How to test a transformer?


To confirm the specifications and performance of transformers, it has to go through different tests. Many tests are carried out before the transformer is shipped at the manufacturer’s premises to ensure the transformer works properly.

There are several tests that can be done on the transformer:

  1. Winding resistance test
  2. Polarity Test
  3. Open Circuit Test
  4. Short Circuit Test
  5. Dvdf Test
  6. Hv Test

Winding resistance test:

For the following reasons, winding resistance measurements are of fundamental importance in transformers:

  • Calculations of the I2R component of conductor losses.
  • Calculation of winding temperature at the end of a temperature test cycle.
  • As a base for assessing possible damage in the field.

Transformers are vibrating. Due to poor design, installation, storage, poor climates, overloading or poor maintenance, accidents or defects occur.

Measuring the winding resistance guarantees the connections are accurate and the measurements of the resistance mean that there are no significant inconsistencies or openings.

Keep in mind that the purpose of the test is to check the gross differences in the connections between the windings and the openings. The experiments are not carried out to replicate the readings of the produced unit conducted in the factory under controlled conditions and likely at other temperatures.

Polarity Test:

Two transformer polarity measurements are used to evaluate the polarity of a transformer for this reason and its low voltage winding and high voltage winding are linked in parallel to each other by the secondary windings of a transformer that are usually attached to its secondary side of a transformer with a high voltage winding fuse.

Open Circuit Test:

The transformer’s secondary winding will be left open-circuited. The main is paired with a wattmeter. For series, an ammeter is connected to the primary winding. A voltmeter is optional as the voltage applied is equal to the reading of the voltmeter. At primary, a graded voltage is applied.

If the voltage applied is regular, the normal flux will be set. As the loss of Iron is a result of applied voltage, there will be a normal loss of iron. Therefore, the loss of iron at the measured voltage is maximum. Use the wattmeter to calculate this overall iron loss.

At the primary winding, current, voltage and pressure are measured to determine the angle of entry and the power factor.

Short Circuit Test:

The test is carried out on the transformer’s high-voltage (HV) side where the low-voltage (LV) side or the secondary side is short-circuited. The supply voltage needed to pump the nominal current through the transformer is usually very small and is of the order of a few percent of the nominal voltage.

The core losses are very small because the voltage applied is only a few percent of the nominal voltage and can, therefore, be ignored. Thus the reading of the wattmeter tests only the loss of a full load of copper.

Dvdf Test:

The DVDF (double voltage double frequency) test is a measure of resistance to overvoltage. A higher than the nominal voltage (2 times) is applied to the transformer for a relatively short time.

Hv Test:

Separate source voltage withstand test (high voltage checks on HV & LV). This check tests the property of insulation between Primary to Earth, Secondary to Earth and Primary & Secondary.