VFD load side components

[aNerdyEngineer]

Our VFDs are normally run at 2kHz carrier frequency running one or multiple motors up to 60Hz (typically). If I have to provide certain components such as breakers or overlaods on the LOAD side of the VFD, do these components need to be rated anything more than the 50/60Hz that I typically see on Square-D breakers?

 

[Jraef]

Technically, yes. But not because of the 2kHz carrier, although that does have an effect, but rather because the devices are technically rated for 50/60 Hz operation, so using them at anything less is a 110.3 violation.

In reality, the high CF and harmonic rich output of a VFD can cause additional heating in bi-metal current sensing elements, such as OL relays and thermal trips on CBs and MPCBs. So what can happen is not failure to trip but rather nuisance tripping. Then the problem become the human response to nuisance tripping; turning up the dial, which then exposes a risk of it NOT tripping when there really is an overload. Side note: the old eutectic melting alloy OL relays are rated for operation at any frequency up to 400Hz.

Some suppliers are now making protective devices tested and listed for use behind VFDs. A-B now has a version of their 140M Motor Protection Circuit Breakers called the “D8V” that is specifically rated for use down stream of VFDs. I’ve heard others are offering that too or will be soon.

 

[aNerdyEngineer]

Thanks Jraef. Sadly in my scenario, I have quite a bit higher amperage than the 140M are limited to. I think the Schneider GV2, GV3, etc. are also similar to AB 140Ms, but not VFD rated however. I also have the issue that I'd rather not have the circuit open on a thermal trip. OL relays are automatically reset-able and already stop our drives from the NO/NC contacts the OL relays have. I was thinking of keeping in the OL relays, but adding in breakers just simply for short circuit and over-current protection of the wires, but I don't want the breaker to trip under normal operation. Our panels are up on cranes and not always accessible to do manual reset.