Motor efficiency and temperature rise

Update:20 Jul 2018
Summary:

Regardless of the form of the frequency converter, diff […]

Regardless of the form of the frequency converter, different levels of harmonic voltage and current are generated during operation, so that the motor operates under non-sinusoidal voltage and current. Rejecting the data introduction, taking the sinusoidal PWM type inverter commonly used at present as an example, the lower harmonics are basically zero, and the remaining higher harmonic components which are about twice as large as the carrier frequency are: 2u+1 (u For the modulation ratio).

Higher harmonics cause an increase in the stator copper loss, rotor copper (aluminum) consumption, iron loss and additional losses, most notably the rotor copper (aluminum) consumption. Since the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, the high-order harmonic voltage will cause a large rotor loss after cutting the rotor bar with a large slip.

In addition, the additional copper consumption due to the skin effect needs to be considered. These losses will cause the motor to generate extra heat, reduce the efficiency, and reduce the output power. For example, if the ordinary three-phase asynchronous motor is operated under the non-sinusoidal power supply output of the inverter, the temperature rise will generally increase by 10%-20%.