All variable frequency drives (VFDs) include functions to protect them from damage caused by short circuits. The VFD’s internal protection comes from its powerful semiconductor and complex circuitry.

In multiple motor applications, NEC requires individual motor overload protection (thermal) on the load side of a VFD. Load-side protection is required because a single VFD can sense only its total connected load and not which individual motor is drawing high current.

Some challenges for overload protection devices at the output of VFDs include that for electronic overload relays, their sensor technology may not be able to measure the load current and harmonics correctly when operating at frequencies outside their nominal sensing range. For standard motor protection circuit breakers (MPCBs), challenges relate to reflection of voltage pulse that causes high dielectric stress on the MPCB magnetic trip coils, which results in accelerated aging.

Widespread use of VFDs in many industrial processes makes it more important than ever to understand how UL standards and testing relate to VFDs. This session will outline typical system requirements for short circuit protection of variable frequency drives, as well as address the issues involved.


Learning objectives:

  • Short circuit protection — what, why and how?
  • Typical scenarios for protecting drives
  • Short circuit current rating (SCRR) and amps interrupting capacity (AIC}
  • Evolution of UL standards and impact on testing