To determine the efficiency of a single phase transformer by open circuit and short circuit test

Experiment No
 

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Aim: To determine Regulation and Efficiency of a single phase transformer by

      open circuit (O.C.) and short circuit (S.C.) tests  

Apparatus: -           

• Single phase transformer 
• Single phase dimmer stat 
• Ammeter (AC) Voltmeter (AC)
• Multi-function meter                                                                                                                                                                  THEORY:-Open Circuit Test: 

The main purpose of this test is to find the iron loss and no load current which are useful in calculating core loss resistance and magnetizing reactance of the transformer. In O.C. test primary winding is connected to a.c. supply, keeping secondary open. Sometimes a voltmeter may be connected across secondary as voltmeter resistance is very high & voltmeter current is negligibly small so that secondary is treated as open circuit.  Usually low voltage side is used as primary and high voltage side as secondary to conduct O.C. test. When primary voltage is adjusted to its rated value with the help of variac, readings of ammeter and wattmeter are to be recorded. Ammeter gives no load current. Transformer no load current is always very small, 2 to 5 % of its full load current. As secondary is open, I2 = 0, hence secondary copper losses are zero. And I1 = I0 is very low hence copper losses on primary are also very low. Thus the total copper losses in O.C. test are negligibly small, hence neglected. Therefore the wattmeter reading in O.C. test gives iron losses which remain constant for all the loads.

Short Circuit Test:   The main purpose of this test is to find full load copper loss and winding parameters (RO1 &XO1 or RO2 & XO2) which are helpful for finding regulation of transformer.


In this test, secondary is short circuited with the help of ammeter. (secondary may be short circuited with thick copper wire or solid link). As secondary is shorted, its resistance is very very small and on rated voltage it may draw very large current. Such large current can cause overheating and burning of the transformer. To limit this short circuit current, primary is supplied with low/reduced voltage (5 – 15% of the rated voltage) which is just enough to cause rated current to flow through primary which can be observed on an ammeter. The reduced voltage can be adjusted with the help of variac. The wattmeter reading as well as voltmeter, ammeter readings are recorded. As the voltage applied is low which is a small fraction of the rated voltage and iron losses are function of applied voltage, hence iron losses  are negligibly small. Since the currents flowing through the windings are rated currents hence the total copper loss is full load copper loss. Hence the wattmeter reading is the power loss which is equal to full load copper losses. 
Procedure: A) O.C. test: 

1) Connect the circuit as shown in circuit diagram.
2) Switch on the supply after checking connection by concerned teacher.
3) Increase the input voltage to the transformer winding upto rated value (230V) slowly using dimmer stat.
4) Measure the primary voltage, primary current, primary circuit power  and secondary voltage of transformer.
5) Reduce the voltage slowly using Variac.
6) Switch off the supply and remove connections.

Procedure : B) S.C. test 
1) Connect the circuit as shown in circuit diagram.
2) Switch on the supply after checking connection by concerned teacher.
3) Increase the input voltage very carefully and slowly so that the current in secondary winding reaches rated value (8.6A) slowly using dimmer stat.
4) Measure the primary voltage, primary current, primary circuit   power and secondary current of transformer.
5) Reduce the voltage slowly using dimmer stat.
6) Switch off the supply and remove connections.


Precaution: 
1) Do not put on the supply until the circuit is checked by concerned teacher.
2) Do not touch any live part of circuit.
3) Be careful for primary & secondary winding rated current.


Result & Conclusion: 

Discussion Questions
1) What is regulation and efficiency of a transformer?
2) Why core losses remain almost constant at any load?
3) Justify-open circuit test gives core losses while short circuit test gives copper losses.
4) What are the advantages and disadvantages of direct loading method over open circuit (o.c.) and short circuit (s.c.) test?
5) Discuss the effect of output power on efficiency and regulation.
6) Why reduced voltage is required for s.c. test?
7)  Why s.c. test is generally performed with L.V. side short circuited?
8) Why o.c. test is generally performed on L.V. side.