207--DCMT Lab manual by Prof.A.A.Malgave

Experiment no. 1

Object

Load test on DC Shunt motor.

Objectives

To study the direct loading test of DC Shunt motor by Rope-Pulley arrangement method.

Theory

            This is direct method of testing the motor in which the motor is loaded gradually from no load to full load. The pulley is fitted on the shaft of a motor. A rope or leather belt is wound round the pulley & its two ends are attached to two spring balances F₁ & F₂. The tension of the belt can be adjusted with the help of the wheel.

            The force acting tangentially on the pulley is equal to the difference between two spring balance readings.

i.e.       F = (F₁ - F₂)    

If         r = radius of pulley in meters,

            N = speed in rpm,

Advantages of Break load test.

1. It is direct method of testing.
2. Temperature rise of machine can be noted.
3. Results are accurate.
4. Any type of motor Shunt, Series or Compound can be tested.

Disadvantages of Break load test.

1. Not suitable for large machines.
2. Limitations due to brake friction, burning of belt etc.
3. Energy is wasted in testing.
4. Requires large time for testing.

Precaution

1.      Keep the rheostat in armature circuit at its maximum position.

2. Keep the rheostat in field circuit at its minimum position.

Apparatus

S.No.	Apparatus	Range	Type	Quantity
1	DC Ammeter	(0-20)A	MC	1
2	DC Voltmeter	(0-300)V	MC	1
3	Rheostat	100Ω/5A	Wire Wound	1
4	Tachometer	(0-2000) rpm	Digital	1
5	Connecting Wires	2.5sq.mm.	Copper	Few

Procedure

1.      Make the connections as shown in the circuit diagram.

2. Switch ON the supply & start the motor with the help of starter

3.      Adjust the rheostat in armature circuit & set the motor speed at its rated value.

4. At No load note the readings of Voltmeter, ammeter & tachometer.
5. Load the motor in steps by moving the wheel on rope-pulley arrangement.
6. At each step note the readings of all the meters & spring balance readings F₁ & F₂.
7. Calculate the Torque, Output Power, Input Power & Efficiency using appropriate formulae.
8. Plot the graph of Efficiency Vs O/P power Torque Vs O/P power Speed Vs O/P power & Speed Vs Torque.

Circuit diagram

Observation Table

Circumference of the Brake drum         =                                 cm.

	Sr.No	Voltage V(Volts)	Current I(Amp)	Spring Balance Reading		(F1-F2) Kg	Speed N(rpm)	Torque T(Nm)	Output Power Pm (Watts)	Input Power Pi (Watts)	Efficiency %
				F1(Kg)	F2(Kg)

 Sample Calculations

                                                    

                    Circumference

i)  R       =   -------------------     =               m                 

                        100 x2π                 

ii) Torque T = (F₁ - F₂) x R x 9.81   =                 Nm    

                                                                                                                

iii)Input Power P_i = VI      =                  Watts                         

                                                                                                                                                                                   

                                       

                                         2πNT

iv)Output Power P_{m      =}     ------------    =                   Watts

                                            60                                                  

                                          Output Power

v)Efficiency η %     =   -------------------- x 100%

                              Input Power

Nature of graph

Hand calculations

Graph-

Result

Average efficiency of shunt motor is, η %   =…………%

Conclusion

From this experiment we studied the direct loading test of dc shunt motor by using pulley and rope arrangement and calculate the percentage efficiency of dc motor.

Questions

1.      What is the purpose of brake test on dc shunt motor?

2.      What are the limitations of brake load test?

3.      Why cooling is necessary for brake drum?

4. Why brake test is used only for small capacity of dc motor?
5. Comment on the nature of efficiency curve?
6. State the importance of performing a load test on dc shunt motor?
7. When does the efficiency of motor become maximum?

Experiment no. 2

Object  

Perform load test on dc compound motor

Objectives

To conduct load test on DC compound motor and to find its efficiency.

Theory

This is direct method of testing dc compound motor. In this method the motor is putting on the direct load by means of belt and water cooled pulley arrangement by adjusting the tension of belt. The load is adjusted to various values of currents. The load is finally adjusted to get full load current the power development gets wasted against the friction between belt and shaft. Due to the breaking action of belt the test is called break test. In the break test compound motor we are mainly used in long shunt compound motor. In this type the shunt field winding is connected across the combination of armature and series of field winding. The resistance of field winding in series ‘R_se’ and shunt field winding is ‘R_sh’. The total current drawn from supply is ‘I_L’.

I_L= I_se +I_sh, I_se= I_a

Apparatus

S.No.	Apparatus	Range	Type	Quantity
1	Ammeter	(0-20)A	MC	1
2	Voltmeter	(0-300)V	MC	1
3	Rheostat	1250Ω/0.8A	Wire Wound	1
4	Tachometer	(0-1500) rpm	Digital	1
5	Connecting Wires	2.5sq.mm.	Copper	Few

Procedure

1.    Connections are made as per the circuit diagram.

2.    After checking the no load condition, and minimum field rheostat position, DPST switch is closed and starter resistance is gradually removed.

3.    The motor is brought to its rated speed by adjusting the field rheostat.

4.    Ammeter, Voltmeter readings, speed and spring balance readings are noted under no load condition.

5.    The load is then added to the motor gradually and for each load, voltmeter, ammeter, spring balance readings and speed of the motor are noted.

6.    The motor is then brought to no load condition and field rheostat to minimum position, then DPST switch is opened.

7. Plot the graph of Plot the graph of Efficiency Vs O/P power Torque Vs O/P power Speed Vs O/P power & Speed Vs Torque.

Precautions

1.    DC compound motor should be started and stopped under no load condition.

2.    Field rheostat should be kept in the minimum position.

3.    Brake drum should be cooled with water when it is under load.

Circuit Diagram

Observation Table

S.No.	Voltage V (Volts)	Current I (Amps)	Spring Balance Reading		(S1~S2)Kg	Speed N (rpm)	Torque T (Nm)	Output Power Pm (Watts)	Input Power Pi (Watts)	Efficiency η%
			S1(Kg)	S2(Kg)

 Sample Calculations

                                  

                    Circumference

i)  R        =     -------------------     =               m                 

                        100 x2π                 

ii) Torque T = (F₁ - F₂) x R x 9.81   =                 Nm    

                                                                                                                

iii)Input Power P_i = VI      =                  Watts                         

                                                                                                                                                                                   

                                       

                                         2πNT

iv)Output Power P_{m      =}     ------------    =                   Watts

                                            60                                                  

                                          Output Power

v)Efficiency η %     =   -------------------- x 100%

                              Input Power

Nature of graph

Hand calculations

Graph-

Result

Average efficiency of Compound motor is, η %   =

Thus load test on DC compound motor is conducted and its efficiency is determined.

Questions

1) Explain the difference between “long shunt” and “short Shunt” compounding?

2) What are the uses of different types of compound motors?

3) What   is   differential   compounding?   How   is   it   different   from   cumulative compounding?

4) How do you reverse the direction of motor?

5) Draw the speed – torque curve for differential. Compound motor

6) In a dc M/C, windage losses vary with speed in the proportion of …..

7) Brake test on dc motors is usually restricted to ….HP motors

8) Why do we pour water in the brake draw during brake test? What is the effect on speed of dc compound motor if the series field winding is shorted?

9) How do you minimize iron losses in a dc machine?

Experiment no. 3

Object

Speed control of DC Shunt motor (Armature and Field control)

Objectives

To control the speed of DC Shunt motor by

a)      Flux or Field current control method.

b)     Armature Voltage or Rheostatic control method.

Theory

            From the voltage equation of dc shunt motor,

We have,

V = E_b + I_a R_a

E_b = V - I_a R_a   ………………………. (1)

Also,

                                    E_b = ØPN  x Z

60                 A

N = E_b 60 A    ……………………...... (2)

         ØPZ

N α  E_b                   ………………............. (3) [Since all other

        Ø                                                             terms are constant.]

Putting eqn (1) in (3).

                       

N α V - I_a R_a                       

           Ø

            Thus, from this equation it is seen that speed of dc shunt motor can be changed by,

1. Changing the flux Ø. i.e. by adding resistance in field circuit.
2. Changing I_a R_a or armature voltage. i.e. by adding resistance in armature circuit.

A)    Speed Control by Changing Flux.

            This method is also called as flux control method or field current control method. In this method a variable resistor is connected in the field circuit with the help of which the field current can be decreased. This causes decrease in flux & thus increase in speed.

            This method is used to get the speed above normal speed. The waste of power due to resistance in field circuit is very small. Therefore in this method the efficiency of motor is not affected much.

B)    Speed Control by Changing Armature Voltage.

            This method is also called as Rheostatic control method or Armature Voltage control method. In this method a variable resistor is connected in the armature circuit with the help of which the armature voltage can be changed (From normal to lesser values). This causes decrease in armature voltage & thus decrease in speed. This method is used to get the speed below normal speed. The waste of power due to resistance in armature circuit is very high. Therefore in this method the efficiency of motor is affected. So this method is used for speed control of short duration.

Apparatus

S.No.	Apparatus	Range	Type	Quantity
1	DC Ammeter	(0-2A)	MC	1
2	DC Voltmeter	(0-300)V	MC	1
3	Rheostat	100Ω/5A, 290Ω/2.8A	Wire Wound	1
4	Tachometer	(0-2000) rpm	Digital	1
5	Connecting Wires	2.5sq.mm.	Copper	Few

Procedure

1. Make the connections as shown in the circuit diagram.
2. Switch ON the supply & start the motor with the help of starter
3. Keep armature voltage V_a at a constant value by varying the rheostat in armature circuit.
4. Change the field current in steps by varying rheostat in field circuit.
5. Note down the corresponding values of Speed at each step.
6. Plot the graph of Speed (N) Vs Field current (I_f).

Precaution

1. Keep the rheostat in armature circuit at its maximum position.
2. Keep the rheostat in field circuit at its minimum position.

Circuit Diagram

 

Observation Table

A)    Speed Control by Changing Flux.

Armature Voltage (constant) =              Volts.

Sr No	Field Current (If) amp	Speed(N) rpm
1
2
3
4
5

Procedure

1.      Make the Connections as shown in the circuit diagram.

2.      Switch ON the supply & start the motor with the help of starter

3.      Keep field current (I_f) at a constant value by varying the rheostat in field circuit.

4. Change the armature voltage (V_a) in steps by varying the rheostat in armature circuit.
5.  Note down the corresponding values of Speed.
6. Plot the graph of Speed (N) Vs Armature voltage (Va).

Precaution

1.      Keep the rheostat in armature circuit at its maximum position.

2.      Keep the rheostat in field circuit at its minimum position.

Circuit Diagram

 

Observation Table

C)    Speed Control by Changing Armature Voltage.

Field Current (constant) =             amp.

Sr No	Armature Voltage Va (Volt)	Speed N (rpm)
1
2
3
4
5

Nature of graph for Flux control method

 

Nature of graph for Arm. voltage control method

 

 Graph

Questions Speed Control by Changing Flux

1. Can you get fine speed control with this method?
2. What are the applications of variable speed motor?
3. State the factors on which Speed of DC Motor depends.
4. What will happen, if the field circuit rheostat is kept maximum while starting the motor?
5. What will happen, if the field circuit gets open during the running condition?
6. What is the effect of saturation on motor speed?
7. Why the speed of dc shunt motor is considered practically constant?

Questions Speed Control by Changing Armature Voltage

1.      State the factors on which Speed of DC Motor depends.

2.      Why the field rheostat kept at minimum initially?

3. Why the armature rheostat kept at maximum initially?
4. What will happen if resistance in armature circuit is very high at starting?
5. Why this method cannot be used to control the speed above rated value?
6. What are the advantages & disadvantages of this method?
7. What are the applications of DC Motor?

Experiment no. 4

Object

To perform the Swinburne’s test on dc shunt motor

  Objectives

          To conduct Swinburne’s test on DC machine to determine efficiency when working as generator and motor without actually loading the machine.

Theory

It is a simple method in which losses are measured separately and from that knowledge efficiency at any desired load can be predetermined in advance. Only running test needed is no load test. However this test is applicable to those machines in which flux is practically constant, i.e. shunt and compound machines.

This method is an indirect method of testing a dc machine. It is named after Sir James Swinburne. Swinburne's test is the most commonly used and simplest method of testing of shunt and compound wound dc machines which have constant flux. In this test the efficiency of the machine at any load is pre-determined. We can run the machine as a motor or as a generator. In this method of testing no load losses are measured separately and eventually we can determine the efficiency.

Calculation of Efficiency When the Machine is Motoring on Load

Power input = VI

Armature copper loss, P_CU = I² R_a = (I - I_sh)²R_a

Constant losses, W_C = VI₀ - (I₀ - I_sh) ² R_a

Total losses = P_CU + W_C

∴ Efficiency of the motor:

η_g =  = =

Calculation of Efficiency When the Machine is Generating on Load

Power input = VI

Armature copper loss, P_CU = I² R_a = (I + I_sh)² R_a

Constant losses, W_C = VI₀ - (I₀ - I_sh)² R_a

Total losses = P_CU + W_C

∴ Efficiency of the generator:

η_g =  = =

Advantages of Swinburne's Test

1.      This test is very convenient and economical as it is required very less power from supply to perform the test.

2.      Since constant losses are known, efficiency of Swinburne's test can be pre-determined at any load.

Disadvantages of Swinburne's Test

1.      Iron loss is neglected though there is change in iron loss from no load to full load due to armature reaction.

2.       We cannot be sure about the satisfactory commutation on loaded condition because the test is done on no-load.

3.      We can’t measure the temperature rise when the machine is loaded. Power losses can vary with the temperature.

4.      In dc series motors, the Swinburne’s test cannot be done to find its efficiency as it is a no load test.

Apparatus 

S.No.	Apparatus	Range	Type	Quantity
1	Ammeter	(0-20) A	MC	1
2	Voltmeter	(0-300) V	MC	1
3	Rheostats	1250W, 0.8A	Wire Wound	1
4	Tachometer	(0-3000) rpm	Digital	1
5	Resistive Load	5KW,230V	-	1
6	Connecting Wires	2.5sq.mm.	Copper	Few

Procedure

1.    Connections are made as per the circuit diagram.

2.    After checking the minimum position of field rheostat, DPST switch is closed and      starting resistance is gradually removed.

3.    By adjusting the field rheostat, the machine is brought to its rated speed.

4.    The armature current, field current and voltage readings are noted.

5.    The field rheostat is then brought to minimum position DPST switch is opened.

Precaution

The field rheostat should be in the minimum position at the time of starting and stopping the motor

Procedure for calculating armature resistance

1. Connections are made as per the circuit diagram.     
2. Supply is given by closing the DPST switch.
3. Readings of Ammeter and Voltmeter are noted.
4.  Armature resistance in Ohms is calculated as R_a = (Vx1.5) /I

Circuit Diagram

Circuit Diagram for calculating armature resistance

Observation Table

Determination of armature resistance

S.No.	Voltage V (Volts)	Current I (Amps)	Armature Resistance Ra (Ohms)

AS MOTOR:

Sr. No.	V (Volts)	IL (Amps)	Ia (Amps)	Ia2Ra (Watts)	Total Losses W (Watts)	Output Power (Watts)	Input Power (Watts)	Efficiency η%

AS GENERATOR:

Sr. No.	V (Volts)	I1 (Amps)	Ia (Amps)	Ia2Ra (Watts)	Total Losses  (Watts)	Output Power (Watts)	Input Power (Watts)	Efficiency η%

Sample Calculations

FORMULAE:

Hot Resistance  R_a = 1.2 X R Ω

            Constant losses       = VI_o – I_ao² R_a watts

            Where I_ao       = (I_o – I_f) Amps

AS MOTOR:

        Load Current I_L         = _____ Amps (Assume 15%, 25%, 50%, 75% of rated current)

            Armature current Ia = I_L – I_f Amps

            Copper loss               = I_a² R_a watts

            Total losses               = Copper loss + Constant losses

           Input Power               = VI_L  watts

           Output Power            = Input Power – Total losses

                                    Output power      

Efficiency η %           =      ---------------------- X 100%

                                                 Input Power

AS GENERATOR:

  Load Current I_L         = _____ Amps (Assume 15%, 25%, 50%, 75% of  rated     current)

  Armature current Ia = I_L + I_f Amps

  Copper loss               = I_a² R_a watts

  Total losses               = Copper loss + Constant losses

  Output Power            = VI_L  watts

  Input Power              = Input Power +Total losses                             

Efficiency η %           =( Output power / Input Power)*100%

                

Hand Calculation

Result

Thus the efficiency of the D.C machine is predetermined by Swinburne’s test.

Therefore % Efficiency of dc shunt motor = …………%        

Questions

1.      What is the purpose of Swinburne’s test?

2.      What are the constant losses in a DC machine?

3.      What are the assumptions made in Swinburne’s test?

4.      Why is the indirect method preferred to the direct loading test?

5.      The efficiency of DC machine is generally higher when it works as a generator than when it works as a motor. Is this statement true or false? Justify your answer with proper reasons.

Experiment no. 5

Object

To perform Hopkinson’s test on a DC machines

Objectives

            To conduct Hopkinson’s test on a pair of identical DC machines to pre-determine the efficiency of the machine as generator and as motor.

Theory

This test is called regenerative test or to back test which can be carried out on two identical d.c. machines mechanically coupled to each other and simultaneously tested. Thus the full load test can be carried out on two identical shunt machines without wasting their outputs. One of the machines is made to act as a motor while the other as a generator. The mechanical output obtained from the motor drives the generator whose electrical output supplies the greater part of input to the motor. The motor is connected to the supply mains only to components for losses since in absence of losses, the motor-generator set would have run without any external power supply. But due to losses, the generator output is not sufficient to drive the motor. Thus motor takes current from the supply to account for losses.

      The switch is kept open. The other machine which is coupled to first will act as load on first which is acting as motor. Thus second machine will act as a generator. The speed of motor is adjusted to normal value with the help of the field rheostat. The voltmeter reading is observed. The voltage of the generator is adjusted by its field rheostat so that voltmeter reading is zero. This will indicate that the generator voltage is having same magnitude and polarity of that of supply voltage. This will prevent heavy circulating current flowing in the local loop of armatures on closing the switch. Now switch S is closed. The two machines can be put into any load by adjusting their field rheostats. The generator current I₂ can be adjusted to any value by increasing the excitation of generator or by reducing the excitation of motor. The various reading shown by different ammeters are noted for further calculations.

       The input to the motor is nothing but the output of the generator and small power taken from supply. The mechanical output given by motor after supplying losses will in turn drive the generator.

Apparatus

S.No.	Apparatus	Range	Type	Quantity
1	Ammeter	(0-1)A (0-20) A	MC MC	1 2
2	Voltmeter	(0-300) V (0-600)V	MC MC	1 1
3	Rheostats	1250W, 0.8A	Wire Wound	2
4	Tachometer	(0-3000) rpm	Digital	1
5	Resistive Load	5KW,230V	-	1
6	Connecting Wires	2.5sq.mm.	Copper	Few

Procedure

1.      Connections are made as per the circuit diagram.

2.      After checking the minimum position of field rheostat of motor, maximum position of field rheostat of generator, opening of SPST switch,

3.      DPST switch is closed and starting resistance is gradually removed.

4.      The motor is brought to its rated speed by adjusting the field rheostat of the motor.

5.      The voltmeter V₁ is made to read zero by adjusting field rheostat of generator and SPST switch is closed.

6.      By adjusting field rheostats of motor and generator, various Ammeter readings, voltmeter readings are noted.

7.      The rheostats and SPST switch are brought to their original positions and DPST switch is opened.

Procedure for calculation of armature resistance

1. Connections are made as per the circuit diagram.     
2. Supply is given by closing the DPST switch.
3. Readings of Ammeter and Voltmeter are noted.
4. Armature resistance in Ohms is calculated as R_a = (Vx1.5) /I

Precaution

1. The field rheostat of the motor should be in the minimum position at the time of starting and stopping the machine.
2. The field rheostat of the generator should be in the maximum position at the time of starting and stopping the machine.
3. SPST switch should be kept open at the time of starting and stopping the machine.

Circuit Diagram

Determination of armature resistance

Observation Table

Sr.No.	Supply Voltage V(Volts)	I1 (Ams)	I2 (Amp)	I3 (Amps)	I4 (Amp)	I1 + I2 (Amp)	Motor Armature Cu Loss W (watts)	Generator Armature Cu Loss W(watts)	Total Stray losses W (watts)	Stray Loss Per M/c w/2 (watts)

AS MOTOR:

Sr. No.	V (Volt)	I1 (Amp)	I2 (Amp)	I3 (Amp)	Motor Armature Cu Loss W (Watts)	Field Loss (Watts)	stray losses /2(Watts)	Total Losses W (Watts)	Output Power (Watts)	Input Power (Watts)	Efficiency %

AS GENERATOR:

Sr. No.	V (Volts)	I1 (Amps)	I2 (Amps)	Motor Armature Cu Loss W (Watts)	Field Loss (Watts)	Stray losses /2(Watts)	Total Losses W (Watts)	Output Power (Watts)	Input Power (Watts)	Efficiency h%

Calculation of armature resistance       

S.No.	Voltage V (Volts)	Current I (Amps)	Armature Resistance Ra (Ohms)

Sample Calculations

FORMULAE:

Input Power                                      =  VI₁ watts

Motor armature cu loss                   =  (I₁+ I₂)² Ra watts

Generator armature cu loss           =  I₂² Ra watts

Total Stray losses W                        =  V I₁ - (I₁+I₂)² Ra + I₂² Ra  watts.

Stray loss per machine                   =  W/2 watts.

AS MOTOR:

Input Power                                      =  Armature input + Shunt field input

                                                            =  (I₁+ I₂) V + I₃V = (I₁+I₂+I₃) V

Total Losses                                      =  Armature Cu loss + Field loss + stray loss

                                                            =  (I₁ + I₂)² Ra + VI₃ + W/2  watts

                                                               

Efficiency h%                                   = (Input power – Total Losses/ Input Power)*100%

                                                               

AS GENERATOR:

Output Power                                   =  VI₂ watts

Total Losses                                      = Armature Cu loss+ Field Loss + Stray loss

                                                            = I₂² Ra + VI₄ + W/2 watts

                                                                                          

Efficiency h%                                   = (Output power/ Output Power+ Total Losses)* 100%

                                                               

Hand calculation

Result

 Thus Hopkinson’s test is conducted on a pair of identical DC machines the efficiency of the machine as generator and as motor are pre-determined

Questions

1.      What is the purpose of Hopkinson’s test?

2.      What are the precautions to be observed in this test?

3.      What are the advantages of Hopkinson’s test?

4.      What are the conditions for conducting the test?

5.      Why the adjustments are done in the field rheostat of generator and motor?

6.      If the voltmeter across the SPST switch reads zero what does it indicate? If it does not read zero value what does it indicate?

7.      What are the other names for Hopkinson’s test?