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Thursday, 15 August 2019

Open Circuit and Short Circuit Test of Transformer Calculation

Transformer test

The performance of transformer can be considered on the source of its corresponding circuit which have four main parameters the corresponding resistance R01 as known to primary (or secondary R02 ) ,the corresponding leakage reactance X01 known to primary (or secondary X02 ), the core loss conductance G0 and the magnetizing susceptance B0 .These constants are factors that can be simply calculated by two test.

1. Open circuit test (O.C)
2. Short circuit test (S.C)

1. Open Circuit (O.C) Test on Transformer Calculation

Open circuit test is used

• To determine no load constants R0, X0
To determine core loss

The connection diagram of O.C. test is shown in the fig. The rated voltage is applied to the primary winding, keeping the secondary is open. The readings of voltmeter, Ammeter and wattmeter are recorded. Let the respect readings are given below.

Ammeter reading = I0
Voltmeter reading = V0
Wattmeter reading = W0

W0 represents the power input to the transformer on no load. It is equal to sum of core loss and copper loss. Since the no load current is small, the copper loss,(I20 R01) very small. Hence the copper loss at no load is neglected. Therefore W0 reading shows the core loss.
From this reading R0, X0 are calculated as followed,

V0I0 cosӨ0 = W0
Power factor, cos Ө0 = W0 / V0 I0
Core loss component of no load current IC = I0cosӨ0
Magnetising component of no load current Im = I0sin Ө0
No load circuit constants

R0 = V0 / Ic,
X0 = V0 / Im

2. Short Circuit (S.C ) Test on Transformer Calculation:

Short circuit test is used to

1 determine load constants R01 (or) R02, X01(or) X02
2 determine copper loss at full load

The connection diagram S.C. test is shown in the fig. A reduced voltage is applied to the primary winding and keeping the secondary short-circuited. The applied voltage is gradually increased until full load current flows through the transformer. The reading of ammeter, voltmeter and wattmeter are recorded. Let the respecting readings are Is ,Vs ,Ws.

Here the core loss is negligibly small since the applied voltage is very small. So the wattmeter reading (Ws) will give the full load copper loss of the transformer.

VsIs cosӨs = Ws
Power factor, cosӨs = Ws / VsIs
Impedance, Z01 = Vs / Is
Full load copper loss, Ws = Is2 R01
Total resistance, R01 = Ws/ Is2 ( R01 is total resistance referred to primary side)
Total reactance is primary side, X01 = √ Z012 - R012

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