To perform the open circuit and short circuit test of a single phase transformer and to draw the equivalent circuit after determining its constants.

Experiment No.: 02

NAME OF THE EXPERIMENT: –

To perform the open circuit and short circuit test of a single phase transformer and to

draw the equivalent circuit after determining its constants.

APPARATUS REQUIRED: –

Sl. No.	Name	Specification	Remarks/Quantity
1	Single phase transformer	5KVA,230V/115V, 21.74A/43.48A, 50 HZ	1 Nos.
2	Wattmeter	(2.5/5) A LPF	1 Nos.
3	Wattmeter	(20/40) A UPF	1 Nos.
4	Ammeter	(0-5) A	1 Nos.
5	Ammeter	(0-30) A	1 Nos.
6	Voltmeter	(0-300) V	1 Nos.
7	Voltmeter	(0-30/75) V	1 Nos.
8	Single Phase Variac	5 KVA,(0-300) V,  30 A	1 Nos.
9	Connecting Wire	PVC insulated copper	As per required

CIRCUIT DIAGRAM: –

THEORY: –

The performance of a transformer can be calculated on the basis of its equivalent circuit which contains four main parameters, the equivalent resistance R₀₁  as referred to primary( or secondary R₀₂), the equivalent leakage reactance X₀₁ as referred to primary, the core-loss conductance G₀ and the magnetizing susceptance B₀. These constants or parameters can be easily determined by two test i.e. Open circuit test and short circuit test. These are very economical and convenient, because they furnish the required information without actually loading the transformer.

The purpose of O.C. test is to determine no load loss or core loss and no load I₀ which is helpful in finding X₀ and R₀. One winding of the transformer, whichever is convenient but usually high voltage winding, is left open and the other is connected to its supply of normal voltage and frequency. A wattmeter (W), Voltmeter (V) and ammeter (A) are connected in the low voltage winding i.e. primary winding in the present case. With normal voltage applied to the primary, normal flux will be set up in the core, hence normal iron loss will occur which is recorded by the wattmeter. As the primary no load current I₀ is small, Copper loss is negligibly small in primary and nil in secondary. Hence, the wattmeter reading represents practically the core loss under no load condition.

For short circuit test, one winding, usually the low voltage winding, is solidly short circuited by a thick conductor (or through an ammeter which may serve the additional purpose of indicating rated load current). A low voltage (usually 5 to 10% of normal primary voltage) at correct frequency (though for Cu losses it is not essential) is applied to the primary and is cautiously increased till full- load current is flowing both in primary and secondary (as indicated by the respective ammeters). Since, in this test, the applied voltage is a small percentage of the normal voltage, the mutual flux ø produced is also a small percentage of its normal value. Hence, core loss is very small with the result that the wattmeter reading represents the full load Cu loss or I²R loss for the whole transformer i.e. both primary Cu loss and secondary Cu loss. If Vsc is the voltage required to circulate rated load current, then Z₀₁= Vsc/I₁.

A two winding transformer can be represented by means of an equivalent circuit as shown below

PROCEDURES: –

Open Circuit Test:

1. Connect all the instruments as per the circuit Diagram.
2. Open circuit the secondary and apply full load voltage to the primary through a  Variac. The copper loss is negligible since there is only no load current flowing. Hence power consumed is the core losses of the core.
3. Note voltmeter, ammeter and wattmeter readings.

Short Circuit Test:

1. Connect as shown in the circuit diagram. 
2. Short circuit the secondary and apply a low voltage to the primary through an auto transformer. The iron losses are negligible since the flux will be very low on account of the primary and secondary.
3. Increase the voltage gradually till full load current flows in the primary. 
4. Note voltmeter and ammeter and wattmeter reading

PRECAUTIONS: –

1. Don’t switch on the power supply without concerning teachers.
2. Single-Phase Auto transformers must be kept at minimum potential point. Before switching on the experiment.

OBSERVATION TABLE : –

For Open Circuit Test:

Sl. No.	V (in Volts.)	I0 (in Amps.)	W0 (in Watts.)	Iw (in Amps.)	Im (in Amps.)	Cosɸ0
01

Calculation:

See the no load phasor diagram below

For Short- Circuit Test:

Sl. No.	V (in Volts.)	Isc (in Amps.)	Wc (in Watts.)
01

Calculation:

Let the total equivalent resistance of primary and secondary referred to primary side be R₁
ohms and the total equivalent leakage reactance referred to primary side be X₁ ohms.

Now draw the equivalent circuit.

CONCLUSION: –

To be written by student.