17-10-2012, 05:18 PM
SUMPNER’S TEST
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THEORY
This test is conducted simultaneously on two identical transformers and providesdata for finding the efficiency, regulation and temperature rise. The mainadvantage of this test is that the transformers are tested under full-loadconditions without much expenditure of power. The power required to conductthis test is equal to the losses of the two transformers. It may be noted that twoidentical transformers are needed to carry out this test.
Operation
(i) The secondaries of the transformers are in phase opposition. With switch S1 closed and switch S2 open (i.e., regulating transformer not in the circuit),there will be no circulating current (I2 = 0) in the secondary loop circuit. Itis because the induced e.m.f.s in the secondaries are equal and inopposition. This situation is just like an open-circuit test. Therefore, thecurrent drawn from the supply is 2 I0 where I0 is the no-load current of eachtransformer. The reading of wattmeter W1 will be equal to the core lossesof the two transformers.
Advantages
(i) The power required to carry out the test is small.
(ii) The transformers are tested under full-load conditions.
(iii) The iron losses and full-load copper losses are measured simultaneously.
(iv) The secondary current I2 can be adjusted to any current value. Therefore,we can find the copper loss at full-load or at any other load.
(v) The temperature rise of the transformers can be noted.
Losses in a Transformer
The power losses in a transformer are of two types, namely;
1.CORE OR IRON LOSSES
2.COPPER LOSSES
These losses appear in the form of heat and produce (i) an increase intemperature and (ii) a drop in efficiency.
PROCEDURE:
1. Connections are made as shown in the circuit diagram.
2. Rated voltage of 110V is adjusted to get in voltmeter by adjusting the variac of the Auto Transformer which would be in zero before switching on the supply at the primary side.
3. The readings of voltmeter, ammeter and wattmeter are noted on the primary side.
4. A voltmeter is connected across the secondary and with the secondary supply off i.e switch S is kept open. The voltmeter reading is noted.
5. If the reading of voltmeter reads higher voltage, the terminals of any one of secondary coil is interchanged in order that voltmeter reads zero.
6. The secondary is now switched on and SPST switch is closed with variac of auto transformer is zero.
7. After switching on the secondary the variac of transformer (Auto) is adjusted so that full load rated secondary current flows.
8. Then the readings of wattmeter, Ammeter and voltmeter are noted.
9. The Percentage Efficiency and percentage regulation are calculated and equivalent circuit is drawn.