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#167398 - 08/11/07 05:47 PM Motor Oveload Protection
BigJohn Offline

Registered: 03/06/04
Posts: 352
Loc: Boston, MA
I recently had a work-order for a motor that was periodically tripping it's overload. Small 480V pump motor with a FLA of 1.0 and a service factor of 1.0 At the time that I looked at it, it was drawing 0.9A on every phase.

The maintenance guy I was with wanted to crank up the adjustable protection on the OL relay, which was set at 1A but topped out at 1.6A. My answer was that given that the SF of the motor was only 100%, while it may stop the tripping overloads, it would seriously reduce the life of the motor. Since this motor has been in service for some time without previously overloading, and considering that this is an intermittant problem, my suggestion was some sort of mechanical restriction in the pump, and probably not an electrical problem.

According to code it seems I was wrong. I don't remember what the actual HP rating of the motor was, but I'm guessing, based on the FLA that it's more than 1HP. So, according to 430.32(A)(1), it would have been safe to set the OL protection to as much as 115%? Were the OLs just set a too close to the operating range of the motor?

As an aside, does anyone have a good tutorial for motor starters and overloads? In case it's not really obvious, I'm not too familiar with them. For example, why does it seem like every motor starter setup has two contactors? One that has the OL protection in it, and then one down-stream from that?

Also, 430.32(A)(1): Motors with a marked temperature rise of 40 degrees C or less can have an OL rated at 125% FLA. Why would you be able to increase the size of the OL protection on a motor that wasn't able to tolerate as much of a heat increase?

Sorry about all the questions, just trying to understand all this.


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#167484 - 08/13/07 07:36 AM Re: Motor Oveload Protection [Re: BigJohn]
JBD Offline

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Why does evryone alsways assume the "overload" fails and the bearings never do?

Motor overload relays should always be sized based on the starter manufacturer's literature, not the NEC. The results should then compared the NEC requirements.

#167489 - 08/13/07 06:46 PM Re: Motor Oveload Protection [Re: JBD]
twh Offline

Registered: 03/11/04
Posts: 899
Loc: Regina, Sask.
If a motor with a rated FLA of 1 amp is drawing .9 amps, is tripping the overloads there are a few possibilities. Check the connections around the overload to make sure they are not loose or showing signs of heat. Check your ammeter to make sure it is accurate at that low of a current. Does the overload trip when you are watching or is something happening that you aren't seeing? Other than that, I would suspect a bad overload block.

Most overloads are rated for a motor with a service factor of 1.15. That is, the 125% is already calculated. It has been a very long time since I saw one that wasn't, and it was old when I saw it.

That having been said, I just put a bunch of Telemecanique overloads that nuisance tripped frequently and the commissioning people adjusted them up 25%, even though the literature indicated that was already done. I guess that, at some point, production gets priority.

I would guess that a 480 volt motor with a FLA of 1 amp would be .5 or .75 HP.

I'll leave the code questions to someone South of the 49th.

Edited by twh (08/13/07 06:47 PM)
Edit Reason: clarity


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