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Electrical Lab Experiments

To conduct the brake test on a DC shunt motor and to draw its performance curves

AIM: To conduct the brake test on a D.C shunt motor and to draw its performance curves.

Name Plate Details:

S.No. Characteristics D.C Motor
1 Voltage 220V
2 Current 20A
3 Speed 1500rpm
4 Power 5HP

Apparatus Required:

S.No. Description Type Range Quality
1 Ammeter MC 0-20A 1
2 Voltmeter MC 0-300V 1
3 Rheostat WW 370 ohm/2A 1
4 Tachmoeter Digital 0-10000rpm 1

Circuit Diagram:

circuit diagram


This test is direct test to find the efficiency of the DC shunt motor. In this test the motor directly loaded by connecting brakes which are with pulley and motor is subjected to rated load and entire power is wasted. belt around the water cooled pulley has its ends attached to spring balances S1and S2.The belt tightening hand wheels h1 and h2 help in adjusting the load on the pulley so that the load on the motor can be varied.

Output power of the motor = (S1-S2)*Re*9.81*w (watts)

S1, S2 = weights on the pulley.

Re= Effective radius of the pulley.

w=motor speed in rad/sec.

If V is the terminal voltage IL is the line current

Power in put =V*IL watts.

Efficiency (%η) = (w (S1-S2)*Re*9.81/V*IL)*100


1. All the connections are as per the circuit diagram.

2. 220V, DC supply is given to the motor by closing DPST switch.

3. Move the 3-point starter handle form ‘OFF’ to ‘ON’ position slowly and motor Starts running.

4. Vary the field rheostat until the motor reaches its rated Speed and take voltmeter and ammeter readings.

5. Apply the land by break drum pulley and for each applications of load the Corresponding Voltmeter (V), Ammeter (I), spring forces S1 & S2 and Speed (N) Readings are noted.

6. Calculate output & efficiency for each reading.

7. Note down all the readings in the tabular form carefully.

8. Remove the load slowly and keep the rheostat as starting position and switch ‘OFF’ the supply by using DPST switch.

Tabular Column:

result tabular column


The graph is drawn between
a) Output in Watts Vs Speed(N) in RPM
b) Output in Watts Vs Torque (T) in N-m
c) Output in Watts Vs Current (I) in A
d) Output in Watts Vs Efficiency (%η) By taking output in Watts on X axis and speed, Torque, current, Efficiency onY- axis .

Model Graph:

electrical characteristics curve


  • Initially 3-point starter should be kept at ‘OFF’ position and later it must be varied slowly and uniformly from ‘OFF’ to ‘ON’ position.
  • The field regulator must be kept at its minimum output position.
  • The brake drum of the motor should filled with cold water.
  • The motor should be started without load.


Viva Questions:

  1. Why a 3-point starter is used for starting a D.C shunt motor?
  2. If a 3-point starter is not available, how can a D.C motor be started?
  3. Explain the function of overload release coil in 3-point starter.


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