EXPERIMENT NO. 3
WYE AND DELTA CONNECTIONS - VOLTAGE
PURPOSE:
To discover the relationship between line and phase voltage for wye and delta connections.
BRIEFING:
As the magnetic field sweeps around in an alternator, it generates a sine wave voltage in coil
A, a separate sine wave voltage in coil B, and a third sine wave voltage in coil C. Taken
separately, each of these is a single-phase voltage. However, since these coils are fixed on the
stator in a definite physical relationship, these three voltages have definite time relationship to
each other.
In AC circuits, we measure time in electrical degrees. The sine wave cycle of the voltage
induced in coil B is identical to that induced in coil A. However, it reaches its peak 120
electrical degrees later than coil A. In fact every part of the Phase B cycle lags Phase A cycle
by 120 degrees.
Similarly, Phase C cycle lags Phase B cycle by 120 electrical degrees. Then 120 electrical
degrees later you have Phase A again. Although the voltage being generated in each of the
coils reverses polarity twice each cycle, there is a definite relationship of coil polarities at every
instant. Coil polarity is actually included in phase relationship. If we reverse the connections
to the terminals of coil B, for example, its voltage would lead coil A voltage by 60” instead of
lagging at by 120°.
DELTA
A delta connection has the three coils connected series aiding (minus to plus). This connection is shown in Figure 3-1 of the PROGRAM PLAN. An examination of the diagram will
quickly show that the voltage across any two lines (line voltage) is the voltage generated in the
coil between them (phase voltage). Therefore, for a delta connection, line voltage equals phase
voltage,
WYE
A second way of connecting a three-phase alternator is the wye, sometimes called a star. The
wye connection is shown in Figure 3-2 of the PROGRAM PLAN. Note that the end of each
coil with the same polarity is joined at a common point, called N (neutral). Now you an obtain
3-1
the phase voltage between a line and neutral. For example, the voltage generated in coil A
(phase A voltage) appears between Ll and N.
The line voltage, on the other hand, as measured between any two lines, is the phasor sum of
the voltages generated in the two coils appearing between those lines. Between Ll to L2, for
example, you have coil A and coil B. Note further that they are series opposing (plus to plus).
The phasor solution is shown below:
‘NB
‘PH
‘AB
LINE VOLTAGE EQUALS
PHASOR SUM OF TWO
COIL VOLTAGES FOR
WYE C O N N E C T I O N
v
J
BN
The first thing we must do is reverse the phase B vector. The voltage between Ll and L2
(V AB) is the vector sum of VAN and VNB Since V BN lags V AN by 120 degrees, then VNB must
lead VAN by 60 degrees. To find the length of vector VAB, we can divide the phasor diagram
into two identical right triangles. Each has an angle of 30 degrees, a side adjacent equal to
half the line voltage, and a hypotenuse equal to the phase voltage.
The solution is as follows:
cos 3
= 0.866 = *#
therefore
VLINE
V P H x 0.866 = ~2
hINE =
2 x 0.866 x V P H
Vm= 1.73 V P H
To summarize, for a wye connection, line voltage equals 1.73 times the phase voltage.
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Bulletin 100AC-EX Experiment Manual for AC Motors
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Differentiate between wye and delta connections.
2. Compute either phase or line voltage, given the other one.
MACHINES REQUIRED
DM-lOOA DC Machine operating as a motor
SM-100-3A Synchronous Machine operating as an alternator
O-125 volt variable DC, 5 amps
O-150 volt variable DC, 1 amp
O-300 volt AC voltmeter
ADDITIONAL MATERIAL REQUIRED
MGB- 1 OODG Bedplate
SLA-IOOD Strobe Tachometer
Couple and clamp machines securely.
step 1.
Place the two machines on the bedplate.
Install guards.
step 2.
Connect the DC machine as a self-excited shunt motor, as shown in Figure 3-1. Do
not turn the power ON yet.
step 3.
Connect the alternator’s field to the DC excitation (150 volt) supply and its armature coils into a delta configuration a shown in Figure 3-1. Do not turn power ON
yet.
3-3
step 4.
Have someone check your connections to be sure they are correct.
step 5.
Turn the motor’s field rheostat fully counterclockwise to its minimum resistance
position. Turn the knobs of the DC power supplies fully counterclockwise to their
zero output position.
VD
4,596
Figure 3-1
Figure 3-2
Step 6.
Turn ON the main AC circuit breaker; the O-125VDC circuit breaker; and the
motor.
Step 7.
Slowly increase the output of the O-125V.DC supply to 125 volts to start the motor.
Step 8.
Set the Strobe-Tachometer for 1800 RPM. Use the motor’s field rheostat to adjust
the motor speed to 1800 RPM.
step 9.
Be sure the alternator’s switch is in the SYNC RUN position. Then turn on the
0- 150V.DC excitation supply.
Step 10. With the voltmeter connected to terminals 3 and 6, increase the field excitation
until the phase voltage is 100 volts. You should get the same reading between
terminals 2 and 5 and between terminals 1 and 4.
Step 11. Read the line voltage between terminals Tl and T2. Record this value in TABLE
3-1. You should get the same reading between terminals T2 and T3 and between
terminals T3 and T1.
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Bulletin 100AC-EX Experiment Manual for AC Motors
step 12. Increase the field excitation until the phase voltage is 120 volts.
step 13. Repeat step 11.
Step 14. Repeat Steps 12 and 13 for 140 volts.
step 15. Turn OFF aI1 circuit breakers. Disconnect the leads from the alternator’s armature
only.
Step 16. Connect the armature coiIs of the alternator in a wye configuration as shown in
Figure 3-2.
step 17. Repeat Steps 4, 5, 6, 7, 8. 9, 10, 11, 12, 13, and 14.
step 18. Turn OFF all circuit breakers. Disconnect all leads.
DELTA CONFIGURATION
r
,
I
PHASE
VOLTAGE
LINE VOLTAGE
MEASURED
COMPUTED
100
120
140
WYE CONFIGURATION
100
120
140
TABLE 3-1
DE-BRIEFING:
1.
Compute the expected line voltage for each of the phase voltages given in TABLE 3-l
for the delta-connected alternator. Enter your results in TABLE 3-1.
2.
Compute the expected line voltage for each of the phase voltages given in TABLE 3-l
for the wye-connected alternator. Enter your results in TABLE 3-l.
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2.
Compute the expected line voltage for each of the phase voltages given in TABLE
3-1 for the wye-connected alternator. Enter your results in TABLE 3-1.
3.
Did your measured line voltage agree with the computed values?
4.
From the standpoint of the number of voltages available, what is the advantage of
the wye connection?
5..
If you obtained 208 line volts from a delta connected alternator, how much voltage
would the armature coil insulation have to withstand?
How much, if the alternator is wye connected?
QUICK QUIZ:
1.
In a delta connection, line voltage is:
a.
Greater than phase voltage.
b. Less than phase voltage.
c. The same as phase voltage.
2.
In a wye connection, phase voltage is:
a. Greater than line voltage.
b. Less than line voltage.
c. The same as line voltage.
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Bulletin 100AC-EX Experiment Manual for AC Motors
3.
In a delta connection, phase voltage is:
a.
Greater than line voltage.
b. Less than line voltage.
c. The same as line voltage.
4.
In a wye connection, line voltage is:
a.
Greater than phase voltage.
b. Less than phase voltage.
c.
5.
The same as phase voltage.
Standard output voltages of the SM-100-3A as a wye-connected alternator are:
a.
120 volts phase to neutral; 208 volts line to line.
b.
120 volts line to line; 208 volts line to neutral.
c.
120 volts phase to neutral; 120 volts line to line.
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