1. University of Saskatchewan 6-1
EE 392 Electrical Engineering Laboratory III
Three-Phase Transformers
Safety
The voltages used in this experiment are lethal. Assemble or modify a circuit only with
the breakers off. Do not apply power to a circuit until the wiring has been checked by an
instructor. Do not touch any node or component of a live circuit. Be careful when
moving near a circuit so that a wire is not accidentally snagged. Loads can become hot
during normal operation; do not touch or allow anything to fall into a load. If an
emergency occurs, the power for the entire laboratory can be disabled using the red
button on the power distribution panel.
Objectives: To determine the equivalent circuit, voltage regulation and efficiency of
transformers and to study non-linearity and harmonic generation of transformers. To
investigate three phase transformer connections using three single-phase transformers.
References:
Electric Machinery Fundamentals by Stephen J. Chapman
Electric Machinery by Fitzgerald, Kingsley and Umans
Procedure: Select three similar single-phase transformers and record the nameplate data
of the transformers.
1. Transformer Equivalent Circuit
i) Perform an open-circuit test on one of the single-phase transformers:
In the open-circuit test, connect the primary winding to the full rated AC voltage. Leave
the secondary winding open. Connect two voltmeters, an ammeter and a wattmeter as
shown in Figure 1. Record the readings of the voltmeters, the ammeter and the wattmeter.
ii) Perform a short-circuit test on the same transformer
Short-circuit the secondary winding, and connect an autotransformer at the primary
Fig. 1 Open-circuit test

2. University of Saskatchewan 6-2
EE 392 Electrical Engineering Laboratory III
Fig. 2 Short-circuit test
winding (Figure 2). Make sure that the autotransformer can handle the rated current of
the primary winding of the transformer under test. Keep the autotransformer output at its
lowest setting at the beginning. You need a fairly low voltage to set up the rated current in
the primary when the secondary is short-circuited. Adjust the input voltage to the primary
winding by adjusting the autotransformer setting until the current in the primary winding
is equal to its rated value. Record the voltmeter, ammeter and wattmeter readings.
iii) Utilizing the readings taken from the open-circuit and short-circuit tests, determine the
equivalent circuit of the single-phase transformer (Figure 3).
2. Load Test:
Connect a toaster box resistor (EE 579.0X) across the secondary winding of the transformer
as in Figure 4. Connect the primary winding to the full rated AC voltage. Adjust the
toaster box resistance such that the secondary current is 60% of its rated value. Measure
the toaster box resistance. Record the voltmeter, ammeter and wattmeter readings.
Utilizing the meter readings, determine the copper loss, the voltage regulation and the
efficiency of the transformer.
Fig. 3 Equivalent circuit of a transformer

3. University of Saskatchewan 6-3
EE 392 Electrical Engineering Laboratory III
Fig. 4 Load test
Compute the copper loss, voltage regulation and efficiency of the transformer from the
equivalent circuit determined in part 1 for a secondary current 60% of its rated value.
3. Determine the magnetization curve for the transformer:
Keep the secondary winding open. Connect a sampling resistor in series with the primary
winding. Supply full rated voltage to the primary winding. With the help of an
oscilloscope, record the wave shape of the input current (voltage drop across the sampling
resistor). Use two channels of the oscilloscope in differential mode.
Three-phase Connection of the Transformers
4. Using the three similar single-phase transformers (same ones that you used in parts 1,
2, and 3) make a three-phase connection. Connect the primary windings in delta and the
secondary windings in wye. Use three toaster box resistors (EE 579.0X) to make a balanced
resistive load. Connect the secondary side of the three-phase transformer to the balanced
resistive load. Connect the primary side to a balanced three-phase rated voltage supply.
Perform load tests at three current levels, 10%, 40% and 100% of the rated current by
varying the resistive load. Take appropriate voltage, current and power readings. From
the experimental data determine the voltage regulation and efficiency of the three-phase
transformer at the three load levels.
With the help of the equivalent circuit obtained in part 1, calculate the voltage regulation
and efficiency of the transformer at three current levels; 10%, 40% and 100% of the rated
value. Compare these results with the ones obtained from the experimental data.
5. Keep the secondary side open. Record the wave shape of the phase and line excitation
current for the following transformer connections:
(a) Δ/Y, (b) Δ/Δ, (c) Y/Δ and (d) Y/Y.
Comment on the wave shape of the phase and line currents.