The power factor is high in which of the following tests in a transformer?

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SSC JE Electrical 14 Nov 2022 Shift 3 Official Paper
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  1. Both OC and SC tests
  2. OC test only
  3. SC test only
  4. Sumpner’s test

Answer (Detailed Solution Below)

Option 3 : SC test only
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Testing of Transformer

1.) Open Circuit Test

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  • An open circuit test is conducted on a transformer to determine iron loss or constant loss.
  • This test is conducted on the LV side keeping the HV side open circuit.
  • This test is performed at rated voltage and rated frequency.
  • ​As the secondary side is open, the entire coil will be purely inductive in nature. So, the value of power factor will be low and is known as the no-load power factor.
  • The voltage at rated frequency is applied to that LV side with the help of a variac of variable ratio autotransformer.
  •  Now with the help of variac, the applied voltage gets slowly increased until the voltmeter gives a reading equal to the rated voltage of the LV side.

2.) Short Circuit Test

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  • A short circuit test is conducted on a transformer to determine copper loss.
  • This test is conducted on the HV side keeping the LV side short circuit.
  • This test is performed at the rated current.
  • Since, the LV is short circuited, so the power factor will be high or unity in nature.
  • A low voltage of around 5-10% is applied to that HV side with the help of a variac.
  • Now with the help of variac applied voltage is slowly increased until the wattmeter, and an ammeter gives a reading equal to the rated current of the HV side.
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