Open Circuit & Short Circuit Test of Transformer


Open Circuit & Short Circuit Test

Open circuit & Short circuit test is also known as the Indirect Test. Open circuit & Short circuit test is performed on the transformer for the Iron loss, no-load current,no-load resistance & reactance and copper loss, impedance ,voltage regulation respectively. When construction of transformer is completed.  perform its electrical characteristics is necessary. By testing we cane perform it.

Performance of a transformer can be determine without any actual loading at any load and power factor by open and short circuit test. This test are very economical because power require is very less to perform this test.

Open Circuit Test :-

Open circuit test is also known as the No-load test of the x’mer. By performing this test we can determine following parameters are,

  • Iron loss.
  • No-load current.
  • No-load resistance & reactance.

open circuit test

Circuit diagram of open circuit est is shown in fig,its perform on single phase transformer. Fir this test primary winding has low no. of turn. hence, is supplied with the low voltage as voltage and frequency rating of the transformer. Secondary is left open for perform this test and it has high voltage winding.
For measuring the no-load current and no-load power, ammeter and watt-meter are connected in primary circuit. They both are low range measuring instruments. Supply voltage is measuring by voltmeter.Watt-meter reading indicate the iron loss.

Hence, loss is dependent on voltage and frequency. This test is performed on low voltage, because in laboratories obtain to high voltage is difficult. When voltage is applied to primary winding according to mutual induction flux will setup in core. And will some iron losses is occur. No-load primary current is very less. Hence, copper loss is negligible. Secondary is open, therefore no copper loss in secondary side.

No-load input power Wo =V I cosΦ (iron loss)
No-load power factor = Wo / VI
Energy component of No-load current (Ie) = I cosΦ
Magnetizing component of No-load current (Im) = I sinΦ
R = V/ Ie
X = V / Im

Short Circuit Test :-

By performing this test we can determine following parameters are,

  • Voltage regulation
  • Full load copper loss
  • Equivalent impedance

short circuit test

Fig shows the circuit of short circuit test,
In this test primary winding is high voltage and secondary is low voltage. For performing this test, low voltage winding is kept short circuited. Then applied voltage at primary winding is reduced with normal frequency. Applied reduces voltage is increased till full load current.

Applied voltage rating is very low, The iron losses will be very small, it can be neglected.Hence total loss occurring during this teat is copper loss of both windings. The losses of  both winding are indicated by the watt-meter.

Full-load copper loss = Wsc
Equivalent resistance = Wsc / Isc2
Equivalent impedance = Vsc / Isc
Equivalent reactance = X = √(Z2+R2)
Short circuit test is performed on the high voltage side with the low voltage winding short circuited.


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