Bulletin IOOAC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 21
POWER FACTOR CORRECTION USING SYNCHRONOUS MOTORS
PURPOSE:
To discover the effect that an over-excited synchronous motor has on system power factor.
BRIEFING:
Power factor, quite simply, is the ratio of power current to the total current.
An induction motor operates at less than unity power factor because it draws both power
current and excitation voltage from the AC power line. Factories that have a lot of induction
motors in use may have a low, lagging power factor.
Three main detrimental effects of a low, lagging power factor are:
1 . Low power factor cuts down system loadability. That is, it reduces the capacity of
the power system to carry kilowatts. The capacity of all apparatus is determined
by the KVA it can carry. Hence, larger generators, transmission lines, transformers, feeders and switches must be provided for each kilowatt of load when power
factor is low than when it is high. Thus, capital investment per kilowatt of load is
higher.
2 . Low power factor means more current per kilowatt. Hence, each kilowatt must
carry a higher burden of line losses, making it cost more to transport each kilowatt
of power.
3 . Low power factor may depress the voltage, reducing the output of practically all
electrical apparatus.
At low, lagging power factor also effects the following:
1 . w Reduces generator capacity and efficiency.
2 . Transformers. Increases the voltage drop across transformers so that voltage regulation of the transformer is impaired.
3
l
Makes larger distribution lines necessary and causes a greater
w
voltage drop in these lines.
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Bulletin 100AC-EX Experiment Manual for AC Motors
4. Power Cost A majority of power companies have penalties in their rates for low,
lagging power factor and incentives for high power factor. The customer thus benefits when he keeps the power factor of his plant high.
Power factor can be corrected with a capacitor. However, if a synchronous motor is run with
a leading power factor, it can perform useful work and correct power factor at the same time.
What this means is that the exciting current, instead of flowing back and forth from induction
motor to power company, flows back and forth between the induction and synchronous motors
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment the student will be able to:
1. Explain how synchronous motors improve power factor.
2. Connect a synchronous motor in a way that improves system power factor.
SM-100-3A Synchronous Machine
IM-100 Induction Motor
DM-1OOA DC Machine
POWER REQUIRED:
Fixed 3# AC Supply
0 - 150 volts, DC, 1 amp
0 - 125 volts DC, 5 amps
0 -
1 amp DC Ammeterr
0 - 300 volts AC Voltmeter
0 - 4 amp AC Ammeterr
Two (2) AC Wattmeters
0 - 2.5 amp D C A m m e t e r r
0 - 150 volt DC Voltmeter
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Bulletin 100AC-EX Experiment Manual for AC Motors
ADDITIONAL MATERIAL REQUIRED:
MGB-100-DG Bedplate
RL-lOOA Resistance Load
PROGRAM PLAN:
step 1.
Place the IM-100 Induction motor on the left side of the bedplate. Install guards
over shaft extensions and clamp the motor securely.
step 2.
Make the motor connections shown in Figure 21-l. Observe polarity when connecting wattmeters. The “two-wattmeter method” of measuring three-phase power will
be used. It may be necessary to reverse the polarity of one of the wattmeters and
subtract its reading from the other one.
I N D U C T I O N MOTOR
4A
,,/?a
600W
m
L 2 T T2
7.0
0
GENERA
RL-100
STATOR
QB
1
Tnm
UK
I
*2
RHEO.
150v +
-
RM
b’
1
f2
42
:I
V
SHUNT
FLD.
o - 1 5ov
DC S U P P L Y
15ov
VARIABLE
30 AC
INPUT
L3AfiA T3 u’
IND.
START
Y3
-
+
o-125v DC SUPPLY
Figure 21-1
Step 3.
Place the DM-1OOA DC machine on the right side of the bedplate. Couple it to the
induction motor securely and clamp it in place.
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Bulletin 100AC-EX Experiment Manualfor AC Motors
Step 4.
Connect the DM-1OOA as shown in Figure 21-1. Note that this is a separately excited shunt generator connection. Turn the field rheostat to its maximum resistance position, fully clockwise.
step 5.
Put the SM-lOO-3A synchronous motor in a place where there will be no danger of
coming in contact with the spinning shaft of the rotor.
Step 6.
Connect the synchronous motor as shown in Figure 21-l.
step 7.
Have someone check your connections to be sure they are correct.
step 8.
Make sure all motor circuit breakers and power supply circuit breakers are OFF.
Then turn ON the main AC circuit breaker.
step 9.
Turn the toggle switch on the synchronous motor to SYNC RUN. Then turn ON
the 0 -125 volt DC excitation supply and adjust the knob until 1.0 amps flow
through the field coil. Return the switch to IND START position.
step 10. Turn ON the induction motor.
step 11. Turn ON the 0 - 150 volt supply and adjust its output to 115 volts. Adjust the DC
machine’s field rheostat until the generator terminal voltage is 120 volts.
step 12. Apply a load to the generator by turning ON five steps on the RL-1OOA.
step 13. Use the generator’s field rheostat and/or the O-150 volt excitation supply to readjust the terminal voltage to 120 volts.
step 14. Read and record in TABLE 21-1 the motor volts, motor amps, and the total watts.
step 15. Turn ON synchronous motor and synchronize it with the line. If necessary readjust the excitation supply until 1.0 amps current flows.
Step 16. Read and record in TABLE 21-2 system volts, system amps and the total system
watts. If necessary, reverse the connections to the voltage coil of one wattmeter.
step 17. Turn OFF all circuit breakers. Disconnect all leads.
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Bulletin 100AC-EX Experiment Manual for AC Motors
TEST RESULTS:
1 VOLTS
AMPS
w #1
w #2
TOTAL WATTS
VA
P.F.
,
TABLE 2 1 - 1
r
1
1
VOLTS
1 W #1
AMPS
w #2
TOTAL WATTS
VA
P .F.
1
TABLE 21-2
DE-BRIEFING:
1.
Compute the total watts for the induction motor alone. Record in TABLE 21-1. Compute the total volt-amperes of apparent power from the equation:
VA = VOLTS x AMPS x 1.73
Record this value in TABLE 21-l.
2.
Compute the system power factor with only the induction motor line using the equation:
P.F. = WATTS/VOLT-AMPERES
Record this value in TABLE 21-1.
3.
Compute the total watts for the synchronous motor in parallel with the loaded inducRecord in TABLE 21-2. Compute the total volt-amperes of apparent
power for the system. Record in TABLE 21-2.
tion motor_
4.
Compute the system power factor with the synchronous motor helping to correct it.
Divide the watts by the volt-amperes. Record in TABLE 21-2.
5.
W a s there much improvement in power factor when the synchronous motor was put
into the circuit?
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Bulletin 100AC-EX Experiment Manual for AC Motors
Explain the change.
QUICK QUIZ:
1.
Induction motors typically have:
a) A high leading power factor.
b) A low leading power factor.
c) A low lagging power factor.
2.
Excitation current furnishes:
a) Total apparent power.
b) Real (true) active power.
c) Reactive (wattless) power.
3.
When the excitation current for an induction motor comes from the AC lines:
a) Excitation current makes up a large share of the total current.
b) Excitation current makes up a small share of the total current.
c) None of the total current is made up of excitation current.
4.
When some of the excitation current for an induction motor comes from a synchronous motor:
a) Excitation current makes up a large share of the total current.
b) Excitation current makes up a small share of total current.
c) None of the total current is made up of excitation current.
5.
Synchronous motors are usually run at:
a) Unity or lagging power factor.
b) Unity or leading power factor.
c) Lagging or leading power factor.
21-6