The purpose of these experiments[1] was to measure the exciting current and determine the core losses in a transformer using the open circuit test (experiment # 2) and to determine the equivalent circuit of a transformer by using the short circuit method (experiment # 3).

Introduction:

In order to determine the equivalent circuit parameters of a transformer two simple tests are used, namely the open-circuit test (experiment #2) and the short-circuit test (experiment #3). By performing these simple tests the values for the equivalent circuit of a transformer (see Figures 1 &2). If it is desired to find the parameter of the exact equivalent circuit (see Figure 3) it is customary to assume R1=a2R2 and X1=a2X2. By making this assumption we can decompose the values of the equivalence resistance and reactance into the primary (winding 1 or high voltage side) and secondary (winding 2 or low-voltage side) components.

Figure 1 – Approximate Equivalent Circuit of a Transformer

Figure 2 – Approximate Equivalent Circuit of a Transformer

Figure 3 – Exact equivalent circuit of a Transformer in phasor form

Experiments # 2 & 3

[1]

During the open-circuit test, the transformer rated voltage is applied to the low voltage side of the transformer with the high-voltage side as shown in Figure 6. Since the high-voltage side is open, the input current Ioc is equal to the exciting current through the shunt excitation branch as shown in the equivalent circuit in Figure 4. Because this current is very small, about 5% of rated value, the voltage drop across the low-voltage winding and the winding copper losses are neglected. The magnitude of the admittance of the shunt excitation branch of the equivalent circuit referred to the low voltage side is calculated by using the following formulas:

1) admittance

|Yo2| = Ioc/Voc

2) phase angle of the admit

-θo2 = - cos-1(Poc/VocIoc)

3) complex admittance

Yo2 = |Yo2|/-θo2 = Gc2 – jBm2

The corresponding resistance and reactance parameters of Figure 2 are derived from the conductance and susceptance, as follows:

Rc2 = 1/Gc2

&

jXm2 = 1/-jBm2

Figure 4 – Equivalent Circuit for open-circuit test

During the short-circuit test, the low voltage side is short circuited and the high-voltage side is connected to a variable, low voltage source. Measurements of power, current, and voltage are made on the high voltage side as shown in Figure 7. Then, the applied voltage is adjusted until rated short-circuit current

flows in the windings. This voltage is generally much smaller than the rated voltage, in the range of .05 to .010 per unit. Therefore, the current through the magnetizing branch is negligible, and the applied voltage may be assumed to occur wholly as a voltage drop across the transformer series impedance as shown in the equivalent circuit of Figure 5.

Figure 5 - Equivalent Circuit for short-circuit test

Equipment:

Hampden Transformer (Model T-100-3A)

Hampden AC Wattmeter

Hampden Variable AC Power Supply

2 Fluke Digital Multimeters

Procedure[2]:

1.

2.

3.

Make the connections shown in Figure 6. Apply 240 volts to the primary side as shown. Measure the input current, voltage, and power and record the data.

Make the connections shown in Figure 7. Apply 120 volts to the primary side as shown. Measure the input current, voltage, and record the data.

Calculate the in-phase and lagging components of the exciting current for each step. The transformer is rated at 240/120 volts, 120VA for the connections made. Calculate the exciting currents’ percentage of the full load current rating. Calculate the magnetizing

Experiment # 2 procedure & results analysis

[2]

branch impedances.

Procedure[3]:

1.

2.

3.

Calculate the full load current for the Hampden Transformer

which is rated at 240/120 volt, 120 VA for the connection shown in Figure 8.

Make the connections shown in Figure 8 and BE SURE that...