EE 280
Experiment #6 – Fall 1999
THREE-PHASE TRANSFORMER CONNECTIONS
The purpose of this experiment is to investigate the methods of changing a three-phase, three wire source to a three
phase, four-wire source.
Laboratory Equipment:
This lab will make use of:
1) Three single phase Hampden 1.0kVA transformers.
2) The resistive load cart
3) The TekMeter as an oscilloscope
4) Various instruments
Procedure:
A) Part 1 - Wye-Wye (Y-Y) Connection
1. Construct the circuit as per your Figure 1. Note that the source is assumed to be a three-wire source, and not
have a neutral connection available. Have your instructor examine your circuit.
2. Energize your circuit. Close switch SS, but leave switch SL open.
3. Sketch the load side neutral-to-line (channel 1) and line-to-line (channel 2) voltage waveforms. Also note the
phase difference between the two waveforms.
4. Record the load side line-to-neutral voltages VAN, VBN, and VCN, and the load side line-to-line voltages VAB, VBC,
and VCA. Also record a source side line-to-line voltage.
5. Close switch SL and load the transformer bank with six 150W resistors per phase.
6. Sketch the load side neutral-to-line (channel 1) and line-to-line (channel 2) voltage waveforms. Also note the
phase difference between the two waveforms.
7. Record the load side line-to-neutral voltages VAN, VBN, and VCN; the load side line-to-line voltages VAB, VBC, and
VCA; the load side line currents IA, IB, IC, and IN; and the power.
8. Use the clamp-on ammeter to measure the phase currents IA, IB, and IC, on the source side of the transformer.
9. Open the resistor switches on B and C phase, leaving only a line-to-neutral load on phase A.
10. Record the load side line-to-neutral voltages VAN, VBN, and VCN; the load side line-to-line voltages VAB, VBC, and
VCA; the load side line currents IA, IB, IC, and IN; and the power.
11. Use the clamp-on ammeter to measure the phase currents IA, IB, and IC, on the source side of the transformer.
12. Turn off the three-phase power, reconnect the six 150W resistors on phase B and C, and remove the three leads
connecting the source to the transformer.
B) Part 2 - Delta-Wye (∆
∆-Y) Connection
1. Reconnect the primaries of the transformers in delta, as per your Figure 2. Have your lab TA check the
connections. There is now no possibility of a neutral connection on the source side. Have your instructor
examine your circuit.
2. Energize your circuit. Close switch SS, but leave switch SL open.
3. Sketch the load side neutral-to-line (channel 1) and line-to-line (channel 2) voltage waveforms. Also note the
phase difference between the two waveforms.
4. Record the load side line-to-neutral voltages VAN, VBN, and VCN, and the load side line-to-line voltages VAB, VBC,
and VCA. Also record a source side line-to-line voltage.
5. Close switch SL and load the transformer bank with six 150W resistors per phase.
6. Sketch the load side neutral-to-line (channel 1) and line-to-line (channel 2) voltage waveforms. Also note the
phase difference between the two waveforms
7. Record the load side line-to-neutral voltages, VAN, VBN, and VCN; the load side line-to-line voltages VAB, VBC, and
VCA; the load side line currents IA, IB, IC, and IN; and the power.
8. Use the clamp-on ammeter to measure the transformer bank phase currents IAB, IBC, and ICA, on the source side
of the transformer.
9. Open the resistor switches on B and C phase, leaving only a line-to-neutral load on phase A.
10. Record the load side line-to-neutral voltages VAN, VBN, and VCN; the load side line-to-line voltages VAB, VBC, and
VCA; and the load-side line currents IA, IB, IC, and IN.
11. On the source side of the transformer use the clamp-on ammeter to measure the transformer bank phase
currents IAB, IBC, and ICA,. and the line currents IA, IB, and IC.
12. Turn off all the power and disassemble your circuit.
Report Questions
1. Discuss the unloaded Y-Y connection (parts A3 and A4). Are the line-to-neutral and line-to-line voltages
sinusoidal? Is the average line-to-line voltage √3 times the average line-to-neutral voltage. Comment on any
discrepancies.
2. Discuss the balanced loaded Y-Y connection (parts A6-A8). Are the line-to-line and line-to-neutral voltages
sinusoidal? Is the average line-to-line voltage √3 times the average line-to-neutral voltage? Draw a phasor
diagram showing the 3 line-to-line voltages, the line-to-neutral voltages, and the line currents. Does the power
equal √3 times the average line-to-line voltage times the average line current? Comment on any discrepancies.
3. Discuss the single-phase loaded Y-Y connection (parts A10 and A11). Draw a phasor diagram showing the lineto-line voltages, the line-to-neutral voltages, and the line currents.
4. Discuss the unloaded ∆-Y connection (parts B3 and B4) Are the line-to-neutral and line-to-line voltages
sinusoidal? Is the average line-to-line voltage √3 times the average line-to-neutral voltage? Is this any different
than the Y-Y connection? Explain.
5. Discuss the balanced loaded ∆-Y connection (parts B6-B8). Are the line-to-line and line-to-neutral voltages
sinusoidal? Is the average line-to-line voltage √3 times the average line-to-neutral voltage? Draw a phasor
diagram showing the 3 line-to-line voltages, the line-to-neutral voltages, and the line currents. Does the power
equal √3 times the average line-to-line voltage times the average line current? Comment on any discrepancies.
6. Discuss the single-phase loaded ∆-Y connection (parts B10 and B11). Draw a phasor diagram showing the load
side line-to-line voltages, the line-to-neutral voltages, and the line currents. Draw a phasor diagram showing the
source side line and transformer phase currents. Include a line-to-line voltage phasor as reference.