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EXPERIMENT
NO. 4
WYE-CONNECTED
ALTERNATORS
PURPOSE:
To discover the effect of balance and unbalanced loads on wye-connected alterrrators.
BRIEFING:
When the amature coils of an alternator are connected in a rrye configuration, each coil ig in
series with one of the lines. Ther€fone, when a load is connect€d the phase cur:ent equals the
line curTent.
Assume we have a three-wire s5rstemwith a balanced load, Iike that shown in Figure 4-1. We
can further assume that this is an equally balanced load. That means the cunent through
each of the linee is the same. The question is: What is golng on at the junction point called
N?
We know fron Kirchhoffs law that the total cur:nent entering a junction must equal the total
current leaving the junction. But, because the thnee 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
curreut is zet! while B and C are equal and opposite to each other. Cunent oomes in through
B and leaves thmugh C. At another instant A is maximum positive while B and C are 50Vo
negative. Current enters through A and leaves through B and C.
But suppoee the load becomes unbalanced. The net current at point N still must be zero. A
change in cunent in one coil affects the cunrent in the others, too. There is a way to beat
that. If we nrn a wire from the altemator's neutral point to the load's neutral point, any
unbalanced cur"nentcan flow through this line. Then, if the load changes in one phase, it will
affect the curtent in that phase only; not the others.
PERFORMANCE
OBJECTIVES:
Upon succeasfulcompletion 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
MACHINESREQUIRED:
DM-100A DC Machine operating as a motor
SM-1OO-3ASynchronousmahcine operating as a generator
POWERREQUIRED:
0-125 volt variableDC, 5 amps
O-150volt variableDC, 1 amp
METERSREOUIRED:
0-150 volt AC Voluneter
(4) 0-0.5 Amp AC Ammeters
ADDITIONALMATERIAL REQUIRED:
MGB-IOODG Bedplate
SLA-I00D Strobe-Tachometer
RLC-IOOResistance/Reactance
Load
RL-IOOAResistanceLoad
PROGRAMPI.AN:
\
Step 1.
Place the two machines on the bedplate. Couple and clamp the machines securely.
IDstall 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 (0-150 volt) supply to the alternator's field and connect
the resistance load to the altemator's annature coils as shown in Figure 4-1. Note
that the RL-100A is in parallel with one of the rcsistance lep of the RLC-100. Do
not tum the power ON yet.
Step 4.
Have someonecheck your oonnectionsto be sure they are correct.
Step 5.
Turn the motor's field rheostat'knob fully counterclockwise to its minimum resistance position. T\rrn the voltage control knobs of the two DC power supplies fully
counterclockwise to their zero output positions.
4-2
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Step 6.
Turn ON the main AC circuit breaker; the 0-125V DC circuit breaker and the
motor.
Step 7.
Slowly increasethe output of the 0-125V DC supply tD L25 volts to start the motor.
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SYNCHRONOUS ALTIRNATOR
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Figure rt-l
Step 8.
Set the Strobe-tachometerto 1800 R.PM and use the motor's field rheostat to
achievethat speed.
Step 9.
Be sur€ the alternator'e switch is in the SYNC run position and that all of the
toggle switches both the RL-100A and on the RLC-100 l,oad Bank are in the
downward (OF.F)position. Then turn ON the 0-150V DC excitation supply.
Step10. Increase the output of the excitation supply until the alternator voltage from phase
to neutral is 120 volts.
Step11. Check the speed and, if necesslaryre-adjust the field rheostat so that the alternator
is being driven at 1800 RPM.
Step12. Disconnect the neutral line at Point N.
Step13. Turn ON three of the RLC-100 Resistance toggle switches. Each switch applies
2000 ohms of nesistancesimultaneously in each of the three legs. In other words, a
balancedload. Then repeat Step 11.
Step 14. Measure the three liue curtents and the curent 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 switches of the RL-100A.
Step 16. Repeat Stepe 11 and 14.
Step 17. Connect the neutral line.
Step 18. RepeatSteps 11 and 14.
Step 19. Turn off dl circuit breaker switches. Disconnect all leads.
TESTRESULTS:
BALANCEDLOAD
NO NEUTRAL
UNBAIANCED UNBAI.ANCED
NO NEUTRAL WITHNEUTRAL
NEUTRALCURRENT
LINE1 CURRENT
LINE2 CURRENT
LINE3 CURRENT
TABLE+1
DE-BRIEFING:
1.
When a thnee phase load is perfectly balanced, the cunent through each armature
coil is the same. What should the cument iu the neutral line be under that circumstance?
whv?
2.
From your observations, is there a need for a neutral line when the Ioad is
balanced?
What led you to fhis sonslusiel?
4-4
3.
What is the main advantage of the neutral line between dternator and load if there
is a possibility of inbalance?
OUICKQUIZ:
1.
According to Kirchhoffs law, the cur:rent leaving a junction must be:
(a) Greater thau the cument entering the junction.
(b) Less than the cument entering the junction.
(c) The same as the curnent entering the junction.
2.
With a balanced load connected. the cunent in the neutral line is:
(a) Low
(b) Zero
(c) Higb
3.
Without a neutral, an increase in load cument in one phase:
(a) Increases the sunent in the other two phases,
(b) Decreasesthe cur:nent 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 curnent in the other two phases.
(b) Decreasesthe curnent in the other two phases.
(c) Has little effect on the current in the other two phases.
5.
In a wye connectionl line current is:
(a) Larger then phase current.
(b) Smaller than phase current.
(c) The same as phase current.
4-5
ET332b
1.
DiscussionPoints for Lab 2
Define what Line-to-Line and phasevoltage are in a wye connection.Also define phaseand line
currents.Give mathematicalrelationships
betweenphaseand line quantities.
2, List andexplaintwo advantages
of usinga wye connectionin a three-phase
system.
3.
Explain why no current flows in the wye-connectedsystemneutral when the system is balanced.
Also explainwhat happensto the neutral current when the load becomesunbalanced.
4.
List and explainthree advantagesifusing a neutral connectionin a three-phasesystem.
Spring2002
dp-l ab 2.doc