Open Circuit Test of Single Phase Transformer

Experiment No.: 4(A)

Experiment Name: Determine Equivalent Circuit Parameters of Single-Phase Transformer by Performing O.C. Test


To perform the O.C. test of a single- phase transformer with the help of an ammeter, voltmeter and a wattmeter and to find out the core loss of the transformer, different parameter at no load condition and to predetermine the performance of the transformer represented by its equivalent circuit.


Transformer is a static AC machine. In this test normally the high voltage winding is left opened and all the meters are placed on the low voltage winding side (depending upon the availability of supply voltage in the lab). At no load, the current taken by the ammeter gives no load current (I0) whose value is very small w.r.t. full load current. The wattmeter gives the reading of the core loss of the transformer, which is constant at any load. The voltmeter indicated the rated voltage of the transformer at which side of the winding of the transformer, the meters are connected.

Experimental Setup for Open Circuit Test on a Single Phase Transformer:

Core loss depends upon the applied voltage. Since normal voltage is applied to low voltage side, so normal flux will be setup on the core, and the normal iron loss (core loss) will be occurred which are recorded by the wattmeter. Hence the wattmeter reading represents practically no load core loss, which is constant at any load.

Let,            W0 = Wattmeter reading which gives iron loss or core loss

                  I0 = No load current

                  V0 = Rated voltage at no load on the low voltage side

                  cos Φ0 = No load power factor

                  Φ0 = Angular distance between I0 & V0

So, Wattmeter reading, W0 =  V0 . I0. cos Φ0

Or, cos Φ0 = W0/V0 I0

By knowing the ‘Cos Φ0’, we can find out sinΦ0

Now, iron loss or working component of the current IW = I0 cosΦ0

The magnetising component of the current Iµ =  I0 sinΦ0

There the no load resistance,

                  R0 = V0 / I

And the leakage reactance, X0 = V0 / Iµ

The open circuit test is done on the low voltage side and the high voltage side is kept open, because it is safe and it requires low range meters which gives low cost for performing this test.

Circuit Diagram:

Procedure of the Experiment:

  1. Connect as shown in the circuit diagram
  2. Set the variac to zero output, and switch on the supply
  3. Set the variac to a suitable voltage output
  4. Watch the wattmeter, ammeter and voltmeter. Increase the variac output voltage gradually till the rated voltage is reached. With rated voltage applied to the primary side, take the readings of the wattmeter, ammeter and voltmeter
  5. Calculate R0 & X0 from this readings

Result & Calculation:

Ammeter reading in Amp (I0)Voltmeter reading in volt (V0)Wattmeter reading in watt(W0)
No load power factor (cosΦ0)IW=I0cosΦ0Iµ=I0sinΦ0R0=V0/IWX0=V0/Iµ
 0.656 A 115 V 30×1=30 W0.397 lag 0.2609 A 0.6019 A 440.78 Ω 191.06 Ω
*M.F.= Multiplying Factor

No load vector diagram:

Apparatus Used:

Sl. No.Name of the ApparatusSpecificationQuantityMaker’s Name
1.Transformer Single Phase, 230/115 V, 5 A 1 
2.Variac 230/0-270 V  1 
3.Digital Multimeter as Ammeter0-750 V AC, 0-1000 V DC, 0-10 A  1Akademika
4.Voltmeter MI type, 0-300 V 1 Sushma
5.Wattmeter Digital 1 CABS Electra
6.Control Panel 230 V, 50 Hz 1 M.E.W.

Remarks: The measuring instruments are to be connected in low voltage side and high voltage side should be opened for open circuit test.

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