Transformer Testing Methods for OC and SC Test

Apr 27 • General • 18073 Views • 3 Comments on Transformer Testing Methods for OC and SC Test

Transformer testing (Open Circuit and Short Circuit Test) Transformer is a static or stationary electromagnetic device consisting of two magnetic fields by means of which the electric power in one circuit is transform into the electric power of same frequency in another circuit. Or we can say that a transformer is a static device which consists of two or more stationary electric circuit interlinked by a common magnetic circuit for the purpose of transferring electrical energy between them. The testing of transformer gives better efficiency and regulation of the transformer.

Transformer Testing Methods for Single phase is tested by two basic methods. They are:-

Distribution Transformer

• Open Circuit Test
• Short Circuit Test

Though there are many more testing methods of a transformer like Polarity test, winding resistance test and load test. These test are done for small rated transformers or mainly for current transformer and potential transformers. For transformer with higher ratings we go for short circuit test and open circuit tests.

Short Circuit Test:-

Open Circuit Test of Transformer

While testing a transformer through the open circuit method we first need to make the secondary open circuited. After that we need to add a watt meter and an ammeter in series to the primary winding. As the voltage supply will be rated voltage so there is no need of providing a voltmeter. When we give the supply voltage to the primary winding, the flux is set-up in the armature of the core of the transformer.

There will be some iron losses in the core of the transformer once the rated supply is provided. Since the impedance of the series winding of the transformer is very small as compared to that of the excitation branch, all of the input voltage is dropped across the excitation branch. Thus the watt meter measures only the iron loss. Since the secondary of the transformer is open, the primary draws only no-load current, which will have some copper loss. This no-load current is very small and because the copper loss in the primary is proportional to the square of this current, it is negligible. There is no copper loss in the secondary because there is no secondary current.

The ratio of output power to the input power of a transformer is known as transformer efficiency at a particular load and power factor is defined as the output delivered by input power. The voltage regulation of transformer is defined as the arithmetic difference in the secondary terminal voltage between no load and full rated load voltage at a given power factor at a same value of primary voltage for both rated load and no load.

Voltage regulation =(no load voltage- full load voltage)*100/ no load voltage.

From open circuit test and short ckt test this can be determined.

Formula tobe used:

W = I V COS X
V = I R
W = I* I R
Xo = V / Im
Ro = V / Iw
Zo = Ro + j Xo
Yo= 1 / Zo

Where,

W= watt meter reading in watt i.e power
V= voltage in volts i.e voltmeter reading
i= ammeter reading in amp i.e current
Xo= reactance
Zo= impedance
Yo= admittance Impedance is the reciprocal of admittance

 Watt meter is connected and it measures the full load copper loss.in open circuit test both iron loss and copper loss occurs.the applied voltage is very few percentage of the nominal voltage so in case of short circuit test the core loss is very les so it can be neglected .the Hysteresis loss is varies with the variation of frequency and Eddy current loss varies its square of these two so to separate these two losses different frequency is applied.as iron loss is the function of applied voltage it is maximum at the rated voltage.

Short Circuit Test:-

Testing of Transformer by Short Circuit Test

In the testing process of transformer by short circuit test we make the secondary side of the transformer short circuited and  we perform the test of HV side i.e high voltage side.The supply voltage is made to circulate rated current throughout the HV side of the transformer.The core losses are very small because applied voltage is only a few percentage of the nominal voltage and hence can be neglected. Thus the wattmeter reading measures only the full load copper loss.This is the short circuit method of performing short circuit test of a single phase transformer.

For more details one can refer to the PDF attached below:-
Method of Transformer Testing

Question and Answers on Testing of Transformer:-  

Q1: Which test is also known as no load test in transformer testing?
Ans: Open circuit test is also known as no load test.

Q2: Which test is known as full load test in transformer testing?
Ans: Short circuit test is also known as  full load test.

Q3: What is the need of open circuit test and short circuit test?
Ans: To know the efficiency and voltage regulation of transformer for better use.

Q4: In which test no load impedance and admittance can be calculated? Ans: In open circuit test this can be calculated.

Q5:  Relation between impedance and admittance?
Ans: Impedance is the reciprocal of admittance.

For more questions on transformers on can download the attached PDF :- Question and Answers on Transformer

Related Links:

• Details of Regulating Transformers
• Power Transformer Protection

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