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Bulletin 100AC-EX Experimen t Manual for AC Motors
EXPERIMENT NO. 4
WE-CONNECTED ALTERNATORS
PURPOSE:
To discover the effect of balance and unbalanced loads on wye-connected alternators.
BRIEFING:
When the armature coils of an alternator are connected in a wye configuration, each coil is in
series with one of the lines. Therefore, when a load is connected the phase current equals the
line current.
Assume we have a three-wire system with a balanced load, like that shown in Figure 4-1. We
can further assume that this is an equally balanced load. That means the current through
each of the lines is the same. The question is: What is going on at the junction point called
N?
We know from Kirchhoffs law that the total current entering a junction must equal the total
current leaving the junction. But, because the three currents are 120’out-of-phase with each
other, there is a constant shifting of value and direction. For example at one instant, phase A
current is zero while B and C are equal and opposite to each other. Current comes in through
B and leaves through C. At another instant A is maximum positive while B and C are 50%
negative. Current enters through A and leaves through B and C.
But suppose the load becomes unbalanced. The net current at point N still must be zero. A
change in current in one coil affects the current in the others, too. There is a way to beat
that. If we run a wire from the alternator’s neutral point to the load’s neutral point, any
unbalanced current can flow through this line. Then, if the load changes in one phase, it will
affect the current in that phase only; not the others.
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Discuss the importance of balanced loads.
2. Explain the function of the neutral wire.
4-1
Bulletin 100AC-EX Experiment Manual for AC Motors
DM-1OOA DC Machine operating as a motor
SM-100-3A Synchronous mahcine operating as a generator
REQUIRED:
O-125 volt variable DC, 5 amps
O-150 volt variable DC, 1 amp
O-150 volt AC Voltmeter
(4) O-O.5 Amp AC Ammeters
MGB-100 DG Bedplate
SLA-IOOD Strobe-Tachometer
RLC-100 Resistance/Reactance Load
RL-1OOA Resistance Load
PROGRAM PLAN:
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 4-1. Do
not turn the power on yet.
Step 3.
Connect the DC excitation (O-150 volt) supply to the alternator’s field and connect
the resistance load to the alternator’s armature coils as shown in Figure 4-l. Note
that the RL-1OOA is in parallel with one of the resistance legs of the RLC-100. Do
not turn the power ON yet.
step 4.
Have someone check your connections to be sure they are correct.
step 5.
Turn the motor’s field rheostat knob 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.
4-2
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Bulletin 100AC-EX Experiment Manual for AC Motors
Turn ON the main AC circuit breaker; the 0-125V DC circuit breaker and the
Step 6.
motor.
Slowly increase the output of the 0-125V DC supply to 125 volts to start the motor.
step 7.
SYNCHRONOUS ALTERNATOR
DC MOTOR
-
RLC-100
SYNC. RUN
IND.
START
-
+
O - 1 SOVDC
SUPPLY
Figure 4-1
Step 8.
Set the Strobe-tachometer to 1800 RPM and use the motor’s field rheostat to
achieve that speed.
step 9.
Be sure the alternator’s switch is in the SYNC run position and that all of the
toggle switches both the RL-1OOA and on the RLC-100 Load Bank are in the downward (OFF) position. Then turn ON the 0-15OV DC excitation supply.
step 10. Increase the output of the excitation supply until the alternator voltage from phase
to neutral is 120 volts.
step 11. Check the speed and, if necessary re-adjust the field rheostat so that the alternator
is being driven at 1800 RPM.
step 12. Disconnect the neutral line at Point N.
step 13. Turn ON three of the RLC-100 Resistance toggle switches. Each switch applies
2000 ohms of resistance simultaneously in each of the three legs. In other words, a
balanced load. Then repeat Step 11.
step 14. Measure the three line currents and the current in the neutral line. Record these
values in TABLE 4-1.
4-3
Step 15. Add load to one phase by turning ON three toggle switch- of the RL-lOOA.
Step 16. Repeat Steps 11 and 14.
Step 17. Connect the neutral line.
Step 18. Repeat Steps 11 and 14.
Step 19. Turn off all circuit breaker switches. Disconnect all leads.
BALANCED LOAD
UNBALANCED
NO NEUTRAL
NO NEUTRAL
UNBALANCED
WITH NEUTRAL
/ NEUTRALCURRENT 1
1 LINE 1 CURRENT
1
I
I
I
I
I
I
1 LINE2 CURRENT 1
1 LINE3 CURRENT 1
TABLE 4-1
1.
When a three phase load is perfectly balanced, the current through each armature
coil is the same. What should the current in the neutral line be under that circumstance?
why?
2.
From your observations, is there a need for a neutral line when the load is
balanced?
What led you to this conclusion?
4-4
Bulletin 100AC-EX Experiment Manual for AC Motors
3.
What is the main advantage of the neutral line between alternator and load if there is
a possibility of inbalance?
1.
According to Kirchhoff’s law, the current leaving a junction must be:
( a 1 Greater than the current entering the junction.
09 Less than the current entering the junction.
(c ) The same as the current entering the junction.
2.
With a balanced load connected, the current in the neutral line is:
( a)
LOW
09 Zero
l
(C ) High
3.
Without a neutral, an increase in load current in one phase:
(a) Increases the current in the other two phases.
(b) Decreases the current in the other two phases.
(c) Has little effect on the current in the other two phases.
4.
With a neutral, an increase in load current in one phase:
(a) Increases the current in the other two phases.
(b) Decreases the current in the other two phases.
(c) Has little effect on the current in the other two phases.
5I
In a wye connection; line current is:
(a) Larger then phase current.
(b) Smaller than phase current.
(c) The same as phase current.
4-5