Week 2
Practical Work
No-Load
1.3
Characteristic
of
Three-phase
Synchronous
Generator

Introduction
Un like DC generator, synchronous must be excited through their rotor in
order to induce a voltage in the stator. A synchronous must be driven at definite
constant speed because the frequency of the generator voltage is determined by
that rotor speed.
The amplitude of the generated voltage is proportional to the rotor speed and
to value of the DC flux. Before a synchronous is ready to deliver voltage to a
load:
a) It must be brought to synchronous speed.
b) It must be excited from a separate DC source
c) Its terminal voltage must be adjusted to the correct value with the field
rheostat.

Apparatus
1. One Dc machine
2. One synchronous machine
3. One power supply
4. One AC instrumentation group
5. One DC instrumentation group
6. One tachometer
1.3.1 Relationship between excitation current and output terminal voltage

Procedure
1. Couple the synchronous generator to Dc motor as prime mover.
2. Connect the dc motor as in figure 1-9
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Week 2
Practical Work
0-1A dc
A
Supply
0-250V
dc
F1
F2
S1
S2
A1
A2
L1
Tach
U1
L2
V1
0-1A dc
U2
A
V2
N
V
z
0-300V
AC
W2
+
Supply
0-250V
dc
_
W1
L3
Figure (1-9)
3. Connect the DC excitation supply to the field coil of the synchronous generator as
shown in Figure (1-9). Do not turn the power ON yet.
4. Turn the field rheostat knob on the motor fully counterclockwise to its minimum
resistance position. Turn the voltage control knobs of the two DC power supplies
fully counterclockwise to their zero output position.
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Week 2
Practical Work
5. Turn ON the main AC circuit breaker; and run the motor.
6. Slowly increase the output of the DC supply to the rated volts to start the motor. Be
sure the alternator's switch is in the SYNC RUN position. Then turn ON the DC
excitation supply.
7. Set the Tachometer for 1500 RPM. Use the motor's field rheostat to adjust motor
speed to 1500 RPM.
8. Adjust the output of the DC supply until the alternator's field current is
approximately 0.1 ampere, and take your readings there.
9. Record the exact value of field current and terminal voltage in TABLE 1-3. that the
voltage across each of the three armature coils is the same, since the being produced
by the same field.
10. Repeat steps 8 and 9 for the following values of alternator field current:
0.2 0.4,
and 0.5 amps.
11. Slowly decrease the excitation voltage until the ammeter reads approximate 0.4
amps. Record the exact value of field current and terminal voltage in TABLE 1-3
12. Repeat step 11 for the following approximate values of field current: 0.3, 0.2, 0.1
and 0 amperes.
13. Turn OFF all circuit breaker switches. Disconnect all leads.
1.3.2 Relationship between speed and output terminal voltage

Procedures
1. Couple the synchronous generator to Dc motor as prime mover.
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Week 2
Practical Work
2. Connect the dc motor as in figure 1-9 Do not turn the power ON yet.
3. Connect the DC excitation supply to the field coil of the alternator as shown in
figure 1-9. Do not turn the power ON yet.
4. Turn the field rheostat knob on the motor fully counterclockwise to its minimum
resistance position. Turn the voltage control knobs of the two DC power supplies
fully counterclockwise to their zero output positions.
5. Turn ON the main AC circuit breaker; DC circuit breaker and the motor.
6. Slowly increase the output of the DC excitation supply to the rated voltage to start
the motor.
7. Increase the speed to the motor by turning the field rheostat clockwise until speed is
1500 RPM.
8. Be sure the alternator's switch is in the SYNC RUN position.
Then turn ON the
DC excitation supply.
9. Adjust the output of the supply until the alternator is generating 240 volts.
10. Adjust the motor's field rheostat until the motor is running at 1400 RPM. Record the
terminal voltage in TABLE 1-4. Repeat Steps 8 and 9 for 1300, 1200 and 1000
RPM.
11. Turn OFF all circuit breaker switches. Disconnect all leads
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If
Vph
(Amp)
(Volts)
Week 2
Practical Work
Worksheet 02
Solve the following questions:
1. Connect the circuit as shown in figure 1-9, Operate,
record your results in Table 1-3 and plot the graph of
terminal voltage versus Field current (If VS Vph)
Table(1-3)
Graph 1-1 : the relationship (If VS Vph)
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Speed (N)
Vph
(RPM)
(Volts)
Week 2
Practical Work
2. Connect the circuit as shown in figure 1-9,
Operate,
record
your
results
in
table (1-4) and plot the graph of Terminal voltage
versus prime mover speed (N VS Vph)
Table(1-4)
Graph 1-2 : The relationship (N VS Vph)
3. What effect did saturation have on terminal voltage?
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Week 2
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4. From your observations would you regard residual magnetism in the rotor core an
important or unimportant factor?
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5. What led you to this conclusion?
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6. Using the equation f = S x P where f is the frequency in hertz; S is speed in revolutions
per second; and P is the number of pairs of poles, compute the frequency of the generated
voltage at 1500 RPM. (if 4 pole machine).
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7. Compute the frequency at 1400 RPM.
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8. Compute the frequency at 1300 RPM
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