SHORT CIRCUIT OR IMPEDANCE TEST

The main purpose of this test is to compute the following:

• Equivalent impedance of the transformer referred to primary and secondary.
• Cu loss of the transformer at any desired load.
• Total voltage drop of the transformer referred to primary and secondary.

Figure 1.37 shows the schematic diagram for the short circuit test of a transformer in which the low-voltage winding is short-circuited. During the short circuit test, we apply 5–10 per cent of the rated voltage to high-voltage side so that the full-load current flow both in primary and secondary. Voltage is slowly increased from zero to a value to get full-load current to flow. Since 5–10 per cent of rated voltage at the primary is easier to achieve smoothly and read by a voltmeter, instruments are always placed on the high-voltage side. A voltmeter, an ammeter and a wattmeter are placed on the high-voltage side. The low-voltage side is directly short-circuited by a thick conductor or by an ammeter.

Let P_SC be the reading of the wattmeter, I_SC be the reading of the ammeter and V_SC be the reading of the voltmeter.

Figure 1.37 Short Circuit Test

We have

and

∴