Open circuit test

Open circuit test

The Open circuit test, or "no-load test", is one of the methods used in electrical engineering to determine the no load impedance in the excitation branch of a real transformer.

Circuit diagram for Open Circuit test



The secondary of the transformer is left open-circuited. A wattmeter is connected to the primary. An ammeter is connected in series with the primary winding. A voltmeter is optional since the applied voltage is same as the voltmeter reading. Rated voltage is applied at primary.

If the applied voltage is normal voltage then normal flux will be set up. As the Iron loss is a function of applied voltage, normal iron loss will occur. Hence the iron loss is maximum at rated voltage. This maximum iron loss is measured using the wattmeter. Since the impedance of the series winding of the transformer is very small compared to that of the excitation branch, all of the input voltage is dropped across the excitation branch. Thus the wattmeter measures only the iron loss.

Since the secondary of the transformer is open, the primary draws only no load current. This no load current is negligible. As the copper losses depend on current they can be neglected.

Current, voltage and power are measured at the primary winding to ascertain the admittance and power factor angle.

Another method of determining the series impedance of a real transformer is the short circuit test.


The current \mathbf{I_0} is very small.

If \mathbf{W} is the wattmeter reading then,

\mathbf{W} = \mathbf{V_1} \mathbf{I_0} \cos \phi_0

The above equation can be rewritten as,

\cos \phi_0 = \frac {\mathbf{W}} {\mathbf{V_1} \mathbf{I_0}}


\mathbf{I_m} = \mathbf{I_0} \sin \phi_0
\mathbf{I_w} = \mathbf{I_0} \cos \phi_0


By using the above equations, \mathbf{X_0} and \mathbf{R_0} can be calculated as,

\mathbf{X_0} = \frac {\mathbf{V_1}} {\mathbf{I_m}}

\mathbf{R_0} = \frac {\mathbf{V_1}} {\mathbf{I_w}}


\mathbf{Z_0} = \sqrt {\mathbf{R_0}^2 +\mathbf{X_0}^2}


\mathbf{Z_0} = \mathbf{R_0} + \mathbf{j} \mathbf{X_0}


The admittance is the inverse of impedance. Therefore,

\mathbf{Y_0} = \frac {1} {\mathbf{Z_0}}

The conductance \mathbf{G_0} can be calculated as,

\mathbf{G_0} = \frac {\mathbf{W}} {\mathbf{V_1}^2}

Hence the susceptance,

\mathbf{B_0} = \sqrt {\mathbf{Y_0}^2 -\mathbf{G_0}^2}


\mathbf{Y_0} = \mathbf{G_0} + \mathbf{j} \mathbf{B_0}


\mathbf{W} is the wattmeter reading

\mathbf{V_1} is the applied rated voltage

\mathbf{I_0} is the no load current

\mathbf{I_m} is the magnetizing component of no load current

\mathbf{I_w} is the core loss component of no load current

\mathbf{Z_0} is the exciting impedance

\mathbf{Y_0} is the exciting admittance


  • Kosow (2007). Electric Machinery and Transformers. Pearson Education India. 
  • Smarajit Ghosh (2004). Fundamentals of Electrical and Electronics Engineering. PHI Learning Pvt. Ltd.. 
  • Wildi, Wildi Theodore (2007). Electrical Machines , Drives And Power Systems, 6th edtn.. Pearson. 

See also

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