Electrical Engineering Technology
SUNY College of Technology, Alfred, NY
ELET 4143 Electrical Machines and Controls
Spring Semester 2008
Lab 6. Load Characteristics of a Separately-Excited and a Self-Excited
Shunt Generators
Introduction:
A dc machine works through the interaction of its field and armature windings. When a prime mover is
coupled to a dc machine, the dc machine becomes a generator of electric power. The load to be powered is
connected across the armature windings. The field winding may be connected in a couple of ways.
The first is called self-excited. Here the field winding is connected in parallel with the load. The
generator supplies its own field winding. No external power is required.
The second field winding connection is called separately-excited. This configuration requires an
external dc power supply.
The prime mover for this laboratory will be a capacitor start single-phase ac induction motor.
Equipment:
Description
Manufacturer
Model Number
Serial Number
ac motor
ac power supply
dc generator
field winding dc
power supply
tachometer
dc voltmeter
dc ammeter
Set-up:
The set-up is similar to the previous experiment generating a B-H curve.
Procedure:
1. Connect the field winding for a self-excited generator.
2. Check that the variable dc supply is set to zero volts.
3. Connect an ac wattmeter to the ac supply for the prime mover.
4. Disconnect the load resistor so as to start at no-load conditions.
5. Turn on the ac power supply, ac motor, and dc machine.
6. Note the speed of the motor with the tachometer. Check this speed at each
measurement point. Ideally the speed will not change.
7. Adjust the field rheostat so that the output voltage is 60 Vdc.
8. Adjust the load resistance to increase the load current in 0.1 amp steps up to 2.5 amps.
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9. Record the load voltage for each load current step in an Excel table. Also record the
input power from the wattmeter.
10. Calculate the load power and overall efficiency for each load current step.
11. Now connect the field winding as a separately-excited generator. Adjust the field
winding excitation for a 60 volt output under no-load.
12. Adjust the load resistance to increase the load current in ¼ amp steps up to 2.5 amps.
13. Repeat the previous measurements.
14. Plot load regulation curves showing the change in load voltage with increasing load
current. This graph should have two curves, one for the self-excited and one for the
separately excited field winding.
15. Plot efficiency curves showing the efficiency with increasing load current. This graph
should have two curves, one for the self-excited and one for the separately excited field
winding.
16. In your conclusion, compare the two means of wiring the field excitation. Suggest
applications for each method.
As a complement to these instructions, the handouts provided by the Hampden machines
manufacturer are attached (Bulletin 250-DC-EX: Experiment 5 and Experiment 6).
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Bulletin 250-DC-EX Student Laboratory Experiments
EXPERIMENT NO. 5
LOAD CHARACTERISTICS OF A SELF-EXCITED
SHUNT GENERATOR
Purpose:
To study the load characteristics of a self-excited shunt generator at rated speed and
voltage and determine the generator’s internal (generated) characteristics.
Apparatus Required:
1
1
1
1
1
1
1
1
1
DCG-100 DC Generator (or DM-1.00A DC Machine as Generator)
DCM-l00 DC Motor (or DM-100A DC Machine as Motor)
HT-100J Tachometer
0 to 150 Volt DC Voltmeter
0 to 2.5 Amp DC Ammeter
0 to 0.5 Amp DC Ammeter
Resistance Load
Ohmmeter
0 to 125 Volt / 5 Amp DC Power Supply
Procedure:
1.- Couple the DC Generator to the DC Motor (or other prime mover capable of maintaining
a constant speed) and make the connections shown in Figure 5-1. Adjust the generator's
field rheostat to its maximum resistance, fully CW position. Adjust the motor's field rheostat
to its minimum resistance, fully CCW position.
2.- Have the instructor check your connections and then start the motor and adjust its
speed to 1800 RPM by slowly increasing the output of the 125 volt DC supply (if the DM100A is used as the motor, slowly increase the output of the 125 volt DC supply to 125 volts
and then use the motor's rheostat to bring the motor up to speed). Adjust the generator's
output to 60 volts DC (130 volts, if the DM-100A is used as the generator) by means of its
rheostat.
3.- Perform a load test on the generator from no-load to approximately 2.5 amperes load. It
will be necessary to adjust the 125 volt DC supply (motor's rheostat, if the DM-l00A is used
as the motor) after each load step is applied to maintain 1800 RPM. Record the armature
volts, load current, field current, and speed in Table 5-1.
4.- Measure the resistance of the armature circuit.
5.- Measure the resistance of the load.
Page 3
Bulletin 250-DC-EX Student Laboratory Experiments
Suggestions for conclusion:
Using the data in Table 5-1, plot the armature terminal voltage as ordinate versus the load
current as abscissa. Explain the shape of this curve.
SPEED
FIELD
AMPS
ARMATURE
VOLTS
ARMATURE
AMPS
0.05
0.1
0.15
0.2
Table 5-1
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Bulletin 250-DC-EX Student Laboratory Experiments
EXPERIMENT NO. 6
LOAD CHARACTERISTICS OF A SEPARATELY-EXCITED
SHUNT GENERATOR
Purpose:
To study the load characteristics of a separately-excited shunt generator at rated speed and
voltage.
Apparatus Required:
1
1
1
1
1
1
1
1
1
DCG-100 DC Generator (or DM-1.00A DC Machine as Generator)
DCM-l00 DC Motor (or DM-100A DC Machine as Motor)
HT-100J Tachometer
0 to 150 Volt DC Voltmeter
0 to 2.5 Amp DC Ammeter
0 to 0.5 Amp DC Ammeter
Resistance Load
0 to 125 Volt / 5 Amp DC Power Supply
0 to 150 Volt / 1 Amp DC Power Supply
Procedure:
1.- Couple the DC Generator to the DC Motor (or other prime mover capable of maintaining
a constant speed) and make the connections shown in Figure 6-1. Adjust the generator's
field rheostat to its maximum resistance, fully CW position. Adjust 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 and then start the motor and adjust its
speed to 1800 RPM by slowly increasing the output of the 125 volt DC supply (if the DM100A is used as the motor, slowly increase the output of the 125 volt DC supply to 125 volts
and then use the motor's rheostat to bring the motor up to speed). Adjust the generator's
output to 60 volts DC by means of its field rheostat.
3.- Perform a load test on the generator from no-load to approximately 2.5 amperes load. It
will be necessary to adjust the 125 volt DC supply (motor's rheostat, if the DM-l00A is used
as the motor) after each load step is applied to maintain 1800 RPM. Record the armature
volts, load current, field current, and speed in Table 6-1.
4.- Measure the resistance of the armature circuit.
5.- Measure the resistance of the load.
6.- Measure the resistance of the rheostat.
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Bulletin 250-DC-EX Student Laboratory Experiments
Suggestions for conclusion:
Using the data in Table 6-1, plot the armature terminal voltage as ordinate versus the load
current as abscissa. Compare this curve to the one obtained for the self-excited shunt
generator.
SPEED
FIELD
AMPS
ARMATURE
VOLTS
ARMATURE
AMPS
0.05
0.1
0.15
0.2
Table 6-1
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