Which of the following losses can be observed in a transformer when it is NOT connected to any load?

Testing of the transformer

1.) Open circuit test

• This test is used to find the iron or core losses of the transformer.
• This test gives the value of core resistance (RW) and determines magnetizing resistance (Xm)
• In this test, the voltmeter, wattmeter, and ammeter are connected to the LV side of the transformer and the HV side is open-circuited.
• This test is performed at rated voltage and reduced current.
• As the secondary of the transformer is open, thus no-load current flows through the primary winding.
• The value of the no-load current is very small compared to the full-rated current. The copper loss occurs only on the primary winding of the transformer because the secondary winding is open. The reading of the wattmeter only represents the core and iron losses.

Wattmeter Reading \(=W_o\)

Voltmeter Reading \(=V_1\)

Ammeter Reading \(=I_o\)

\(I_c={V_1\over R_c}\)

\(I_m=\sqrt{I_o^2-I_c^2}\)

The value of magnetizing resistance is given by:

\(X_m={V_1\over I_m}\)

2.) Short circuit test

• This test is used to find the copper loss of the transformer.
• In this test, the voltmeter, wattmeter, and ammeter are connected to the HV side of the transformer and the LV side is short-circuited.
• This test is performed at rated current and reduced voltage.
• A low voltage of around 5-10% is applied to that HV side with the help of a variac. Now with the help of variac applied voltage is slowly increased until the wattmeter, and an ammeter gives a reading equal to the rated current of the HV side.

The short circuit current is given by:

\(I_{sc}={V_{sc}\over \sqrt{R^2_{sc}+X^2_{sc}}}\)

\(I_{sc}={V_{sc}\over \sqrt{R^2_{sc}+(2\pi fL)^2_{sc}}}\)

From the above expression, the short-circuit current is inversely proportional to the frequency.