Difference between Allowed Temperature and Actual Operating Temperature of Electrical Insulating Materials

In electronic appliances, the weakest link is the insulating material, and high temperature is an important factor in the aging of insulating materials. Therefore, general electrical equipment stipulates the highest temperature for its operation, ……

In electronic appliances, the weakest link is the insulating material, and high temperature is an important factor in the aging of insulating materials. Therefore, general electrical equipment stipulates the highest temperature for its operation, but various insulating materials have different working temperatures in accordance with their own performance characteristics.

The allowable temperature of the insulating material is different from the actual operating temperature. The main reason can be considered from three aspects: through the resistance, the measurement results reflect the average value of the copper temperature of the entire winding. The main principle is to measure the cold state and the thermal resistance of the conductor, and calculate the average temperature rise according to the relevant formula. The calculated data is reduced by 5 to 15 °C according to the actual maximum temperature.

Secondly, the measurement is carried out by a thermometer. The measurement result reflects the local surface temperature of the winding insulation. This figure is about 15 °C lower than the actual maximum temperature of the winding insulation, that is, the “most hot spot”. Since the mercury thermometer generates heat due to eddy current loss in an alternating magnetic field, an alcohol thermometer is used in the alternating current motor.

The most common method used in medium and small motor field applications is the thermometer method. The last cause of the error is the embedding of a copper or platinum resistance thermometer or thermocouple in a winding test, a core or other component that requires the highest temperature measurement. The result of this measurement is the temperature at which the temperature measuring element contacts. Large motors often use this method to monitor the operating temperature of the motor.

Regarding the reason why the allowable temperature of the insulating material is different from the actual working temperature, in addition to the above, the temperature measured by the electric resistance method of the closed fan-cooled motor is 0 to 15% higher than that measured by the thermometer method; the protective type is 10% 20% higher, which should also be noted.

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