.
Bulletin 100AC-EX Experiment Manual for AC Motors
~-
EXPERIMENT NO. 2
EFFECT OF SPEED ON AN ALTERNATOR
PURPOSE:
TO discover the effect of speed on the terminal voltage and frequency of generated voltage.
BRIEFING:
The armature coils of an alternator are wound on the stator. There are no salient poles on
the stator; the conductors are imbeded into slots. The coils are arranged into distinct coil
groups. These coil groups are arranged on the stator to produce three-phase power. The coils
are known as A, B, and C.
Assume the magnetic field from the rotor is sweeping around. As it passes under Phase A
coil, Phase A voltage is at peak. 120 electrical degrees later, it passed under Phase B coil.
Then, 120 electrical degrees later, it passes under Phase C coil. If there is only one group of
three coils on the stator, a full cycle in completed during one revolution of the rotor.
The rotor has one pair of salient poles (north and south) for each coil group on the stator.
An alternator is known by the number of rotor poles. Thus, the lowest number is 2. We call
that a two-pole alternator. The Hampden SM-lOO-3A is a four pole machine. That means
that two cycles of AC are produced each time the rotor completes one revolution.
The frequency of the output voltage depends on the number of poles and the speed at which
the rotor is being driven.
Frequency (Hertz) = Speed (RPM) * ‘Greze * no. of pairs of poles
.
For example, a 2-pole (1 pair of poles) alternator driven at 3600 RPM has a frequency of
60Hz.
Speed affects terminal voltage also. The faster the rotor is driven, the faster the magnetic
flux lines get cut by the stationary armature conductors. Therefore, the larger the terminal
voltage for the same field current.
2-l
Upon successful completion of this experiment the student will be able to:
1. Vary the output voltage and frequency of an alternator by changing speed.
2. Compute alternator frequency, given the number of poles and speed.
DM-1OOA DC Machine operating as a motor
SM-100-3A Synchronous Machine operating as an alternator
O-125 volt variable DC, 5 amps
O-150 volt variable DC, 1 amp
O-300 volt AC voltmeter
O-1 amp DC ammeter
Frequency meter (if available)
ADDITIONAL MATERIAL REQUIRED
MGB-1OODG Bedplate
HT-1OOJ Tach-generator
step 1.
Place the two machines on the bedplate. Couple and clamp the machines securely.
Install guards.
step 2.
Connect the DC machine as a self-excited shunt motor, as shown in Figure 2-1. Do
not turn the power ON yet.
step 3.
Connect the DC excitation (150 volt) supply to the field coil of the alternator as
shown in Figure 2-1. Do not turn the power ON yet.
2-2
Bulletin 100AC-EX Experiment Manual for AC Motors
step 4
Have someone check your connections to be sure they are correct.
Step 5
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.
DC MOTOR
SHUNT
D_AMPER
ARMATURE
+’
I
I
0 - 1 2 5 V D C
SUPPLY
3s A L T E R N A T O R
’
/
IND.
START
I
-START
SW.
ARMATURE
2
,’
+
iACH
I
i
+
O - l SOVDC
Figure 2-1
Step 6.
Turn ON the main AC circuit breaker; the O-125V.DC circuit breaker and the
motor.
step 7.
Slowly increase the output of the O-125V.DC supply to 125 volts to start the motor.
Step 8.
Increase the speed to the motor by turning the field rheostat clockwise until speed
is 1800 RPM.
step 9.
Be sure the alternator’s switch is in the SYNC RUN position. Then turn ON the
0- 15OV.DC excitation supply.
Step 10. Adjust the output of the 0-150V.DC supply until the alternator is generating 120
volts.
Step 11. Adjust the motor’s field rheostat until the motor is running at 2000 RPM.
Step 12. Record the terminal voltage in TABLE 2- 1.
Step 13. Repeat Steps 11 and 12 for 1600 RPM.
2-3
Step 14. Turn OFF all circuit breaker switches. Disconnect all leads.
TEST RESULTS:
SPEED
I
TERMINAL VOLTS
I
1800
I
120
I
2ooo
I
I .
I
1600
,I
TABLE 2-1
DE-BRIEFING:
1.
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 1800 RPM. (The SM-100-3A is a 4 pole machine).
2.
Compute the frequency at 2000 RPM.
3.
Compute the frequency at 1600 RPM.
4.
What relationship did you observe between terminal voltage and speed?
5.
Is it better to energize the alternator’s field before or after it reaches full speed?
What led you to this conclusion?
QUICK QUIZ:
1.
When speed increases, the frequency:
a. Increases.
b. Decreases.
c. Remains the same.
2-4
Bulletin 100AC-EX X
Experiment Manual for AC Motors
2
l
When speed increases ,terminal voltage:
a. Increases.
b. Decreases.
c. F&mains the same.
3.
To obtain a higher terminal voltage at the same frequency, you should increase:
a. Speed.
b. Armature Resistance.
c. Field Current.
4.
The terminal voltage of an alternator depends on:
a. Direction of Rotation.
b. Direction of Field Current.
c. Speed and Field Current.
5.
In three-phase power, each phase voltage lags behind the one in front of it by:
a. 30 Electrical Degrees.
b. 120 Electrical Degrees.
c. 360 Electrical Degrees.