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Deliberate O/L Due to Motor Service Factor?

Deliberate O/L Due to Motor Service Factor?

Deliberate O/L Due to Motor Service Factor?

(OP)
My greetings to all

Is it correct, that for a given 100% mechanical load, a motor of a lower HP rating maybe used as long as the overloading does not go beyond the motor service factor?

For example, I have a 80HP mechanical load, say a 100HP at S.F. = 1.0 could have been sufficient, but one may say that a 75HP motor at S.F. = 1.15 can be used.

Is this correct even if the motor will be operated continuously at 80HP?

RE: Deliberate O/L Due to Motor Service Factor?

There is no clear definition of what will occur when running in the service factor beyond it being acceptable to expect some kind of reduction in motor life. So, my answer would be that is is not acceptable. Others may feel it is acceptable, especially those looking to make a few bucks by specifying the smaller motor.

RE: Deliberate O/L Due to Motor Service Factor?

As long as the motor has proper overload protection, I suppose this would be "allowed" per the NEC.  However, as LionelHutz says, you can expect a reduced motor life when operating continuously in the service factor region.  I would consider it a poor design.  

The service factor rating is intended to account for unknowns and unexpected changes in loading, generally brief in nature.   

David Castor
www.cvoes.com

RE: Deliberate O/L Due to Motor Service Factor?

I would consider the nature of the mechanical load. Steady or varying, also the actual mechanical load versus the calculated mechanical load.
Is this a critical load?
Then consider the stability of the energy supply, are low voltage conditions common?
Will this motor run 24/7, 40 hours per week or less.
What are the costs and difficulties if the motor must be up sized to 100 HP at a later date? (The load information may not have been precise). Is it a "drop-in" change or must mounts and gearboxes be changed.
Location, is this motor close to the service or is it on the end of long conductors where voltage drop may be a factor.
I may compare the frame sizes of a 75 HP/SF 1.0, a 75 HP/SF 1.15 and  a 100 HP/SF 1.0 motor.
Lastly compare the price.
Some may consider it prudent to consider these factors whenever a motor is to be closely matched to load requirements.
After determining and considering these factors you may make an informed decision and justify your decision should your design be challenged.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Deliberate O/L Due to Motor Service Factor?

I wouldn't do it unless the supplier said it was okay.

I have done it.  ... Ran a 10HP CNC spindle at 13HP for 6 seconds during a much longer cycle, as documented allowable by the CNC supplier.  Great fun putting aluminum chips on the ceiling...

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Deliberate O/L Due to Motor Service Factor?

Just to chime in on what has already been said, "Service Factor" has had a somewhat nebulous definition over the years and people take it for what they want it to mean for the most part. As a general rule, I think what dpc said is what USED TO be the "official" NEMA MG-1 statement on SF; that you can use it continuously, but you should expect reduced motor life and performance. Before that, it used to say something like "for a short time", without definition, but they removed that. As of the most recent changes however (I don't have my copy here), even that expectation of reduced life and performance statement has been watered down to be even more meaningless. Most likely that was a way of appeasing motor mfrs selling to equipment OEMs who wanted to continue selling marginally rated motors to keep the cost down.

Bottom line, what you decide on will have long term effects on total cost of ownership and that should be given MORE weight than initial capital expense in my humble opinion.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
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