It is necessary to consider the following factors to determine the leakage current of the system, and to select appropriate leakage circuit breakers and necessary measures to improve the phenomenon of leakage circuit breaker tripping after power transmission. 

It is necessary to consider the following factors to determine the leakage current of the system, and to select appropriate leakage circuit breakers and necessary measures to improve the phenomenon of leakage circuit breaker tripping after power transmission. 

The frequency converter is a high frequency square wave voltage output, because there is an equivalent capacitance between the internal coil of the motor and the motor shell, thus generating leakage current. If there is no grounding or poor grounding, there will be leakage.

The calculation formula for selection of rated current of general leakage circuit breaker is as follows: 

I△n ≧ 10*〔Ig1+Ign+3*(Ig2+Igm)〕 

Ig1、Ig2：Leakage current of cable in commercial operation. 

Ign：Leakage current of noise filter at input side of frequency converter. 
Igm：Leakage current of motor in commercial operation. 

According to the relevant variation parameters of the above formula,The factors that will affect the leakage current are： 

• Leakage current of cable (two parts) 
  Leakage current of cable length of leakage circuit breaker and filter. 
  Leakage current of cable length of frequency converter and motor

• Leakage current of filter (including frequency converter)

• Leakage current of motor 

Leakage current value of each part (unit: MA)：

• Leakage current of cable = a * (actual cable length / 1000m); cable manufacturer shall provide leakage current value a for every 1000m of each wire diameter.

• Leakage current of filter (including frequency converter) - provided by frequency converter supplier. For example, the filter used by delta vfd055b43b is 26tdt1w4b4, and the maximum leakage current is 70ma.

• Leakage current of motor - provided by motor supplier.

Some sites use frequency converter to control the motor, there will be leakage problems, leakage voltage has dozens of volts to 200 volts.In view of this problem, here is a theoretical analysis and explanation of the causes of this failure.

According to the functional block diagram of inverter control motor operation (Figure 1), the three-phase power supply is sent to IGBT through capacitor filter after rectifier bridge of inverter, and then to control motor operation through three-phase alternating current with adjustable output frequency and voltage of inverter bridge.

The three-phase alternating current with 120 degrees difference flows through the three-phase stator winding of the motor, generating a rotating magnetic field, which makes the motor rotor automatically rotate under the effect of the rotating magnetic field of the stator winding。

我As we all know, the three-phase stator winding of the motor generates a rotating magnetic field after passing through the current, and according to the principle of electromagnetic induction, the shell of the motor will generate induced electromotive force. The magnitude of the induced electromotive force depends on the switching frequency of IGBT and c * DV / dt (related to the switching speed of IGBT); because the high-performance control requires a high switching frequency, the switching speed is required to be faster, then the DV / DT is larger.

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 the induced electromotive force (i.e. the induced voltage) of the motor shell is, and the higher the control accuracy and dynamic response of the frequency converter to the motor is, the greater the feeling of being electrified after touching the human body; on the contrary, the lower the switching frequency of IGBT is, the lower the effective value of the induced electromotive force (the induced voltage) of the motor shell is, and from the body touch to Then the feeling of being electrified is smaller.

Therefore, the switching frequency of some domestic low-end inverter IGBT is designed to be low. After controlling the motor, the induction voltage of the motor shell is low, but its control performance is poor and its dynamic response is slow. The performance and dynamic response of our inverter are better, so the switching frequency and switching speed of IGBT are higher, and the induced electromotive force is higher.

Because the asynchronous motor runs, the motor shell will have induction voltage (so-called leakage), so the motor manufacturer will install the grounding terminal in its junction box when the motor leaves the factory, so as to facilitate the user to connect the earth during application to eliminate its induction electromotive force (i.e. eliminate the induction leakage voltage), so as to solve the feeling of being electrified when the human body contacts the motor.

Of course, because the switching frequency of the power frequency is about 50Hz when the power frequency operates the motor, it is very low, so in general, there is almost no sense of leakage (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 sense of leakage.