Short Circuit Test

 Aim:- To perform short circuits test on a single phase transformer to calculate:

1.      The equivalent circuit parameters of the transformer.

2.      The short circuit loos/copper variable use of the transformer.

Apparatus Required:-

S. No.	Name of Equipment	Type	Range	Quantity
1.	Single Phase Transformer	-	5kVA, 230V 21.74 A, 50Hz	1
2.	Voltmeter	PMMI		1
3.	Ammeter	PMMI		1
4.	Wattmeter	Dynamometer	128ohm, 2.5Amp	1
5.	Single Phase Varic	-	230V, 28A, 50Hz	1

 

 Theory:- In this test high voltage winding (secondary) is connected to a supply of normal voltage and frequency through variac and the low voltage winding (primary) is short circuited as shown in Figure. The high voltage winding is supplied to a very low voltage such that the rated current would flow in short circuit branch. This test is performed at rated current flowing in both the windings. Since the applied voltage is very less therefore iron loss occurring in the transformer under short circuit condition is very less and can be neglected. Therefore the total losses occurring under short circuit condition are mainly the copper loss of both the windings which are indicated by the wattmeter connected in the circuit.

Hence,

                             Wattmeter reading in short circuit condition = W_sc

                             Voltmeter reading in short circuit condition = V_sc

                             Ammeter reading in short circuit condition  =  I_sc

 

From the observation of short circuit test of single phase transformer, the short parameters Ryand Of the transformers equivalent circuit referred to particular winding can be calculated as follows;

                                         Total copper loss = W_sc

                    Equivalent resistance referred to HV winding R_eq = W_i/(I_sc)²

                    Equivalent resistance referred to HV winding Z_eq  = V_sc/I_sc

                     Equivalent resistance referred to HIV winding X_eq =√ Zeq^2-Req^2

Circuit diagram.

                

                                                  

    

Procedure:- 

1.     Make the connections as per the circuit diagram.

2.     Make sure that the secondary side of transformer is sorted.

3.     Keep the variac at 0 position before switch on the supply.

4.     Switch on AC supply.

5.     By varying the variac apply full load current to the transformer and note the reading of voltmeter, wattmeter, and ammeter.

6.     Keep the various at zero position and switch off the supply.

Observation Table:-

 

S. No.	Voltmeter Reading (Vsc)	Ammeter Reading    (Isc)	Wattmeter Reading               (W)
1.	11	21.5	70

Calculation:-

Least count of voltmeter = 0.5V

Least count of ammeter = 0.5V

M.F. (multiplying factor) of wattmeter = 2        

                                        Copper loose  = wattmeter reading M.F.

                                                               = 70 x 2

                                                               = 140

      Short circuit current = 21.5 amps

                                 

Precautions:-

1.      All the connection should be tight and clean.

2.      Special care should be taken while selecting the range of the metres for conducting short circuit test.

3.      While conducting the short circuit test the voltage applied should be initially set at zero and then increase slowly. if a little higher voltage than the required voltage be applied (by mistake) there is a danger of transformer being damaged.