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 PSC be the reading of the wattmeter, ISC be the reading of the ammeter and VSC be the reading of the voltmeter.
Figure 1.37 Short Circuit Test
We have
and
∴