Test wiring method for measuring the DC resistance of transformer windings

Measurement of winding DC resistance

1) For the primary winding of voltage transformers, it is advisable to use a single arm bridge for measurement;

2) For the secondary winding of voltage transformers and the primary or secondary winding of current transformers, it is advisable to use a double arm bridge for measurement. If the DC resistance of the secondary winding exceeds 10 Ω, a single arm bridge should be used for measurement;

3) A DC resistance tester can also be used for measurement, but it should be noted that the test current should not exceed 20% of the rated current of the coil to avoid heating the coil and increasing the DC resistance, which will affect the accuracy of the measurement.

4) Test wiring:

Connect the first and last ends of the tested winding to the electric bridge separately. When using a double arm electric bridge (or digital DC resistance tester) with the non tested winding suspended, the current terminal should be on the outer side of the voltage terminal;

5) When changing wiring, the power supply of the bridge should be disconnected and the tested winding should be fully discharged after short circuiting before disassembling the measurement terminal. If the discharge is not sufficient and the measurement terminal is forcibly disconnected, it is easy to cause overvoltage and damage the main insulation of the coil. Generally, digital DC resistance measurement devices have automatic discharge and warning functions.

When measuring the DC resistance of the secondary winding of the intermediate transformer of a capacitive voltage transformer, the connection between the primary winding and the voltage divider capacitor and the external connection wire of the secondary winding should be disconnected. When the primary winding of the intermediate transformer and the voltage divider capacitor are connected internally but cannot be separated, the DC resistance of the secondary winding may not be measured.

Key points of the experiment:

The measured current should not exceed 50% of the output current calculated based on the rated load of the winding.

When the coil turns are large and the inductance is large, the data displayed by the instrument should be stable before reading. After the measurement is completed, the measurement circuit should be disconnected after the DC resistance testing instrument is fully discharged.

Record the ambient temperature and relative humidity of the air during the test.

The DC resistance measurement value should be converted to the same temperature for comparison.

Result judgment

There is no significant difference compared to previous test results and test results of similar equipment.