Variable frequency drive (VFD) is a kind of power control equipment which uses frequency conversion technology and microelectronics technology to control AC motor by changing the frequency mode of motor power supply.

The inverter is mainly composed of rectifier (AC to DC), filter, inverter (DC to AC), brake unit, drive unit, detection unit, micro processing unit, etc. The inverter adjusts the voltage and frequency of the output power supply by breaking the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, so as to achieve the purpose of energy saving and speed regulation. In addition, the inverter has many protection functions, such as over-current, over-voltage, overload protection, etc. With the continuous improvement of industrial automation, inverter has been widely used.

1. The problem of motor insulation strength

At present, many small and medium frequency converters adopt PWM control mode. Its carrier frequency is about several thousand to more than ten kilohertz, which makes the stator winding of the motor bear a high voltage rise rate, which is equivalent to imposing a steep impulse voltage on the motor, which makes the turn to turn insulation of the motor withstand a more severe test. In addition, the rectangular chopper impulse voltage generated by PWM converter superimposed on the motor operating voltage will pose a threat to the motor ground insulation, and the ground insulation will accelerate aging under high voltage repeated impact.

2. Harmonic electromagnetic noise and vibration

The vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex when the ordinary asynchronous motor is powered by frequency converter.

Frequency conversion power supply is to convert the alternating current in the city power supply through AC → DC → AC, and the output is pure sine wave, and the output frequency and voltage can be adjusted within a certain range. It is different from the variable frequency speed controller used in motor speed regulation and the common AC stabilized voltage power supply. The ideal AC power supply is characterized by stable frequency, stable voltage, zero internal resistance and pure sine wave (no distortion). The frequency conversion power supply is very close to the ideal AC power supply. Therefore, the frequency conversion power supply is increasingly used as the standard power supply in advanced developed countries, so as to provide the best power supply environment for the electrical appliances, and facilitate the objective assessment of the technical performance of the electrical appliances. There are two main types of variable frequency power supply: linear amplifier and SPWM switch. The time harmonics and the inherent space harmonics of the electromagnetic part of the motor interfere with each other, forming various electromagnetic exciting forces. When the frequency of electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, which will increase the noise. Due to the wide range of operating frequency and wide range of rotating speed, the frequency of various electromagnetic force waves is difficult to avoid the natural vibration frequency of each component of the motor.

3. Adaptability of motor to frequent starting and braking

Since the motor can be started without impact current at a very low frequency and voltage, and can be quickly braked by various braking methods provided by frequency converter, which creates conditions for frequent starting and braking. Therefore, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings about the mechanical structure and insulation structure Fatigue and accelerated aging problems.

4. Cooling problem at low speed

First of all, the impedance of asynchronous motor is not ideal. When the power supply is the device that provides power to electronic equipment, also known as power supply device, it provides the electrical energy required by all parts of the computer. When the frequency is lower, the loss caused by higher harmonics in the power supply is larger. Secondly, when the speed of ordinary asynchronous motor decreases, the cooling air volume decreases proportionally to the third power of the speed, resulting in the deterioration of the low-speed cooling condition of the motor, the rapid increase of temperature rise, and the constant torque output is difficult to achieve.