What is the difference between variable frequency motor and industrial frequency motor?
Posted on2025-03-18
Ordinary asynchronous motors are designed according to constant frequency and constant voltage, and cannot fully meet the requirements of variable frequency speed regulation. The following are the effects of frequency converters on motors.
1. The efficiency and temperature rise of motors.
Regardless of the type of frequency converter, high-frequency harmonic voltage and current will occur during operation, causing the motor to operate under non-sinusoidal voltage and current. According to the information, taking the currently commonly used sinusoidal wave PWM frequency converter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components that are about twice the carrier frequency are: 2u+1 (u is the modulation ratio), High-order harmonics will cause the increase of stator copper loss, rotor copper (aluminum) loss, iron loss and additional losses of the motor. The most significant is the rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, after the high-order harmonic voltage cuts the rotor bars with a higher slip, very high rotor losses will occur. In addition, the additional copper loss caused by the skin effect must also be considered. These losses can cause the motor to generate additional heat, reduce efficiency, and reduce output power. For example, if an ordinary three-phase asynchronous motor is operated under the non-sinusoidal power supply output by the inverter, its temperature rise will generally increase by 10%-20%.
2. The insulation strength of the motor.
At present, many small and medium-sized inverters use PWM control, and their carrier frequencies are about several thousand to more than ten kilohertz. This requires the stator winding of the motor to undergo a very high voltage rise rate, which is equivalent to applying a voltage to the motor. The high-steepness impulse voltage puts the inter-turn insulation of the motor to a more severe test. In addition, the rectangular chopping impulse voltage generated by the PWM inverter is superimposed on the motor's operating voltage, which poses a threat to the motor's insulation to the ground. The insulation to the ground will age faster under repeated high-voltage shocks.
3. Harmonic electromagnetic noise and vibration When ordinary asynchronous motors are powered by inverters, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors become more complicated. The various time harmonics contained in the variable-frequency power supply interfere with the inherent spatial 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 inherent vibration frequency of the motor body, resonance will occur, thereby increasing the noise. Due to the wide operating frequency range of the motor and the high speed variation range, the frequencies of various electromagnetic force waves are difficult to avoid the inherent vibration frequencies of the various components 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 in a way without impact current, and can use various braking methods provided by the inverter for rapid braking, which creates conditions for frequent starting and braking. However, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating forces, which brings fatigue and accelerated aging problems to the mechanical structure and insulation structure.
5. Cooling problem at low speed
First, the impedance of the asynchronous motor is not ideal. When the power supply frequency is low, the loss caused by high-order harmonics in the power supply is high.
Secondly, when the speed of ordinary asynchronous motors is reduced, the cooling air volume decreases in proportion to the cube of the speed, causing the low-speed cooling condition of the motor to deteriorate, the temperature rise to increase sharply, and it is difficult to achieve constant torque output.
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