Difference between variable frequency motor and ordinary motor

In many situations, many mechanical devices require frequency conversion speed regulation for electric motors. Some companies choose frequency conversion speed regulation motors for speed regulation, while others directly use the most common three-phase asynchronous motors for speed regulation. What are the differences between frequency conversion motors and ordinary motors? Let's take a look at three major aspects below.

I. Ordinary asynchronous motors are designed for constant frequency and constant voltage, and cannot fully meet the requirements of frequency conversion speed regulation. The following are the effects of the frequency converter on the motor:

1. Motor efficiency and temperature rise issues

Regardless of the type of frequency converter, harmonic voltages and currents of varying degrees are generated during operation, causing the motor to operate under non-sinusoidal voltage and current. According to the information, taking the commonly used sinusoidal PWM type 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 an increase in the stator copper loss, rotor copper (aluminum) loss, iron loss, and additional losses of the motor, the most significant being the rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to the base frequency, the high-order harmonic voltage will cut the rotor bars with a large slip, resulting in large rotor losses. In addition, the additional copper loss caused by the skin effect must also be considered. These losses will cause the motor to generate extra heat, reduce efficiency, and reduce output power. If an ordinary three-phase asynchronous motor is operated under the non-sinusoidal power supply output by the frequency converter, its temperature rise will generally increase by 10%--20%.

2. Motor insulation strength issues

Currently, many small and medium-sized frequency converters use PWM control. Its carrier frequency is about several thousand to ten thousand Hz, which causes the motor stator windings to withstand a very high voltage rise rate, which is equivalent to applying a very steep impact voltage to the motor, putting a severe test on the inter-turn insulation of the motor. In addition, the rectangular chopper impact voltage generated by the PWM frequency converter superimposed on the motor operating voltage will threaten the motor's ground insulation, and the ground insulation will accelerate aging under repeated high-voltage impacts.

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, and ventilation factors will become more complex. The various time harmonics contained in the frequency converter power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces. When the frequency of the electromagnetic wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, thereby increasing the noise. Because the operating frequency range of the motor is wide and the speed change range is large, it is difficult for the frequencies of various electromagnetic waves to avoid the natural vibration frequencies of various components of the motor.

4. Motor's adaptability to frequent starting and braking

Due to the use of frequency converter power supply, the motor can start at a very low frequency and voltage in a way without impact current, and can use various braking methods provided by the frequency converter for fast braking, creating conditions for frequent starting and braking. Therefore, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating forces, bringing fatigue and accelerated aging problems to the mechanical structure and insulation structure.

5. Cooling problems at low speeds

First, the impedance of the asynchronous motor is not ideal. When the power frequency is low, the loss caused by high-order harmonics in the power supply is large. Second, when the speed of an ordinary asynchronous motor is reduced, the cooling air volume decreases proportionally to the cube of the speed, causing the low-speed cooling condition of the motor to deteriorate, the temperature rise increases sharply, and it is difficult to achieve constant torque output.

6. Working principle of frequency conversion motor

The following figure (a) is a photo of a disassembled fan motor. The fan uses a frequency conversion motor, which can be identified from the location of the coil. The following figure (b) is the control circuit board of the frequency conversion motor. The control chip integrates DSP functions and drivers, simplifying the circuit structure. By programming the control chip, the motor speed can be changed.