Lab Information -- Direct Current Machine Experiments
EXPERIMENT NO. 7
EFFICIENCY AND LOSSES OF A SHUNT GENERATOR
PURPOSE:
To determine the magnitude and sources of power losses in a DC generator and to calculate
efficiency.
APPARATUS REQUIRED:
1. One DM-100/A Machine as Generator
2. One DM-100/A Machine as Motor
3. One SCA-100 D Tachometer
4. Two 0 to 150 Volt DC Voltmeter
5. One 0 to 1 Amp DC Ammeter
6. One 0 to 150 Volt/1 Amp DC Power Supply
7. One 0 to .5 Amp DC Ammeter
8. One 0 to 125 Volt/5 Amp DC Power Supply
PROCEDURE:
1. Couple the DC motor to the DC generator and make the connection shown in Figure 1. Adjust
the generator's field rheostat to maximum resistance, fully CS4W position and the motor's field
rheostat to its minimum resistance, fully CCW position. Adjust the output of the 150 volt DC
supply to 125 volts.
2. Have the instructor check your connections.
3. Start the motor and adjust its speed to 1800 RPM by slowly increasing the output of the 125
voltage supply to 125 volts and then use the motor's rheostat to bring the motor up to speed.
Adjust the generator's voltage to 125 volts by means of its field rheostat.
4. Record the motor's volt and amps, and the generator volts and shunt field circuit volts and
amps.
5. Stop the motor and uncouple it from the generator. Restart the motor and adjust its speed back
to 1800 RPM. Record the motor volts and amps. The difference in the power inputs to the motor
in steps 3 and 4 equals the no load losses of the generator.
6. Compute the I2R losses of the armature for values of 25%, 50%, 75%, 100% and 125% of the
generator's rated current (2 amp for DM machine).
7. Compute the output power for each of the above values of load current. (125 volts is used in
this lab).
8. Compute the total copper losses (12R armature) losses plus stray power loss plus field losses.
9. Compute the efficiency for each value of load according to this formula:
% efficiency = Output Power
Output Power + Copper losses
FIGURE 7