Direct connect fan motor overload ratings?

[AJ2002]

Is there a Code or Standard that allows motors that move air across itself to be overloaded more than the service factor? An example is a fin/fan motor that is rated at 3.2 amps full load (575Vac)operating at 3.8amps. This is an ~19% increase.

Thanks in advance for your comments/references.

 

[itsmoked]

Code shmode.

That's a substantial overload.

It all comes down to the motor temperature. If it is somehow below the maximum allowed you will be OK. This could be by the motor running in a cold environment or thru massive air flow. Or thru an overbuilt or lucky build.

If you can measure the temperature and find it acceptable then you're good to go.

If it is over temperature then you are rapidly carving running hours off.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yes, but you would need to measure the temperature on the INSIDE of the motor, i.e. at the windings. Outside (case) temeprature is not always an accurate means of determining winding hot spots.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

True.

Are the Temp Rise numbers printed on the nameplate referring to some random unspecified internal location or the case temperature? Seems since the rest of the plate info seems to be for the guy wrenching the motor into its location that putting a rise number on that cannot be measured nor even guessed about, would be pretty useless to me.

I expected them to refer to the motor's external case after thermal equilibrium is reached and excluding non normal conditions like having high velocity air blown on the case.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Not a random unspecified internal location but a very specific internal location known as the hot spot. That location which gets the hottest. Whether or not the manufacturer actually knows where it is or not is a different matter.

 

[jraef]

Were you to order the motor with embedded RTDs, you could determine the hot spot (within reason). But as to the information on the nameplate, that is for determining the design of the insulation. Anything relevant to that is essentially inferred. By that I mean that if it says Class H insulation, that gives you an idea of ambient temperature performance relative to Class F insulation, but without embedded sensors, it is otherwise useless information to the average wrench turner. Someone with a lot of experience and a really good sensitive ohmeter could possibly determine actual temperature, but I have never seen anyone do that.

Motor temperature information link


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

What a shame. Obviously they should've conferred with me before the industry standard was set.

Thanks for the article Jeff. Interesting read. I may try that sometime since I have an ohmmeter that reads way low.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

I'll put your name on the next NEMA standards committee nomination list.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[fangas]

When I was testing motors "rise by resistance" was always used. Even when we had implanted T/C's in the windings.

It is pretty tricky though. The motor has to sit in the same, well know, temperature (25C) for at least a day, preferable more. Cold esistance is taken. Then the motor is run at load until it is stable (usually a few hrs). Then disconnected. And then hot resistance taken immediately, with a millohmeter, using Kelvin leads.

Immediately, means, right now! Gotta be quick, as the resistance will start changing the instant power & load is removed. And invalidate your measurements.

Ed

 

[itsmoked]

[red]"Then the motor is run at load until it is stable"[/red]

Noted how? External case temp?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

That explains why I have never seen it done in the field. I didn't really know all the details of what was involved. Thanks.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[fangas]

Keith,

We attached T/C's to the end turns and on the frame, Both PE and OP ends. they in turn ran into a chart recorder. Stable was defined as flat lined for 1 hr.

there was also a T/C hanging loose, in free air, to track the ambient and make sure that it hadn't effected the heat run.

Ed

 

[itsmoked]

fangas: Nice test!

Can you comment on any number you saw as the T_diff between your generally calculated winding temp verse the outer case temps?

A typical difference? It would be nice to have some rough idea for someone stuck out in the trenches trying to make a call.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fangas]

Sorry Keith,

I'ts been waaay too long since I've seen any numbers. I'm not really sure there is any.

There's too many varibles involved. Hp., Frame size, Frame materials, TENV, TEFC, and .......

Ed

 

[ozmosis]

I thought there is a code within NEC that defines Motor Overload Protection (430.31 - 430.44). These devices measure current and will assume the motor temperature running is high if the motor is overloaded. How come the overload has not protected the motor in this instance? it doesn't know what the ambient temperature is, it is simply looking at the drawn current.
I know this is not answering the original question but if the motor is pulling too much current then you should protect it, whether it is getting too hot or not.
or have I missed something somehwere?

 

[rovineye]

It is fairly simple in my world. IEEE Std 45 (shipboard) requires the overload to be set at 115% of motor full load current rating. Makes no difference as to sf.