Winding resistance Measurement

Winding Resistance is an important measurement in electrical machines.  Winding resistance tells us about the condition of the winding.  Any fault in the winding such as an open circuit or an inter-turn short circuit will be reflected in the winding resistance value.  Besides, winding resistance is used to measure I2R losses in the winding.

The Winding Resistance is measured by winding measurement test kits.  In earlier times, winding resistance was measured using the Kelvin Bridge. The Kelvin Bridge is an arrangment 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.

The machine to be tested is disconnected from the lines and de-energized.  The measurement are usually taken phase-to-phase.  The three readings should be within 1% of the average value.

Winding resistance can change with temperature.  The measurement are usually taken at the cold temperature known as the cold resistance.  The transformer or the motor is allowed to cool for a few hours and the temperature taken.

Based on the measurement taken at a particular temperature, the resistance at any other temperature may be calculated from the following formula

Rs= Resistance value to be calculated at a specific temperature
Rm= Resistance valued measured
Tm= Temperature at which the resistance was measured
Ts= Temperature at which the resistance is to be calculated
Tk= Winding Material Constant ( 234.5 °C for copper or 225 °C for aluminum)

The windings can store a huge amount of electromagnetic energy when a current is passed through them during measurement. When the test current is stopped, there may be a voltage kickback from the winding.  The test equipment should be able to absorb the voltage kick and safely discharge it.