Open Circuit Test (O.C.Test)

To ascertain / determine the followings
1. Iron loss at desired voltage and frequency (Pi)

2. No load current (I0), finding out the percentage of full load current.
 Magnetising component of no load current, Im
 Loss component of no load current, Iw.
 No load power factor cos φ0 and phase angle φ0.

3. No load circuit parameters
 No load resistance (R0) –taking core loss into account
 Magnetising reactance Xm.

To carry out open circuit test it is the LV side of the transformer where rated voltage at
rated frequency is applied and HV side is left opened as shown in the circuit diagram. The
voltmeter, ammeter and the wattmeter readings are taken and suppose they are V

respectively. During this test, rated flux is produced in the core and the current drawn is
the no-load current which is quite small about 2 to 5% of the rated current. Therefore low
range ammeter and wattmeter current coil should be selected. Strictly speaking the
wattmeter will record the core loss as well as the LV winding copper loss. But the winding
copper loss is very small compared to the core loss as the flux in the core is rated. In fact this
approximation is built-in in the approximate equivalent circuit of the transformer referred
to primary side which is LV side in this case.





Since power factor at no load is very low. Therefore, for O. C. Test low power factor
wattmeter is preferred and wattmeter terminals M- & C are to be shorted for minimising
error due to connection of wattmeter.

However, due to non linear characteristics of the magnetisation curve the wave shape of the
magnetising current is nonsinusoidal.