Open and Short Circuit Test of Transformer

In this article, we will go through the Open and Short circuit tests of a Transformer. Open and short circuit tests are performed on a Transformer to evaluate its performance characteristics and parameters which mainly determine the Voltage regulation of the transformer, the Transformer’s efficiency, and the Equivalent circuit of the Transformer. They determine Iron and copper losses in the transformer. In this tutorial, we will go through the open and short circuit tests on a transformer with their circuit diagrams and an example and also we will tabulate the differences between the two.

Basic Terminologies

• Short circuit: An electrical connection in which current flows along a path that has very little resistance. This may result in high current flow, damage to the equipment, heating, etc.,
• Open circuit: It is an electrical connection in which there is a break in the path in which electric current flows. This may result in High resistance, no current flow, etc.,
• Transformer: An electrical device that is used to transfer electrical energy from one circuit to another without physical connection through electromagnetic induction.
  Now, let us learn open circuit tests and short circuit tests step by step with an example for each.

Open Circuit Test of Transformer

It is a method in electrical engineering used to determine the iron or core loss and it determines the exciting current of the transformer when it operates at rated voltage. The circuit diagram of this is shown below. In this, the secondary winding or the high voltage side is left open and the low voltage side is used to perform the test completely. Using this test we calculate no-load circuit parameters(X_0, R₀) and iron loss.

Open CIrcuit Transformer

In the circuit diagram, the voltmeter(V), ammeter (A) and wattmeter(W) were all connected on the low-voltage side of the transformer, which is supplied at rated voltage(V1). The secondary winding side is left open, so which a small amount of current(I₀) is flowing in primary winding. Here, I₀ is called as no – load current. The reading of the wattmeter gives the iron loss.

The iron loss of transformer is given by (P_i),

where (P_i) = V₁I₀cosΦ₀ (where, cosΦ₀ is called as no – load power factor. )

True power(P) is given as , ( P) = E²_p / R₀

so, no – load resistance (R₀) = E²_P /P = V₁ / I₀

Apparent power(S) is given as , (S) = E_P I₀

also, S = sqrt(P² + Q² ) (where P=true power, Q=reactive power )

so, Q = sqrt((E_P I₀ )² – P² ——— eq1)

no- load reactance (X₀ ) can be calculated as,

(X₀) = E²_P / Q

Example: An open circuit test is conducted on a transformer and noted the following recordings E_S = 57V, E_P = 110V , I₀ = 150mA , P= 9.0W, Calculate the no load resistance(R₀ ) and no load reactance(X₀ )

Solution:

Given, R₀ ,I₀ ,E_p, E _s , P

now, according to the formula R₀ = E²_P / P

so, R₀ = (110V)² / 9 => 1.35 kΩ

now, on substituting the required values in eq1 , we have

sqrt((110V*150mA)² -(9W)² = 14.41vars

and, X₀ = E² _P / Q = (110v)² / 14.41vars = 840Ω

Short Circuit Test of Transformer

It is a method in electrical engineering used to determine the copper or winding loss and also it determines the impedance of the transformer . The circuit diagram of this is shown below. In this, the secondary winding (low voltage side is shorted by a thick conductor) and on primary side(high voltage side) Ammeter(A), Voltmeter(V) and wattmeter(W) are connected.

Transformer

so, generally short circuit test is performed on high winding side and short circuited on low winding side. It will read copper losses because of high current passing through windings.

we have Power losses ( W _{s. c}) = I² _{s. c} * R_e

Hence, R_e = W _{s c} / I²_s.c

we know that, Impedance( Z _e ) = V_{s .c} / I_{s. c}

Reactance(X _e ) = sqrt((impedance)² – (resistance)² )

Example:

A short circuit test is conducted on a transformer and noted the following recordings V _{s .c} = 7V , I_{s .c} = 1.5mA , W _{s .c} = 4W, Calculate the Reactance(X _e ) , resistance(R_e ) ?

Solution:

Given that, V _{s. c} = 7V , I _{s. c} = 1.5mA , W _{s . c} = 4W

now, resistance ( R_e ) = W _{s. c} / I² _{s. c}

so, Resistance (R_e ) = 1.8Ω

Impedance(Z _e ) = V _{s .c} / I _{s. c} => 7/1.5

Now, Reactance( X _e ) = sqrt((impedance)² – (resistance)² )

X _e = 1.46Ω

Differences Between Open Circuit Test and Short Circuit Test

OPEN CIRCUIT TEST	SHORT – CIRCUIT TEST
also known as no – load test	also known as Impedance test
main aim is to calculate the iron loss and exciting current of the transformer	main aim is to calculate the copper or winding loss and the impedance of the transformer
primary winding is connected to rated voltage source	primary winding is connected to low- voltage source
secondary winding is open – circuited	secondary winding is short – circuited
voltmeter is connected across the primary winding to measure the applied voltage	voltmeter is connected across secondary winding to measure secondary voltage
ammeter is connected in series with the primary winding to measure primary current	ammeter is connected in series with primary winding to measure short – circuit current

Conclusion

Both the open circuit test and short circuit test are done to measure the efficiency of the transformer through various parameters in each case, voltage regulation. In this article, we have covered this topic in detail. We have covered Open CIrcuit Test and Short Circuit Test both separately with proper example.