SYNCHRONOUS GENERATOR OPERATION
ECE 454/554: Power Systems Laboratory
Contributors:
Dr. Tim A. Haskew
Objectives
 Be able to determine the equivalent circuit for a three-phase synchronous generator
from test data.
 Understand the operational characteristics of three-phase synchronous generators.
References
Electromechanical Energy Devices and Power Systems, Zia A. Yamayee and Juan L.
Bala, Jr., John Wiley and Sons, Inc., New York, New York, 1989.
- Sections 7.1-7.5
Pre-Lab Assignment
1) Open- and short-circuit test data for a three-phase, 2 pole, 60 Hz, Y-connected, 480 V,
300 kVA generator is provided below. The generator has negligible armature
resistance.
Field Amps (A)
OC Voltage (V)
SC Current (A)
0
0
0
1
125
71
2
250
143
3
350
214
4
425
286
5
480
357
6
520
429
7
540
500
8
560
571
9
580
643
Determine:
(a) The unsaturated synchronous reactance.
(b) The saturated synchronous reactance.
2) The generator in problem 1 has rotational losses of 24 kW. The generator supplies a
250 kVA load at a power factor of 0.8 lagging. Compute:
(a) The line current.
(b) The magnitude and phase angle of the back-emf.
(c) The power required by the prime mover.
(d) The torque required by the prime mover.
(e) The voltage regulation of the generator.
Equipment List
Quantity
1
1
1
1
1
1
1
1
1
1
1
1
1
Description
Benchtop Digital Multimeter
Three-Phase Synchronous Generator/Motor
DC Motor/Generator
Three-Phase Power Supply Module
Strobotach
Three-Phase Voltmeter
Oscilloscope
Phase Shift Indicator
Three-Phase Watt/VAR Meter
Three-Phase Resistive Load
Three-Phase Ammeter
Three-Phase Inductive Load
Three-Phase Capacitive Load
Number
Fluke 45
EMS 8241
EMS 8211
EMS 8821
---EMS 8426
---EMS 8909
EMS 8446
EMS 8311
EMS 8425
EMS 8321
EMS 8331
Procedure
1) Using a digital Ohmmeter (Fluke 45), measure the 3 line-to-line resistances of the
three-phase synchronous generator (EMS 8241). Record your measurements below,
and use them to compute the phase winding resistance.
Quantity
Rab
Rbc
Rca
Ra =
Value
Units




2) Connect the dc motor (EMS 8211) in a shunt configuration with the field rheostat in
the field circuit. Use the 8 A dc output on the power supply module (EMS 8821) to
power the motor. The synchronous generator (EMS 8241) should be belt driven by
the dc motor. Use the 2 A dc output of the power supply module for excitation of the
generator field winding. Note that this adjustment can be accomplished with
armature voltage and field current control on the dc motor. Using the strobotach,
adjust the dc motor speed to 1800 RPM. You can sense the speed with the phase-shift
indicator (EMS 8909), which should be installed on the shaft of the generator. Be
sure that the generator field current is at a minimum value by setting the generator
field rheostat to the maximum value. Connect the three-phase voltmeter (EMS 8426)
across the armature of the generator to measure the line-to-line voltage magnitudes.
With the system rotating at 1800 RPM, gradually increase the field current on the
synchronous generator in discrete steps. Record each value of field current and the
corresponding average line voltage magnitude. You should vary the field current up
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Synchronous Generator Operation
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to the rated value in about 10 equal increments. The setup is illustrated below. Note
that external field resistance may be required for the generator.
a
A2
dc
motor
synch.
gen.
A1
V(ac)
b
V(ac)
c
V(ac)
F1
F2
A(dc)
0-120 V
120 V
Field Current (A)
Vab (V)
Vbc (V)
Vca (V)
VL (V)
3) With the dc motor still serving as the prime mover, adjust the generator field rheostat
to the minimum setting (minimum field current). Place the three-phase ammeter
(EMS 8425) in series with the generator armature, and apply a three-phase short
circuit after the ammeters, as illustrated below. From the nameplate data of the
generator, compute the rated line current for the device. Record this value below.
Irated =
A
Bring the system up to speed at 1800 RPM. Then, gradually ramp up the generator
excitation in discrete steps while measuring the short-circuit current. Be careful not
to allow the armature current to exceed the rated value. Record the field current and
average short-circuit current in about 4 equal intervals.
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a
A2
dc
motor
synch.
gen.
A1
F1
A(ac)
b
c
A(ac)
A(ac)
F2
A(dc)
0-120 V
120 V
Field Current (A)
Ia (A)
Ib (A)
Ic (A)
IL (A)
With the results from steps 2 and 3, plot the occ and scc on the same graph. Using the
graphs, compute the unsaturated and saturated synchronous reactances for the
synchronous generator.
4) Adjust the field current on the generator (under no load) to provide rated output
voltage. Connect the a-phase line-to-neutral output to the oscilloscope. Vary the
speed of the generator by varying the armature voltage of the dc motor to achieve the
speeds indicated below. At each speed record the armature voltage frequency and the
peak line-to-neutral voltage magnitude. Plot the frequency vs. speed and the rms
magnitude of the open-circuit line voltage vs. speed.
Speed (RPM)
1800
1500
1200
900
f (Hz)
Peak LN Voltage (V)
RMS Line Voltage (V)
5) For this step, the generator is again to be operated at 1800 RPM. The phase shift
indicator (EMS 8909) should still be placed on the shaft of the generator to adjust
speed. Connect armature of the generator to a purely resistive load (EMS 8311) that
is Y-connected. The three-phase watt/VAR meter (EMS 8446) and the three-phase
voltmeter (EMS 8426) should be used to monitor the electrical output of the
generator. A dc ammeter should be used to monitor the generator field current. The
load resistance is to be varied as indicated in the table below. For each value of
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resistance, be sure to adjust the generator speed to 1800 RPM and the generator
terminal voltage to the rated value.
VL (V)
Load Resistance ()
1200
600
300
If (A)
P3 (W)
Using the measured load voltage and power with the equivalent circuit developed
above, compute the required field current. How does the calculation compare with
the measurement using both the unsaturated and saturated synchronous reactances of
the machine for all three load resistance?
6) Now, use a resistive load bank (EMS 8311) and an inductive load bank (EMS 8321)
to form a load of 300 + j300 . Operate the generator at 1800 RPM with rated
terminal voltage. Monitor the line-to-neutral voltage of the generator with the threephase ac voltmeter (EMS 8426) and the three-phase complex power output of the
generator with the three-phase watt/VAR meter (EMS 8446). Record your
measurements. Switch the load off, and record the new output line-to-neutral voltage
once the speed has been stabilized to 1800 RPM.
VLN (V)
120
Full-Load
P3 (W)
Q3 (VAR)
No-Load
VLN (V)
Use the appropriate equivalent circuit under full load to verify your measurements
treating the load voltage and complex power as knowns. Use the full- and no-load
voltage measurements to compute the voltage regulation.
7) Use a resistive load bank (EMS 8311) and a capacitive load bank (EMS 8331) to form
a load of 300 - j300 . Operate the generator at 1800 RPM with rated terminal
voltage. Monitor the line-to-neutral voltage of the generator with the three-phase ac
voltmeter (EMS 8426) and the three-phase complex power output of the generator
with the three-phase watt/VAR meter (EMS 8446). Record your measurements.
Switch the load off, and record the new output line-to-neutral voltage.
VLN (V)
120
Full-Load
P3 (W)
Q3 (VAR)
No-Load
VLN (V)
Use the appropriate equivalent circuit under full load to verify your measurements
treating the load voltage and complex power as knowns. Use the full- and no-load
voltage measurements to compute the voltage regulation.
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8) Connect the fixed 208/120 V output of the three-phase power supply (EMS 8821)
through the watt/VAR meter (EMS 8446) to the armature of the generator. Connect
the generator field to the fixed 120 V dc supply. Set the field rheostat at the nominal
mark, but leave the field switch open. Start the power supply.
The synchronous generator has a squirrel-cage imbedded in the rotor. Hence,
with the field winding open-circuited, the machine can be started as an
induction motor.
When the machine has reached steady-state speed, close the field switch. This will
synchronize the synchronous motor. Now, record the real and reactive power
absorbed by the synchronous motor at minimum and maximum field current.
Minimum Field Current
Maximum Field Current
P3 (W)
Q3 (VAR)
Questions
1) Why does a synchronous machine equivalent circuit based on the saturated
synchronous reactance not provide accurate results for light loading conditions?
2) Explain the frequency versus speed and voltage versus speed plots obtained in step 4.
3) Explain, in detail using the equivalent circuit, how the excitation of the synchronous
generator alters the direction of reactive power flow in step 8.
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