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VFD Electromagnetic Interference

Electromagnetic interference (EMI), also called electrical noise, is the unwanted signals generated by electrical & electronic equipment. EMI VFD problems range from corrupted data transmission to electric motor drive damage. Modern VFDs using IGBT switches for motor frequency control are very efficient because of their high switching speed. Unfortunately the high-speed switching also results in much higher EMI being generated. All VFD manufacturers detail installation procedures that must be followed in order to prevent excessive noise on both sides of the VFD. Some of these noise suppression procedures include the following:
  • Use a shielded power cable to connect the variable frequency drive to the motor.
  • Use a built-in or external EMI filter.
  • Use twisted control wiring leads to provide a balanced capacitive coupling.
  • Use shielded cable to return the noise current flowing in the shield back to the source, instead of through the signal leads.
  • Maintain at least 8-inch separation between control & power wires in open air, conduit, or cable trays.
  • Use a common-mode choke wound with multiple turns of both signal & shield.
  • Use optical isolation modules for control signal communications
Inherent in all motor cables is line-to-line & line-to ground capacitance. The longer the cable, the greater is this capacitance. Electrical spikes occur on the outputs of PWM VFDs due to currents charging the cable capacitances (how to select VFD cables?). Higher voltages, such as 460 VAC, along with higher capacitances, result in larger current spikes. These spikes can shorten the lives of inverters & motors. For this reason cable length must be limited to that recommended by the manufacturer.

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