The inverter output is controlled by PWM (Pulse Width Modulation, similar to high-speed switching), which results in high-frequency leakage current. If a general residual current circuit breaker is to be installed on the primary side of the inverter, it is recommended to select a residual current circuit breaker with a sensitivity current of 200mA or more and an operating time of 0.1 seconds or more for each inverter . However, it is not guaranteed that the residual current circuit breaker will not trip. The following factors must be considered to determine the magnitude of the system leakage current and to select an appropriate residual current circuit breaker and necessary measures to improve the phenomenon of residual current circuit breaker tripping after power is supplied.
The general formula for calculating the rated current of a residual current circuit breaker is as follows :
I△n≧10*〔Ig1+Ign+3*(Ig2+Igm)〕
Ig1, Ig2: Leakage current of cables during commercial operation.
Ign: Leakage current of the noise filter on the input side of the frequency converter.
Igm: Leakage current of the motor during commercial operation.
Based on the relevant variable parameters in the above formula, the factors that affect the magnitude of leakage current include:
(1) Leakage current of the cable (in two parts)
The leakage current is the length of the cable used for the leakage circuit breaker filter.
The leakage current of the inverter motor cable .
(2) Leakage current of the filter (including the frequency converter).
(3) Leakage current of the motor.
Leakage current values for each part (unit: mA)
(1) The leakage current of the cable = A * (actual cable length / 1000m); the cable manufacturer provides the leakage current value A per 1000m for each wire diameter.
(2) Leakage current of the filter (including the inverter) - provided by the inverter supplier. For example: Delta VFD055B43B uses a 26TDT1W4B4 filter, and its maximum leakage current is 70mA.
(3) Motor leakage current - provided by the motor supplier.
