Today, we will analyze five aspects: the efficiency and temperature rise of the motor, the insulation strength of the motor, the harmonic electromagnetic noise and vibration, the adaptability of the motor to frequent starting and braking, and the cooling problem at low speed. We explain the influence of the frequency converter on the motor.
1. The efficiency and temperature rise of the motor
Regardless of the type of frequency converter, different levels of harmonic voltage and current are generated during operation, so that the motor runs under non-sinusoidal voltage and current. According to the information, taking the commonly used sine wave PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components about twice the carrier frequency are: 2u+1 (u is the modulation ratio). Higher harmonics will cause the increase of stator copper loss, rotor copper (aluminum) loss, iron loss and additional loss, the most notable being the rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, a large rotor loss will be generated after the high-order harmonic voltage cuts the rotor bar with a large slip. In addition to this, additional copper losses due to the skin effect need to be considered. These losses will make the motor additionally generate heat, reduce the efficiency, and reduce the output power. For example, if an ordinary three-phase asynchronous motor is operated under the condition of a non-sinusoidal power supply output by the inverter, its temperature rise will generally increase by 10%~20%.
2. Motor insulation strength problem
At present, many small and medium-sized inverters use PWM control methods. Its carrier frequency is about several thousand to ten kilohertz, which makes the motor stator windings have to withstand 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 more resistant. ordeal. In addition, the rectangular chopper impulse voltage generated by the PWM inverter is superimposed on the motor operating voltage, which will pose a threat to the motor's ground insulation, and the ground insulation will accelerate aging under the repeated impact of high voltage.
3. Harmonic electromagnetic noise and vibration
When an ordinary asynchronous motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The time harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, a resonance phenomenon will occur, thereby increasing the noise. Due to the wide operating frequency range of the motor and the wide range of rotational speed variation, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequency of each component of the motor.
4. The adaptability of the motor to frequent starting and braking
After the inverter is used for power supply, the motor can be started at a very low frequency and voltage without inrush current, and the various braking methods provided by the inverter can be used for rapid braking, in order to achieve frequent starting and braking The conditions are created, so the mechanical system and 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 insulation structure.
5. Cooling problem at low speed
First of all, the impedance of the asynchronous motor is not ideal. When the frequency of the power supply is low, the loss caused by the higher harmonics in the power supply is large. Secondly, when the speed of the ordinary asynchronous motor is reduced again, the cooling air volume decreases in proportion to the cube of the speed, resulting in the deterioration of the low-speed cooling condition of the motor and the sharp increase in temperature rise, making it difficult to achieve constant torque output.
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