The temperatures rise at various parts of the motor due to the heat loss dissipated in the motor. An excessive temperature rise will cause the deterioration of the insulation, and so the temperatures of the coils and adjacent core are to be kept below the specified temperature rise limit. There are the following three different methods for temperature measurement.

Thermometer Method

This is a method of measuring temperatures by fitting thermometers to the measuring parts of a motor. Usually, thermometers are fitted to the frame or to the bearings. This method is widely used for temperature measurement.

Resistance Method

This is a method of measuring the mean temperature of the coil by utilizing the change of the coil resistance with the temperature; the temperature is calculated in the following way.


R2 : Coil resistance in heated condition (temperature t2)
R1 : Coil resistance in cold state (temperature t1)
t2 : Coil temperature immediately after test (ºC)
t1 : Temperature obtained at the time of measuring R1in cold state
t0 : Reference ambient temperature at the last of the test (ºC)
t : Temperature rise of coil (ºC)
T : Constant                T = 234.5 for copper
 T = 230.0 for aluminum

T = ……….A1

Embedded Resistance Temperature Detector (RTD) Method

This is a method of measuring temperatures of winding and bearings by means of RTDs embedded. (Only applied to the 6.9 KV motors.) The temperature settings for alarm and trip are recommended as follows:

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