Nov 21, 2013

Open Circuit Test or No Load Test for Transformer


The determination of this take a look at is to work out no-load loss or core loss and no-load Io that is beneficial to find the results of X0 and R0.

One winding of the transformer either is convenient however sometimes high voltage winding is left open and also the any is connected to its provide of usual voltage and frequency. A wattmeter-W, voltmeter-V and an ammeter-A are connected within the low-tension winding i.e. primary coil within the case. With traditional voltage applied to the first, usual flux are found out within the core, thus general iron losses can occur that is recorded by the watt-meter. Because the primary no-load current I0 (as measured by ammeter) is tiny (usually two to ten percent of rated load current), copper loss is negligibly tiny in primary and zero in secondary (it being open). Hence, the watt-meter reading represents much the core loss beneath no-load condition.

It ought to be noted that since I0 is itself terribly tiny, the pressure coils of the watt-meter and also the voltmeter are connected specified the present in them doesn't withstand the present coil of the watt-meter.

Every so often, a high-resistance voltmeter is connected across the secondary. The reading of the voltmeter provides the induced e.m.f. within the secondary coil. This helps to seek out transformation magnitude relation K.

The no-load vector diagram is shown below. If W is that the watt-meter reading in the above figure,

then,     W = V1I0cosø0
          cosø0 = W/(V1I0)
          X0=V1/ Iµ and R0=V1/IW

Or since the current is almost all-exciting current when a transformer is on no-load (i.e. Io=Iµ) and as the voltage drop in primary leakage impedance is small, hence the exciting admittance Yo of the transformer is given by I0=V1Y0.

Then the exciting Conductance G0 is given by W = V12G0 
  or  G0=  W/V12
The exciting Susceptance B0 = Sqr.(Y02 – G02)


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