1、 The inverter output is controlled by PWM (pulse width modulation, similar to high-speed switch), so leakage current with high frequency will occur.

The PE points of frequency converter, front power supply transformer and motor must be connected together, and then be grounded.

The frequency converter is a high frequency square wave voltage output, because there is an equivalent capacitance between the motor inner coil and the motor shell, resulting in leakage current. If not grounded or poorly grounded, there will be leakage.

The factors affecting the leakage current are as follows:

(1) Leakage current of cable, leakage current of leakage breaker and filter. The leakage current of the cable length of frequency converter and motor.

(2) Leakage current of filter (including frequency converter).

(3) Leakage current of motor.

Some site use frequency converter control motor, there will be leakage problems, leakage voltage has dozens of volts to 200 volts. In view of this problem, the causes of this phenomenon are analyzed as follows:

2、 According to the functional block diagram of motor operation controlled by frequency converter, the three-phase power supply is transmitted to the inverter bridge (IGBT) after being rectified by the converter rectifier bridge, and then the motor is controlled by the three-phase AC with adjustable output frequency and voltage of the inverter bridge.

The alternating current with a difference of 120 degrees between three phases flows through the three-phase stator coil winding of the motor to produce a rotating magnetic field, which makes the rotor of the motor rotate automatically under the action of the rotating magnetic field of the stator winding.

After the three-phase stator winding of the motor flows through the current, the rotating magnetic field is generated. According to the principle of electromagnetic induction, the shell of the motor will produce the induced electromotive force. The magnitude of the induced electromotive force depends on the switching frequency of IGBT and C × DV / dt (related to the speed of IGBT switch). If the induced electromotive force is large, people will feel shocked when they touch it. Theoretically, the higher the switching frequency of IGBT is, the higher the effective value of induced electromotive force (i.e. induced voltage) of motor shell, and the higher the control accuracy and dynamic response of frequency converter to motor, the greater the feeling of being electrified after human touch; on the contrary, the lower the switching frequency of IGBT, the lower the effective value (induced voltage) of induced electromotive force (induced voltage) of motor shell, and the higher the control precision and dynamic response of inverter to motor And then the less you feel electrified.

Due to the operation of asynchronous motor, the motor shell will have induced voltage (i.e. the so-called leakage). Therefore, the motor manufacturer will install a grounding terminal in the junction box when the motor leaves the factory, so as to facilitate the user to connect the ground to eliminate its induced electromotive force (i.e. eliminate the induced leakage voltage), so as to solve the feeling of being electrified when the human body contacts the motor.

When the motor is running at power frequency, the switching frequency of power frequency is about 50 Hz, which is very low, so there is almost no sense of leakage in general (unless the motor insulation is very poor). When the frequency converter is controlled, the switching frequency is much higher than the power frequency, so when the frequency converter controls the motor rotation, the motor shell will have the feeling of leakage.