Open Circuit and Short Circuit Test of Transformer

What are the transformer’s testing methods?

Basically, the efficiency and regulation of the transformer on any load condition and at any power factor condition can be predetermined by indirect loading methods which include not an actual load on the transformer but the equivalent circuit parameters are determined by the following tests.

  1. Open Circuit test (O.C. Test)
  2. Short Circuit test (S.C. Test)

The parameters which are calculated from these test results without loading the transformer are very effective in finding the regulation and efficiency of a transformer at any load and power factor condition. The main advantage of these tests is that rather we load the transformer to full load condition, we just perform these tests to calculate the efficiency and regulation of the transformer which brings us the performance of these tests without losing much power.

Open Circuit Test:

In this test, the transformer primary winding is connected to Single-phase AC power supply and Ammeter, Voltmeter, Wattmeter, and Variac. Basically, the Variac is a potential divider circuit like the potentiometer which divides the potential on the output by varying the knob. The primary winding is usually subjected to the low voltage side and secondary winding to the high voltage side for the conduction of the open-circuit test.

The primary winding is excited by the rated voltage which is adjustable precisely with the help of the variac and the wattmeter and ammeter measure the input power and current, respectively. The voltmeter at the primary winding measures the rated primary winding voltage which is applied with the rated frequency.

Sometimes when primary winding is subjected to the rated voltage, the voltmeter is connected to the secondary winding to measure the secondary voltage which is V2 = E2. There is a specific reason behind this which is as we know the voltmeter resistance is very high and when connected, the current through it is always negligibly small due to which secondary winding high voltage side is considered to be open.

When primary winding voltage is adjusted to the rated voltage with the help of variac, the observed readings of ammeter, voltmeter, and wattmeter are recorded as follows.

Vo voltsIo amperesWo watts


Vo = Rated Voltage

Io = Input Current = no-load current

Wo = Input Power

In the open-circuit test, the secondary side is open and there is no load connected to it. So, the current drawn by the primary winding will be no load current Io. For a transformer, no-load current Io has two components,

Magnetizing component = Im = Iosinϕo

Active component = Ic = Iocosϕo

cosϕo = No load power factor

Therefore, the input power can be written as

Wo = VoIocosϕo

As we know the secondary side is open i.e. there is no load connected, I2 = 0 and due to this its reflected current I2 is also zero. Hence, we can say that the primary winding current I1 = Io. The transformer no-load current is always very small hardly 2-4% of its full load current. As I2 = 0, the secondary side copper losses are zero and I1 = Io is very low, the copper losses on the primary side are also very low. Thus, we can say that in the open circuit test, the total copper losses are very low which are negligible. Due to this, the input or primary side of the transformer is subjected to the rated voltage at rated frequency; the flux density in the core s at its maximum value due to which we can say that core losses are occurring only in the transformer and the total input power is supplying the core losses measured by the wattmeter i.e. Wo. Hence, the wattmeter in the open circuit test measures core losses in the transformer which is constant for all the loads.


Wo = Pi = Core losses = VoIocosϕo


cosϕo = Wo/VoIo = No-load Power Factor

The main thing to remember for the conduction of this test is that it always uses wattmeter of low power factor because no-load power factor is very low, otherwise there might occur an error in the results observed.

Short Circuit Test:

In this test, the secondary side is short-circuited with the help of the thick copper wire or a solid link. As it is a general observation that high voltage side is always low current side and low voltage side always high current side; therefore, we can say that in short circuit tests the primary and secondary are high and low voltage sides, respectively.

When secondary is short-circuited, its resistance is extremely small and it draws very large current on the rated voltage due to which overheating and burning of the transformer could be happened. To limit this very large current, the primary side is supplied with just enough voltage or low voltage just to cause this current to flow at the rated one in the primary through which can be measured by the ammeter. This low voltage can be varied by the variac and this test is also called low voltage test or reduced voltage test.

When primary winding voltage is adjusted to the low voltage with the help of variac, the observed readings of ammeter, voltmeter, and wattmeter are recorded as follows.

VSC voltsISC amperesWSC watts

Now we know that the currents flowing in the primary and secondary are the rated ones, then the total copper loss is full load copper loss and the applied voltage is the fraction of the rated voltage and we know that the core losses are the function of the rated voltages. The total core losses are very small due to which wattmeter measures the copper losses as core losses are very small.


WSC = (PSC) Full load = Copper losses

From test readings, we can write,


cosϕSC = WSC/VSCISC = Short Circuited Power Factor

WSC = ISC2*R1e

R1e = WSC/ ISC2


Z1e = VSC/ISC = (R1e2 + X1e2)1/2

X1e = (Z1e2 – R1e2)1/2

Thus, we get the equivalent circuit parameters R1e, X1e & Z1e from the short circuit test. Knowing the transformation ratio K, we can also find the equivalent parameters referred to the secondary side which are R2e, X2e & Z2e.

Thanks for reading our open Circuit and Short Circuit Test of Transformer

Related Topics;

  1. Efficiency and losses of a transformer
  2. Equivalent circuit of the transformer
  3. 3-phase transformer
  4. Parallel operation of Transformer
  5. All About Transformer