown in circuit diagram. 2. Keeping the rheostat in its maximum resistance position close the DPST Switch and switch ON

220V D.C Supply. 3. By adjusting the rheostat for different values of current note down the meter readings. 4. Observing the precautions switch OFF the supply.

Tabular Columns:

Field Current=0.6A Field Current=0.8A

VL

(V) Ia

(A) N

(rpm) IL = Ia + If

(A) WC (W)

WC/N (W/A)

VL

(V) Ia

(A) N

(rpm) IL = Ia + If

(A) WC (W)

WC/N (W/A)

To find Losses:

Speed N (rpm)

Hysteresis Losses AN

Eddy Current Losses BN2

Mechanical Losses WC – (Wh + We)

DC MACHINES LAB-I

Dept of E.E.E 35



1

2

Avg=

Formulae:

WC = VLIL – Ia2Ra

Wi = Wh + We Wh = AN We = BN2 Wmech = WC – (Wh + We) Model Graphs:

Result:

DC MACHINES LAB-I

Dept of E.E.E 36

VIVA VOICE QUESTIONS: 1) What are the types of losses that occur in a dc machine?

2) Why the test is conducted on no load?

3) Whether the test is direct or indirect?

4) Why the motor is operated in more than the rated speed?

5) Why this method is not suitable for series machines?

6) What are the advantages and disadvantages of this test?

8) What are the different types of iron losses?

9) How the eddy current losses occur in a dc machine?

10) How the hysteresis current losses occur in a dc machine?

DC MACHINES LAB-I

Dept of E.E.E 37

SPEED CONTROL OF A D.C SHUNT MOTOR Aim: To obtain the speed characteristics of D.C Shunt Motor by

1. Armature Controlled Method. 2. Field Controlled Method.

Apparatus:




(0-5)A MC 1





Circuit Diagram:

Precautions:

1. Field rheostat must be kept in minimum resistance position. 2. Armature rheostat must be kept in maximum resistance position. 3. Starter arm must be in OFF position.

Procedure: Armature Controlled Method:

1. Connect the circuit as shown in circuit diagram.

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Dept of E.E.E 38

2. Observing the precautions switch ON 220V D.C supply. 3. Start the motor with the help of starter. 4. By adjusting the field rheostat set the field current to a constant value. 5. By adjusting the armature rheostat for an armature voltage note down the speed and voltmeter

readings. 6. Repeat step 5 for another constant field current.

Field Controlled Method:

1. By adjusting the armature rheostat set the voltage to a constant value. 2. By adjusting the field rheostat for a field current note down the speed and armature current

readings. 3. Repeat the above step for another constant armature voltage.

Tabular Columns: Armature Controlled Method:

Field Current=0.8A Field Current=0.6A

Va (V) Speed (rpm)

Va (V) Speed (rpm)


Armature Voltage=160V Armature Voltage=200V

If (A) Speed (rpm) If (A) Speed (rpm)

DC MACHINES LAB-I

Dept of E.E.E 39

Model Graphs: Armature Controlled Method:


Result:

DC MACHINES LAB-I

Dept of E.E.E 40


1) What is speed equation of DC shunt motor?

2) What is the no load speed of DC shunt motor?

3) What are the various speed control techniques of a dc motor?

4) Why DC shunt motor is called Constant speed motor?

5) What happens when the field of dc shunt motor gets open circuited during running

condition?

6) Why field rheostat is kept minimum position at starting condition?

7) Which method we can obtain speed of motor is above its rated speed?

8) Which method we can obtain speed of motor is below its rated speed?

9) what versus we can draw speed curve field controlled method?

10) what versus we can draw speed curve armature controlled method?

DC MACHINES LAB-I

Dept of E.E.E 41

SWINBURNE’S TEST Aim: To predetermine the efficiency of a D.C Shunt Machine when run both as generator and motor. Apparatus:



(0-30)V MC 1


(0-2)A MC 1

3 SPSTS - Knife 1





Circuit Diagram:


DC MACHINES LAB-I

Dept of E.E.E 42

Precautions: 1. Field rheostat must be kept in minimum resistance position. 2. Armature rheostat must be kept in maximum resistance position. 3. SPST Switch must be kept in closed position.

Procedure:

1. Connect the circuit as shown in circuit diagram. 2. Observing the precautions close the DPST Switch and switch ON 220V D.C supply. 3. Adjust the speed of motor to its rated value by adjusting field and / or armature rheostat. 4. Now open the SPST Switch & note down all the meter readings. 5. Observing the precautions switch OFF the supply.


1. Connect the circuit as shown in circuit diagram. 2. Keeping the rheostat in its maximum resistance position close the DPST Switch and switch ON

220V D.C Supply. 3. By adjusting the rheostat for different values of current note down the meter readings. 4. Observing the precautions switch OFF the supply.

Tabular Columns:

S. No. Supply Voltage

(V) Line Current

IL (A) Shunt Current

If (A)

1



1

2

3 Avg=

Machine when run as Motor:

S. No. Voltage

(V)

IL

(A) If

(A) Ia

(A) Input (W)

WCu (W)

WC (W)

Output (W)

Ŋ (%)

WT

1

2

3

4

DC MACHINES LAB-I

Dept of E.E.E 43

Machine when run as Generator:

S. No. Voltage (V) IL

(A) If

(A) Ia

(A) Input (W)

WCu (W)

WC (W)

Output (W)

Ŋ (%)

WT

1

2

3

4

Formulae: Motor: Ia = IL - If Input = VLIL WC = VLIL - Ia

2Ra

WCu = Ia2Ra

WT = WCu + WC Output = Input - WT Ŋ = (Output/Input) X 100 Generator: Ia = IL + If Output = VLIL

WC = VLIL - Ia2Ra

WCu = Ia2Ra

WT = WCu + WC Input = Output + WT Ŋ = (Output/Input) X 100 Model Graph:

DC MACHINES LAB-I

Dept of E.E.E 44

Result:

DC MACHINES LAB-I

Dept of E.E.E 45

VIVA VOICE QUESTIONS: 1) What is the necessity of Swinburne’s test?

2) Why this test is conducted on no load?

3) Why this test is not suitable for series machines?

4) Whether the test is a direct method or indirect method?

5) What are the advantages of Swinburne’s test?

6) What is meant by the efficiency?

7) What is rated current of motor as well as generator of dc shunt machine?

8) How do you estimate the half load efficiency?

9) How do you estimate the half load copper losses?

10) Explain Faradays Laws of Electro Magnetic Induction

DC MACHINES LAB-I - crectirupati.com LAB MANUAL_0.pdf · The speed control techniques of DC motors...

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