Does the inverter have any effect on the motor?
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 effect of the inverter on the motor:
1, the efficiency of the motor and the problem of temperature rise
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. According to the information, to the present
For example, a commonly used sine wave PWM type inverter has a low harmonic of substantially zero, and the remaining higher harmonic components that are about twice as large as the carrier frequency are: 2u+1 (u is a 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%.
2, motor insulation strength problem
At present, many small and medium-sized inverters use PWM control. His carrier frequency is about several thousand to ten kilohertz, which is equivalent to applying a steep shock voltage to the motor, so that the inter-turn insulation of the motor is subjected to a harsh 4-6 times voltage superimposed on the operating voltage of the motor. It will pose a threat to the insulation of the motor to the ground. The repeated impact of the ground insulation on the high voltage of the motor must withstand a very high voltage. The rectangular chopper generated by the PWM inverter accelerates the aging.
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.
