Analysis of leakage problems in frequency inverters

1. Causes of leakage in frequency inverters

According to the functional block diagram of the frequency inverter controlling the operation of the motor (above), the three-phase power supply is rectified by the frequency inverter rectifier bridge, filtered by capacitors and sent to the frequency inverter bridge (IGBT), and then outputs three-phase AC with adjustable frequency and voltage to control the operation of the motor through the frequency inverter bridge.

Three AC currents with a phase difference of 120 degrees flow through the three-phase stator coil winding of the motor, generating a rotating magnetic field, so that the rotor of the motor automatically rotates under the action of the rotating magnetic field of the stator winding.

After the current flows through the three-phase stator winding of the motor, a rotating magnetic field is generated, and according to the principle of electromagnetic induction, the shell of the motor will generate an induced electromotive force. The size of this induced electromotive force depends on the switching frequency of the frequency inverter IGBT and C×DV/DT (related to the switching speed of the IGBT). If this induced electromotive force is large, people will feel like being electrocuted when they touch it. Theoretically, the higher the switching frequency of the IGBT, the higher the effective value of the induced electromotive force (induced voltage) of the motor housing, and the higher the control accuracy and dynamic response of the frequency inverter to the motor, and the greater the feeling of being electrocuted after the human body touches it; conversely, the lower the switching frequency of the IGBT, the lower the effective value of the induced motive force (induced voltage) of the motor housing, and the smaller the feeling of being electrocuted after the body touches it.

Since the asynchronous motor is running, there will be induced voltage (the so-called leakage) on the motor housing, so the motor manufacturer will install a grounding terminal in its junction box when the motor leaves the factory, so that the user can connect to the ground when using it to eliminate its induced electromotive force (eliminate the induced leakage voltage) to solve the feeling of being electrocuted when the human body touches the motor.

When the motor is running at industrial frequency, the switching frequency of the industrial frequency is about 50HZ, which is very low, so under normal circumstances, there is almost no feeling of leakage (unless the motor insulation is very poor). When the frequency inverter is controlled, because its switching frequency is much higher than the industrial frequency, when the frequency inverter controls the motor to rotate, the motor housing will feel leakage.

2. Factors affecting the magnitude of leakage current

The frequency inverter output is controlled by PWM (pulse width modulation, similar to high-speed switching), so high-frequency leakage current will occur. The following are factors that determine the magnitude of system leakage current

(1) Leakage current of the cable

Leakage current of the leakage circuit breaker and the cable length of the filter.

Leakage current of the cable length of the frequency inverter and motor.

(2) Leakage current of the filter (including the frequency inverter).

(3) Leakage current of the motor.