Oversized Motor

[Kiljoy]

A little background first:

We have been purchasing a centrifugal pump/motor combination for several years. We’ve frequently had problems in the field with the branch circuit fuses blowing during startup. They are Type J and sized at 175% of nameplate FLA. That being said, the motor is a 460Vmotor that has nameplate FLA of 2.9 and it runs about 3.1A while pumping hydraulic oil.

Anyway, I went out to get the locked rotor code from the motor nameplate on the power unit so I could calculate inrush for fuse sizing. It was blank as were a few other bits of typical information. I wrote the model and catalog numbers down and could not find them anywhere in the catalog. So, I called our Leeson dealer suspecting a custom motor. I was correct. The pump manufacturer is apparently taking a 3HP Jet Pump motor and de-rating it to 2HP on the nameplate so they can get a 50 deg. C rating versus 40 deg. C.

Has anyone ever heard of this before? I’m certainly going to bump the fuse size up a bit on future machines, but I’m afraid an inspector might have an issue at some point since they would not be aware of the “de-rating”. Nameplate FLA for this motor (as standard) is 3.8A.

Am I assuming correctly that a 3HP motor will have more inrush than a 2HP regardless of the size stamped on the nameplate? I’m wondering how much difference the 1HP would make for a 5A type J fuse.

 

[jraef]

Yes, de-rating for temperature is done all the time.

Yes, it is effectively a 3HP motor when starting and yes, that can pose some fusing / circuit breaker sizing issues with regards to magnetization current, which is likely what is causing your fuses to blow. If I'm not mistaken, the NEC allows you to up-size the SCPD if you can demonstrate that a lower rating will not allow the motor to start. This is assuming of course that you have a motor controller with thermal overload protection in it. Remember, in a motor controller circuit, it is the OL relay that is protecting both the motor and conductors from long-term over current, the fuses or CB are only providing short circuit protection.

But if your situation is one of those where someone is trying to get away with sizing the fuses to also act as the OL protection, then you have fallen into the folly of that approach.


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[rbulsara]

Code does not preclude using a larger fuse to accommodate starting. I would just use a larger fuse, it is only for short circuit protection. Small motors routinely have 15 or even 20A fuses/circuit breakers as long as they have proper overload protection.

And yes, if the motor is de-rated for temperature, it would not change its load/torque characteristics and for me it would be a legitimate reason to upsize the fuses. Not all motor mfrs make design changes to every size of their products. I had routinely seen 15 HP motors for 12.5HP motors (just a nameplate change).

Rafiq Bulsara

http://www.srengineersct.com

 

[Kiljoy]

Thanks guys! It certainly explains our ongoing problem in the field. I suspect that when the oil is cold, we have more problems. Once the machine is in production, the oil stays pretty warm and probably never cools down enough for a problem past initial startup. Typically, the machines that use this circulation pump have a larger production output and are rarely shut down for more than a few hours. The hydraulic tanks are about 400 gallons.