Bulletin 100AC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 20
SYNCHRONOUS MOTOR V-CURVES
PURPOSE:
To discover the effect that changing the excitation current of a synchronous motor has on its
stator current.
Synchronous motors have the unique ability to run at different power factors.
Motors actually require electric power for two reasons. The first is to supply power current
that gets converted to mechanical power for the load and rotational losses. The second kind of
electrical input to a motor is the excitation current. Excitation current stores energy in the
magnetic field and releases it back to the source. Excitation current, which does no actual
work, is ninety degrees out of phase with power current.
Induction motors must draw both the power current and the excitation current from the AC
lines. That’
s why typical induction motors operate with 0.8 lagging power factor.
Synchronous motors, on the other hand, have a separate source of excitation current. If you
wanted to supply less than normal excitation current to the DC field coil on the rotor, a
synchronous motor would run at 0.8 lagging power factor, the same as induction motors. This
is seldom done, however.
Instead the excitation current is increased to point where it magnetizes the rotor, stator, and
air gap so that no excitation current is taken frome th AC lines at all. The entir
e stator
current, therefore, is converted to mechanical power. The synchronous motor has a unity
power factor.
Excitation current can, however, be increased above normal. Now, not only does the motor
not take any excitation current from the AC lines, it actually supplied excitation current to the
AC lines. Typical synchronous motors can run at 0.8 P.F., leading.
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Demonstrate change in synchronous motor power factor.
2. Explain the effect of excitation current in terms of V-curves.
20-1
Bulletin 100AC-EX Experiment Manual for AC Motors
SM-100-3A Synchronous Machine
DYN-lOOA-DM Dynamometer
POWER REQUIRED:
Fixed 3$ AC Supply
0 - 150 volt Variable DC, 1 amp
0 - 125 volt Variable DC, 5 amps
METERS REQUIRED:
two (2) 0 - 150 volt DC Voltmeter
0 - 1 amp DC Ammeter
0 - 2.5 amp DC Ammeter
0 - 300 volt AC Voltmeter
0 - 2 amp AC Ammeter
ADDITIONAL MATERIAL REQUIRED:
MGB-lOO-DG Bedplate
RL-1OOA Resistance Load
PROGRAM PLAN:
step 1.
Place the two machines on the bedplate: motor on the left, dynamometer on the
right. Couple and clamp the machines securely. Install guards.
step 2.
Connect the Synchronous motor as shown in Figure 20-l. Note that the stator is
wye-connected.
step 3.
Connect the dynamometer as shown in Figure 20-l. Note that this is a separatelyexcited shunt generator connection. Be sure that all of the load switches on the
RL-1OOA are in the downward (OFF) position.
step 4.
Have someone check your connections to be sure they are correct. Adjust the field
rheostat on the dynamometer to its maximum resistance position, fully clockwise.
20-2
Bulletin lOOAC-EX Experiment Manual for AC Motors
S Y N C H R O N O U S M O T OR
-
IND.
START
INPUT
A
A
0-125V D C S”;PLY
DC GENERATOR
4+
RHEO.
0-150V DC
SUPPLY
SHUNT
Figure 20-1
step 5.
With the motor circuit breaker OFF, turn ON the main AC and the 0 - 125 VDC
excitation supply.
Step 6.
Put the motor switch in the SYNC RUN position and adjust the excitation supply
until 1.0 amps flow in the field coil. Then return the switch to the IND START
position.
step 7.
Turn ON the motor circuit breaker. The motor should now be running as an induction motor.
step 8.
Move the motor’s toggle switch to the SYNC RUN position. The motor should now
be synchronized with the stator’s revolving field.
Step 9.
With 1.0 amperes flowing in the DC field, record stator current in TABLE 20-l.
20-3
Bulletin 100AC-EX Experiment Manual for AC Motors
step 10. Reduce the value of rotor current by 0.1 amps. Read and record stator current in
TABLE 20-l.
step 11. Continue to repeat Step 10 until the motor pulls out of synchronism.
step 12. When the motor pulls out of synchronism, first switch to IND START, then increase rotor current and m-synchronize.
step 13. Turn ON the 0 - 150 volt DC supply and adjust its output to 115 volts.
step 14. Adjust the dynamometer field rheostat until its terminal voltage is 120 volts.
step 15. Turn ON load steps 1 through 4 on the RL-1OOA.
Step 16. Repeat step 14.
step 17. Repeat Steps 9, 10, 11, 12, and 14 for TABLE 20-2.
step 18. Turn ON load steps 1 through 8 on the RL-1OOA.
Step 19. Repeat Steps 9, 10, 11, 12, and 14 for TABLE 20-3.
step 20. Turn OFF all circuit breakers. Disconnect all leads.
TEST RESULTS:
r
’ FIELD AMPS
1.0
0.9
0.8
0.7
0.6
0.4
0.5
0.3
0.2
j STATOR AMPS
0.1
I
TABLE 20-1 - NO LOAD
I
I FIELD AMPS
1.0
1
0.9
0.7
0.8
0.6
0.4
0.5
0.2
0.3
STATOR AMPS
TABLE 20-2 - HALF LOAD
t
FIELD AMPS
1.0
0.9
1
0.8
0.7
STATOR AMPS
TABLE 20-3 - FULL LOAD
20-4
0.6
0.5
0.4
1
0.1
Bulletin 100AC-EX Experiment Manual fo AC Motors
r
1.
From the data you recorded in TABLE 20-1, plot a curve on the graph provided showing how stator current changes as the field excitation current changes with no load on
the motor. Label this curve NO LOAD.
2.
From the data you recorded in TABLE 20-2, plot a curve on the same graph showing
how stator current changes with field excitation at half load. Label this curve HALF
LOAD.
3.
From the data you recorded in TABLE 20-3, plot a curve on the same graph showing
how stator current changes with field excitation at full load. Label this curve FULL
LOAD.
4.
Connect with a dotted line the lowest point of the three curves. Label this line UNITY
POWER FACTOR.
5.
On the left side of the unity power factor line, label the area LAGGING POWER
FACTOR. On the right side of the line, label the area LEADING POWER FACTOR.
QUICK QUIZ:
1
l
As excitation current was decreased from 1.0 amps to pull-out, the stator current:
a) Went up then down.
b) Went down then up.
c) Remained the same.
2.
At the lowest point on the stator current curve:
a) Current is leading voltage.
b) Current is lagging voltage.
c) Current is in-phase with voltage.
3.
Normal excitation is when the synchronous motor has:
a) Unity power factor.
b) Leading power factor.
c) Lagging power factor.
20-5
Bulletin 100AC-EX Experiment Manual for AC Motors
4
l
Under-excitation produces a:
a) Unity power factor.
b) Leading power factor.
c) Lagging power factor.
5.
Over-excitation produces a:
a) Unity power factor.
b) Leading power factor.
c) Lagging power factor.
FIELD AMPS
20-6