Common asynchronous motor is designed by constant frequency constant pressure, It is impossible to fully adapt to the requirements of variable frequency speed regulation. The following is the effect of the frequency converter on the motor.
1. The efficiency and temperature rise of the motor
No matter what kind of frequency converter, the harmonic voltage and current are generated at different degrees in the operation. Take the commonly used PWM model inverter as an example, the low-order harmonic is basically zero, the remaining harmonic frequency components are about twice as high as higher harmonic component: 2u+1(u is modulation ratio).
Higher harmonic will cause the motor stator copper consumption, rotor copper (aluminum) consumption, iron consumption and additional loss increases, the most notable is the rotor copper (aluminum) consumption. Because the asynchronous motor is rotated at the synchronous speed corresponding to the base wave frequency, therefore, The high harmonic voltage cuts the rotor strip with a larger slip, there will be a lot of rotor consumption. Besides, additional copper consumption due to skin effect is also considered. These consumption result in extra heat, lower efficiency, and reduced output power, if the ordinary three-phase asynchronous motor is operated on the non-sinusoidal power supply of the inverter, its temperature rise generally increases 10%~20%.
2. Motor insulation strength issue
Current medium and small frequency converter, many are using PWM control methods. Its carrier frequency is about a few thousand to a dozen kilohertz, this allows the motor stator winding to withstand a high voltage rise, it is equivalent to applying a steep impact voltage to the motor, the insulation of the electric motor will be subjected to a more severe test. in addition, the rectangular chopper impact voltage generated by the PWM converter is superimposed on the operating voltage of the motor, electric motor is a threat to the insulation of the ground, the insulation of the ground will accelerate aging under the repeated impact of high pressure.
3. Harmonic electromagnetic noise and vibration
Ordinary asynchronous motors are powered by inverter, it can make the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors more complicated. The frequency harmonic and the intrinsic space harmonic interference of the electromagnetic part of the motor are contained in the frequency conversion power supply, all kinds of electromagnetic excitation force are formed. When the frequency of electromagnetic waves is consistent or close to the natural vibration frequency of the motor, there will be resonance, and increase the noise. The operating frequency of the motor is wide, the speed varies widely, the frequency of various electromagnetic waves is difficult to avoid the natural vibration frequency of each component of the motor.
4. The motor is adaptable to frequent start and braking
It is powered by a frequency converter, the motor can be started at very low frequencies and voltages with no shock current, and a variety of braking modes for quick braking can be used, conditions are created for frequent activation and braking, therefore, the mechanical system and electromagnetic system of the motor are under cyclic alternating forces, fatigue and accelerated aging of mechanical structures and insulation structures.
5. Cooling problems at low speed
First, the impedance of the asynchronous motor is not ideal, When the power frequency is lower, the high frequency harmonic caused by the power supply is larger. Secondly, the normal asynchronous motor speed decreases, the cooling air volume decreases with the three times of the rotating speed, the low - speed cooling of the motor has deteriorated, temperature rise has increased sharply, constant torque output is difficult to achieve.