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Variable frequency drive leakage current

In I/O cables and motors of a variable frequency drive (VFD), the stray capacitance C is present. High-speed switching in the main circuit lets the leakage current flow from the stray capacitance C to the ground. The amount of the leakage current differs depending on the stray capacitance C, carrier frequency, etc. Accordingly, the low-noise type variable frequency drives generate the larger leakage current.

Effects of leakage current
1) The leakage breaker or leakage relay may operate unintentionally due to the leakage current between grounds. The leakage current includes many high frequency components that have a relatively small influence on human body.
2) The external thermal relay may operate unintentionally due to the leakage current between cables.
VFD leakage current

Measures
  • Decrease the carrier frequency of the variable frequency drive. (Note that doing so causes a louder motor noise.)
  • Install leakage breakers designed for the harmonics and surge suppression to the variable frequency drive's own system and other system. (Switching speed does not have to be decreased.)
  • Decrease the stray capacitances between grounds. (Use cables or wires that are insulated with materials of low relative permittivity.)
Selecting a leakage breaker
Select a leakage breaker designed for the harmonics and surge suppression. The main task of a current circuit breaker or a current protection device in general is to interrupt a circuit in the event of a variable frequency drive malfunction. The electronic switch measures the incoming and outgoing current of a device or system. If it establishes that these two currents are not identical, it will interrupt the circuit, as the current flow has (possibly) taken a hazardous direction somewhere - due to an insulation failure.

This current can be caused by a person touching a conductor and the body of this person directing the circuit towards the earth. The current circuit breaker protects the person by very quickly interrupting the circuit, thereby preventing any further injury.

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Variable Frequency Drives control AC motor for energy savings by adjustable speed, for short VFD, also named variable speed drives and frequency inverter.