Variable frequency motor features

- Jan 30, 2019-

Ordinary asynchronous motors are designed according to constant frequency and constant voltage, and it is impossible to fully adapt to the requirements of frequency control. The following is the influence of the inverter on the motor.

1. Motor efficiency and temperature rise problems Regardless of the type of frequency converter, different levels of harmonic voltage and current are generated during operation, so that the motor operates under non-sinusoidal voltage and current. According to the rejection data, the lower harmonics of the sine wave PWM inverter are basically zero, and the higher harmonics are mainly concentrated near the integer frequency of the carrier frequency. Due to the high harmonic impedance, the current waveform flowing through the motor is close to a sine wave. The right picture shows the PWM voltage waveform and current waveform of the inverter output.

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 is generally increased by 10% to 20%.

2, motor insulation strength problem

Small and medium-sized inverters mostly use PWM control. His carrier frequency is about several thousand to ten kilohertz, which makes the stator winding of the motor to withstand a high voltage rise rate, which is equivalent to applying a steep shock voltage to the motor, so that the inter-turn insulation of the motor is more resistant. A harsh test. In addition, the rectangular chopper surge voltage generated by the PWM inverter is superimposed on the operating voltage of the motor, which poses a threat to the insulation of the motor to the ground, and the ground insulation will accelerate the aging under the repeated impact of high voltage.

3. Harmonic electromagnetic noise and vibration

When the ordinary asynchronous motor is powered by the inverter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. Each time harmonic contained in the variable frequency power supply interferes with the inherent spatial harmonics of the electromagnetic part of the motor to form various electromagnetic exciting forces. When the frequency of the electromagnetic force wave coincides with or is close to the natural vibration frequency of the motor body, a resonance phenomenon occurs, thereby increasing noise. Since the operating frequency range of the motor is wide and the range of the rotational speed is large, the frequencies of various electromagnetic force waves are difficult to avoid the natural vibration frequency of each component of the motor.

4, the ability of the motor to adapt to frequent start and brake

Since the inverter is powered, the motor can be started with no inrush current at very low frequency and voltage, and can be quickly braked by various braking methods provided by the inverter, in order to achieve frequent starting and braking. The conditions are created, so that the mechanical system and the electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and the insulating structure.

5, cooling problems at low speed

First of all, the impedance of the asynchronous motor is not ideal. When the power frequency is lower, the loss caused by the higher harmonics in the power supply is larger. Secondly, when the normal asynchronous motor is reduced in speed, the cooling air volume is proportional to the cube of the rotational speed, which causes the low-speed cooling condition of the motor to deteriorate, and the temperature rise sharply increases, making it difficult to achieve constant torque output.

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