1. 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 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 aging under the repeated impact of high voltage.
2, the efficiency of the motor and the problem of temperature rise regardless of the form of the inverter, in the operation of different degrees of harmonic voltage and current, so that the motor operates under non-sinusoidal voltage, current. Rejecting the data introduction, taking the sinusoidal PWM type inverter commonly used at present as an example, the lower harmonics are basically zero, and the remaining higher harmonic components which are about twice as large as the carrier frequency are: 2u+1 (u For the modulation ratio).
Higher harmonics cause an increase in the stator copper loss of the motor, copper (aluminum) consumption, iron loss and additional losses in the rotor. Zui is significantly 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%.
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 is equal to or close to the natural vibration frequency of the motor body, a resonance phenomenon will occur, 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 adaptability of the motor to frequent starting and braking. After the inverter is used for power supply, the motor can be started in a low frequency and voltage without impact current, and various braking methods provided by the inverter can be utilized. The rapid braking creates conditions for frequent starting and braking. Therefore, 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 problem at low speed First, the impedance of the asynchronous motor is not ideal. When the power frequency is lower, the loss caused by 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.
2, the efficiency of the motor and the problem of temperature rise regardless of the form of the inverter, in the operation of different degrees of harmonic voltage and current, so that the motor operates under non-sinusoidal voltage, current. Rejecting the data introduction, taking the sinusoidal PWM type inverter commonly used at present as an example, the lower harmonics are basically zero, and the remaining higher harmonic components which are about twice as large as the carrier frequency are: 2u+1 (u For the modulation ratio).
Higher harmonics cause an increase in the stator copper loss of the motor, copper (aluminum) consumption, iron loss and additional losses in the rotor. Zui is significantly 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%.
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 is equal to or close to the natural vibration frequency of the motor body, a resonance phenomenon will occur, 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 adaptability of the motor to frequent starting and braking. After the inverter is used for power supply, the motor can be started in a low frequency and voltage without impact current, and various braking methods provided by the inverter can be utilized. The rapid braking creates conditions for frequent starting and braking. Therefore, 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 problem at low speed First, the impedance of the asynchronous motor is not ideal. When the power frequency is lower, the loss caused by 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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