In the past, winding resistance measurements were taken using the Kelvin Bridge, an arrangement of resistors which enables the measurement of very low resistances. Winding Measurement Kits work by injecting known current through the winding and measuring the voltage drop across the winding.
Prior to being tested the transformer or motor is usually disconnected from the lines, de-energized, and measurements are taken phase-to-phase. The three readings should be within 1% of the average value.
Winding resistance can change with temperature, therefore, the equipment is allowed to cool for a few hours and the temperature is recorded. Measurements taken at the cold temperature is known as the cold resistance.
A reliable three phase transformer winding resistance meter is the Vanguard TRM-403 Transformer Resistance Meter. The unit allows the user to connect all test cables to the transformer bushings, then measure the transformer resistance value for each of the phases, without the need to disconnect and reconnect cables for each phase.
For a Delta transformer, the Vanguard TRM-403 can measure the phase resistance readings and provide the individual Delta winding resistance values. The Vanguard TRM-403 can also provide the individual winding resistance values for a Wye transformer without the neutral terminal.
The TRM-403 can perform a special test to collect data automatically for up to 90 minutes (at 60-second sampling intervals) or 45 minutes (at 30 second sampling intervals). The test data is recorded with a time stamp.