Why adding a motor filter?

The high-speed motor drive is a power supply that provides voltage regulation and frequency regulation for high-speed motors. It generally consists of a rectifier circuit, a smoothing circuit and an inverter circuit. Among them, the inverter circuit is to invert the direct current into alternating current with adjustable frequency and amplitude through the inverter bridge formed by the power transistor, which is the core part of the converter. The output voltage signal of the inverter circuit uses the change of the pulse width to obtain the sinusoidal fundamental wave voltage with different amplitudes, which is what we usually think of as a PWM wave. The waveform is shown in the figure below. The high-frequency voltage signal has clutter of various frequencies, which causes the current ripple to be too large and affects the motor heating. The high-frequency PWM wave will also cause the voltage signal to reflect on the cable, and the voltage spike at the motor terminal to damage the motor insulation. Therefore, for special scenarios, a motor filter must be added to avoid the above problems.

There are following options on the output side of the drive:

1) Output reactor, when the length of the cable output from the converter to the motor is greater than the specified value of the product, an output reactor should be added to compensate for the charging and discharging effect of the coupling capacitor, when the long cable of the motor is running, to avoid overcurrent of the converter. The function of adding an output reactor at the output end of the converter is to increase the cable distance from the converter to the motor. The output reactor can effectively suppress the instantaneous high voltage generated when the IGBT of the converter switches on and off, and reduce the adverse effects of this voltage on cable insulation and motor. . At the same time, in order to increase the distance between the converter and the motor, the cable can be appropriately thickened to increase the insulation strength of the cable, and unshielded cables should be selected as far as possible.

2) Output dv/dt reactor, output dv/dt reactor is to limit the rise rate of the output voltage of the converter to ensure the normal insulation of the motor.

3) Sinusoidal wave filter, which makes the output voltage and current of the converter approximate to a sinusoidal wave, reducing the harmonics THD coefficient of the motor and the insulation voltage of the motor.