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(X0, R0) and iron loss.
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 (Pi),
where (Pi) = V1I0cosΦ0 (where, cosΦ0 is called as no – load power factor. )
True power(P) is given as , ( P) = E2p / R0
so, no – load resistance (R0) = E2P /P = V1 / I0
Apparent power(S) is given as , (S) = EP I0
also, S = sqrt(P2 + Q2 ) (where P=true power, Q=reactive power )
so, Q = sqrt((EP I0 )2 – P2 ——— eq1)
no- load reactance (X0 ) can be calculated as,
(X0) = E2P / Q
Example: An open circuit test is conducted on a transformer and noted the following recordings ES = 57V, EP = 110V , I0 = 150mA , P= 9.0W, Calculate the no load resistance(R0 ) and no load reactance(X0 )
Solution:
Given, R0 ,I0 ,Ep, E s , P
now, according to the formula R0 = E2P / 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
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.
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