1. The efficiency of motor and the problem of temperature rise.

No matter what kind of converter, it produces different degrees of harmonic voltage and current in operation, which makes the motor run under non-sinusoidal voltage and current. Taking the sinusoidal PWM converter, it is widely used at present, as an example, the low order harmonics are basically zero, and the remaining high order harmonics, which are about twice the carrier frequency 2u + 1 (u is the modulation ratio).

High order harmonics will lead to the increase of stator copper consumption, rotor copper (aluminum) consumption, iron consumption and additional loss, the most significant of which is rotor copper (aluminum) consumption. Because the asynchronous motor rotates at the synchronous speed close to the fundamental frequency, the high harmonic voltage cuts the rotor guide bar with a large slip, which will produce a lot of rotor loss. In addition, the additional copper consumption caused by skin effect should also be considered. These losses will cause the motor to heat up, reduce the efficiency and reduce the output power. If the ordinary three-phase asynchronous motor is run on the non-sinusoidal power supply output by the frequency converter, its temperature rise will generally increase by 1. 0%--20%.

2. Insulation strength of motor

At present, many of the small and medium-sized frequency converters are controlled by PWM. Its carrier frequency is about thousands to ten thousand Hz, which makes the stator winding of the motor bear a high voltage rise rate, which is equivalent to applying a steep impulse voltage to the motor, which makes the inter-turn insulation of the motor bear a more severe test. In addition, the rectangular chopping impulse voltage generated by PWM frequency converter is superimposed on the operating voltage of the motor, which will pose a threat to the insulation of the motor to the ground, and the insulation to the ground will accelerate the aging under the repeated impact of high voltage.