Breaker sizing for VFD

[thewellguy]

I install submersible motors for well pumps. According to the motor manufacturer a 20 Hp 460 3 phase motor can pull up to 30.3 amps and should be installed on a 70 amp breaker. My question is if I am putting the motor on a ABB VFD model acs550-u1-031a-4 which is only good for 31 amps, what size breaker do I need? I would think that if the VFD is only good for 31 amps then I would be able to put it on a 40 amp breaker. I have asked the pump manufactuer and gotten several different answer. Thanks for your help.

 

[alexit]

Per the drive manual:

ACS550-U1-031A Recommended Input Protection Fuses

IEC269 gG (A)= 40
UL Class T (A) = 40

See page 12

 

[jraef]

The breaker sizing from the pump mfr is based on assuming an Across-the-Line start. They have no other responsibility to interpret your application differently, unless of course they sold you the pump in a package with the VFD.

Looking at the Across-the-Line scenario, the motor starter is going to provide the running overload protection so all the breaker must do is to protect the conductors from short circuit; in breaker parlance that is the instantaneous magnetic trip function. That protection can be as high as 13 times the FLA, but is typically lower that that, but probably should be at least 6 time FLA to avoid nuisance tripping. Many thermal magnetic circuit breakers (the most common sort for field installations) do not have adjustments to the magnetic trip elements, and they are sometimes fixed at 4 times the amp rating. So a 40A TMCB might have mag trips set at 160A, too low to allow a motor with 30 FLA to start. A 70A breaker might have trips set at 280A, which still falls well within the 13x FLA for that motor, so it is unlikely to trip on startup.

All those issues are irrelevant when supplying a VFD however, the VFD will limit current to levels far lower and you do not need to worry about starting inrush creating nuisance tripping.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[thewellguy]

Thanks for the help