.
Bulletin 100AC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 7
PARALLELING ALTERNATORS
PURPOSE:
To learn the technique
s of bringing an alternator on-line and having it assume a share of the
load.
Power companies usually have two or more alternators at each generating station. If both
their
voltages,”,frequencies, and phases are identical. Additionally,
alternators are
“online
there are usually a number of generating stations in any power system The stations are also
interlocked. Then, a number of systems are tied together in a network, or power grid. This
provides what is called the
“infinite bus.
”
When any alternator si sbrought on line, its voltage, frequency, and phase must match those
of the infinite bus Once on line, it. is locked-in to the bus and can pick up its share of the
load. Just how much load is handled by each of the alternators is controlled by computers.
To bring an alternator on line and parallel it with those already on line, you first must have
it spinning at the proper speed. This produces the proper frequency.
Second, you must provide the proper excitation. That produces the proper voltage level.
Third, you must be sure the phase sequence of the new alternator matches that of the on-line
alternators.
Fortunately, there is a simple device to help with phasing and frequency. It consists of three
lamps, one in each phase, which operates from the difference in voltage. When the two voltages have the same phase sequence and frequency, there is no difference between the voltages
at any point in any of the cycles. At that point, all lamps are dark.
If one frequency is greater, the lamps flash together at a rate equal to the difference in
frequency If either
.
phase sequence is reversed, the lamps do not go bright and dark together,
but flash alternately.
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Bulletin 100AC-EX Experiment Manual for AC Motors
In the first part of this experiment you will parallel the alternator with the voltage distribution system in your lab. In the second part, you will parallel two alternators.
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Explain the conditions necessa ry to parallel alternators.
2. Demonstrate the proper procedure for bringing an additional alternator on line.
ITH DISTRI:
DM-1OOA DC Machine operating as a motor
SM-100-3A Synchronous Machine operating as an alternator
POWER REQUIRED: .
O-125 volt variable DC, 5 amps.
O-150 volt variable DC, 1 amp
Fixed 3@ AC Supply
(2) 0 - 300 volt AC Voltmeter
0 - 0.5 amp AC ammeter
0 - 300 watt AC wattmeter (300V - 2A)
0 - 1.0 amp DC ammeter
.
ADDITIONAL MATERIAL REQUIRED:
MGB-1OODG Bedplate
SLA-1OOD Strobe Tachometer
SYN-100 Phasing Lamps
PROGRAM PLAN:
step 1.
Place the two machines on the bedplate. Couple and clamp the machine securely.
Install guards.
7-2
Bulletin 100AC-EX Experiment Manual for AC Motors
step 2.
Turn on the main AC circuit breaker and measure the line voltage. Write it down
for future reference. Then turn power OFF.
step 3.
Make the connections shown in Figure 7-l. Note that the DC machine is connected
as a self-excited shunt motor. The wye-connected alternator is connected directly
to one side of the SYN-100.
DC MOTOR
THREE PHASE ALTERNATOR
SHUNT
DAMPER
ib-0-5
I
1
RHEO.
456
0 T O 125 ’
vet
1
SUPPLY 1
I
I
-m
START
SW.
1
I
1
I
IND.
L
3
I
1
L
+
P L:
A
+
1A
-
+
0 T3 150 VDC S U P P L Y
B
SYN-100
30
INPUT
Figure 7-1
step 4.
Have someone check your connections to be sure they are correct. Be sure the
switch on the SYN-100 is in the OPEN position.
step 5.
Turn on the main AC circuit breaker.
Step 6.
Turn the voltage control knob of the 125V DC supply fully counterclockwise to its
zero output position. Then turn on the supply’s and the motor’s circuit breakers.
7-3
Bulletin 100AC-EX Experiment Manual for AC Motors
step 7.
Slowly increase the output of the 125V DC supply to start the motor.
step 8.
Set the strobe-tachometer to 1800 RPM (1500 RPM for 50Hz systems) and use the
motor’s field rheostat to achieve that speed.
step 9.
Turn the voltage control knob on the excitation supply fully counterclockwise
zero output position. Then turn on the supply.
to its
step 10. Slowly increase the field excitation until the terminal voltage of the alternator
reads exactly the same 5 the line voltage measured in Step 2.
step 11. At this point the lights on the SYN-100 should be flashing OFF and ON together.
If they are flashing alternately turn the excitation supply to zero, interchange any
two leads from the alternator to the SYN-100. Then repeat Step 10.
Step 12. When the lights are bright, the two voltages are 180” out of phase. When they are
out, the two voltages are exactly in phase. Adjust the speed of the motor until the
lights are out.
step 13. Push the toggle switch of the SYN-100 to CLOSE. Your alternator is now in parallel with the supply and can supply power to it. Its speed and terminal voltage are
locked in and cannot be changed independently.
step 14. Read the current and power. If the wattmeter reads downscale, reverse the polarity of the voltage coil (It is recommended that the power be turned off before
switching the leads). Record your readings in TABLE 7-1.
step 15. Normally, adjusting the DC motor’s field rheostat would change motor speed. Attempt to increase motor speed by turning the field rheostat knob clockwise about
45’. Read and record in TABLE 7-1 the alternator’s tirminal voltage, speed, current, and power. Return the knob to its original position.
Step 16. Repeat Step 15 for a counterclockwise rotation of about 45’. Use a minus (-) sign if
power is being supplied to the alternator.
Step 17. Normally, adjusting the alternator’s field excitation would change terminal voltage.
Attempt to increase the voltage by increasing excitation current by 0.1 ampere.
Read and record in TABLE 7-1 the terminal voltage, speed, current, and power.
Return to the original excitation current value.
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Bulletin 100AC-EX Experiment Manual for AC Motors
Step 18. Repeat Step 17 for a reduction of 0.1 amps in excitation current. Use a minus (-)
sign if power is being supplied to the alternator.
step 19. Turn the voltage control knob of the O-125 volt supply fully counterclockwise to its
zero output position. There is no voltage being applied to the motor. Make a note
of what happens for the DE-BRIEFING.
step 20. Turn OFF all circuit breaker switches. Disconnect all leads.
MACHINES REQUIRED:
2- DM-1OOA DC Machines operating as motors.
2 . SM-100-3A Synchronous Machines operating as alternators.
POWER REQUIRED:
2 - 0 to 125 volt variable DC supplies, 5 amp.
2 - 0 to0 150 volt variable DC supplies, 1 amp.
METERS REQUIRED:
2 - 0 to 300 volt AC Voltmeters.
2 - 0 to 0.5 amp AC ammeters.
ADDITIONAL MATERIAL REQUIRED:
MGB-1OODG Bedplates
SLA-1OOD Strobe Tachometer
SYN-100 Phasing Lamp
RLC-100 Resistance/Reactance Load
PROGRAM PLAN:
step 1.
Place a motor and an alternator on each bedplate. Couple and clamp securely.
Install guards.
step 2.
Make the connections shown in Figure 7-2. Have someone check your connection
to be sure they are correct.
7-5
DC MOTOR
+’
I
I
0
TO
125
SHUN
THREE PHASE ALTERNATOR
_ DAMPER
RHEO.
VDC i
SUPPLY
I
2
-L
START
SW.
/
ARMATURE
/
/
I
TACH
0 TO 150 VDC S U P P L Y
START
SW.
ALTERNATOR
NO.2
0 TO 150 VDC S U P P L Y
Figure 7-2
step 3.
Turn the field rheostat knob on each motor fully counterclockwise to their mini-
mum resistance position. Turn the overload switch on each alternator OFF. Be
sure the alternator switch is in the SYNC RUN position. All of the resistance
toggle switches on the RX-100 Load Bank should be downward (OFF). Be sure
the switch on the SYN-100 is in the OPEN position.
step 4.
Call your two M-G sets No. 1 and No. 2. Start motor #l as follows: Turn the
voltage control knob on the 0-125V DC supply fully counterclockwise to zero output. Turn on the main AC, the 0-125V DC, and the motor circuit breakers. Turn
the voltage control knob fully clockwise to 125 volts.
step 5.
With the SLA-1OOD directed at its shaft, use the motor’s field rheostat to increase
its speed to 1800 RPM.
7-6
Step 6.
Turn the O-150 volt supply to zero then turn on its circuit breakers. Slowly increase the excitation until Alternator No. 1 is generating 208V AC.
step 7.
Turn on the circuit breaker switch of Alternator No. 1 and apply a resistive load
until the ammeter reads aproximately 0.25 amps.
step 8.
If necessary, readjust speed to maintain 1800 RPM by using the motor’s field
rheostat. Also readjust the excitation to maintain 208 volts output.
step 9.
Repeat Steps 4, 5, and 6 for Motor No. 2 and Alternator No. 2. Turn on the circuit
breaker switch on Alternator No. 2.
Step 10. The phasing lamps on the SYN-100 should now be flashing on and off together. If
they flash alternately, the phase rotation of the two alternators are not identical.
To correct this condition, simply interchange any two leads from the output of
Alternator No. 2.
Step 11. With the lamps flashing together, adjust the speed of Alternator No. 2 until the
flashing stops. If all lamps are lit, the voltages are 180° out of phase. Speed should
be adjusted until all lamps are out.
Step 12. Check to be sure the terminal voltage of both alternators is 208 volts. Then parallel the alternators by closing the switch on the SYN-100. Alternator No. 2 is now
floating on the line.
Step 13. Adjust the field rheostat of Motor No. 2 until Alternator No. 2 is carrying half of
the load.
Step 14. Reduce the speed of Alternator No. 1 with the Motor’s field rheostat until it is
floating on the line. Then turn OFF its circuit breaker switch.
Step 15. Reduce the load to zero. Turn off all circuit breaker switches. Disconnect all leads.
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Bulletin 100AC-EX Experiment Manual for AC Motors
4.
What four conditions must match in order to bring an alternator on line?
3.
4.
5.
What w a s happening when the wattmeter read in a negative direction and you had to
reverse the polarity of the voltmeter coil connection?
QUICK QUIZ:
1.
When an alternator is “floating’on the line, it is:
(a) Supplying a portion of the load current.
(b) Receiving AC power from the lines.
(c) Neither supplying nor receiving AC power.
2.
The speed of the prime mover determines:
(4 Frequency
(b) Phase Rotation
(c) Phase relationship with bus
3.
The field excitation of the alternator determines:
(a) Frequency
(b) Voltage
(c) Phase Rotation
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Bulletin 100AC-EX Experiment Manual for AC Motors
4.
The synchronizing (phasing) lamps operate from the difference between two voltages.
When they remain dark:
(a) Voltages are equal and 180’out of phase.
(b) Voltages are equal and in-phase.
(c) Voltages are unequal and out of phase.
5.
When an alternator is paralleled with the infinite bus, you cannot change:
(a) The load current it supplies.
(b) The power it supplies.
(c) The frequency of its output.
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