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Winding Ohmmeters:
Technical Support

Measuring DC resistance of large and middle size transformers

The inductance value of a transformer depends on the current injected in the windings. When the transformer is saturated the inductance is minimized. A transformer is normally designed to reach saturation when the current is 1,2 times the peak value of the no-load current. The no-load current is normally in the range of 0,2 to 5 percent of the nominal winding current. When measuring the DC resistance the test current should be at least 1,2 times the no-load current of the transformer. This is to ensure that the transformer core is saturated. Note also that the test current is NOT to exceed approx 10-15 percent of the nominal winding current.

Four-wire (Kelvin) measurement method is used because it provides the best possible measurement. This ensures that the resistance of the connecting current cables is not included in the measurement.
The test current is passed through the windings using high current cables. The voltage drop across the windings is measured using sensing cables. The placing of cables is very important: the current cables should always be outside the sensing cables (see drawing). The resistance is calculated using Ohm's law: voltage drop divided by the test current (U=R*I i.e. R=U/I).
Always use the test instrument to discharge the energy in the windings after the measurement and before the test current is switched off.

Procedure steps:

1. Make certain that the transformer is disconnected properly and that local safety regulations are followed.
2. Ground the test instrument.
3. Make sure that the test instrument is switched off.
4. Connect current and sensing cables according to the drawing.
5. If using a test instrument with temp sensors, connect temp sensors properly.
6. Switch the test instrument on.
7. Set the test current to a minimum of 1,2 times the no-load current. Also make sure that the test current does NOT exceed approx 10-15 percent of the nominal winding current.
8. Perform the measurement.