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Some discussion about howling of the inverter when the motor is running at high frequency

Jan 19, 2021

Some discussion about howling of the inverter when the motor is running at high frequency

The output cable contains considerable high-order harmonic voltage and current, which causes the input voltage of the motor to be distorted, and the stator and rotor electromotive force high-order harmonics further increase. As a result, the phase electromotive force is severely distorted, and the * value rises a lot, causing the motor coil Severe heating, aging and even breakdown of insulation.


    In addition, the high-order harmonic magnetic field generated by the high-order harmonic generates additional torque, which makes the motor produce obvious vibration and sharp noise. High-order harmonics greatly shorten the mechanical life and insulation life of the motor.


    The harmonics output by the inverter are the main reason for the high-frequency whistling of the motor. When the modulated pulse frequency of the power switch of the inverter is increased, the output current is closer to the sine wave, and the frequency of the whistling sound is increased by an extraordinary. The audible sound wave range of the ear will greatly reduce the howling sound, but when the switching frequency of the IGBT is too high, it will increase the loss on the IGBT. At this time, the inverter needs to be derated for use. Generally, when the switching frequency of IGBT is 4Khz, if you hear howling, it is recommended to increase the switching frequency of IGBT appropriately.


    Now the whistling sound of variable frequency motors in operation often gives people an illusion that it is caused by frequency modulation of the inverter, but it is not necessarily. You should first confirm the source of the howling sound, and then deal with it.


    *Step, the inverter adopts V/F control to run the motor to the maximum speed;


    The second step is to operate OFF2 of the inverter to free stop;


    In the third step, it is judged that the motor speed decreases with the free stop process.


    If the howling or vibration still exists, then there is nothing to control the inverter. It is a problem with the production of the motor, and there is a resonance point in the operation of the mechanical transmission. It shows that it is a mechanical manufacturing problem, not an electrical control problem.


    If the motor runs smoothly and there is no whistling during the free stop process, that is the inverter control problem, which can be solved by adjusting the control parameters of the inverter. One of the most important is to use the frequency converter to calculate the dynamic and static mathematical models of the motor. If the mathematical model established by the inverter is accurate, the running quality will be good. For controlling the motor, its electromagnetic noise is completely acceptable. There is no doubt about this.


    Knowledge extension


    The matching problem of inverter and motor


    Choosing the right inverter for the entire motor motion system is a headache for engineers.


    In general, the selection of the frequency converter should be considered in accordance with the type of controlled object, speed range, static speed accuracy, starting torque, etc., so as to meet the process and production requirements, while being easy to use and economical. .


    The general experience is: choose the inverter as large as the motor, and sometimes it can be a larger specification.


    The high-power inverter has a lower power factor. *Add an AC reactor at the incoming line of the inverter. One way is to improve the power factor, and the other is to suppress high frequency harmonics.


    If you frequently start and brake frequently, install a brake unit and a brake resistor.


    If you need to reduce noise, you can choose a water-cooled inverter;


    If braking is required, a braking chopper and braking resistor are required. Or you can choose four-quadrant products, which can feed back energy to the grid and save energy;


    If there is only a DC power supply on site, you can choose a pure inverter product (using DC power supply) to drive the motor.


    The ultimate basis for inverter selection is that the current curve of the inverter includes the current curve of the mechanical load.