exceeding motor fla 1

[thekman]

I've checked the data sheet for a paricular motor, but not seeing anything, tried google,etc.....got a 3HP 208-230 3ph w/ FLA of 8.4a. I'm looking for a way to figure how long a motor this size's FLA can be exceeded. (other than startup)

I will see these motors running as high as 175% of FLA for several seconds, and there is no apparent damage, but what is the long term damage? How can I figure the longest time at which what current will be ok? I hope my question makes sense...I'm looking for some sort of current/time curve...if one exists. Maybe even a rule of thumb...

 

[thekman]

actually more like 150%....(not 175%)

 

[waross]

Try some RMS HP calculations. See the Cowern papers for an explanation.

http://baldor.com/support/literature_load.asp?LitNumber=PR2525

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

 

[thekman]

Thanks Bill, I'll have a look.

 

[jraef]

If you look at a NEMA Class 10, 20 or 30 OL curve, that gives you an approximation of the thermal damage curve of the winding insulation. In most cases it's fairly conservative actually, so if you never exceed it, you should be good.

So which one do you use? The motor mfr would be the best source, but IN GENERAL, Class 10 would be for 99% of IEC (metric frame) motors and most submersible pump motors, Class 20 is good for most standard NEMA (North American) motors, and Class 30 would be for what are typically called "Mill Duty" motors (somewhat rare). So if you are picking out a 208-230V motor rated in HP, that is most probably a NEMA motor, so look at a Class 20 OL trip curve, you can get them from most motor controller manufacturers. In essence, Class 20 means it will guarantee tripping in 20 seconds at 600% current. The other end of the curve is usually 2 hours at 140% FLA (IIRC), then it's an i²t curve between them.


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[electricpete]

Some items from NEMA MG-1 may also be relevant:
12.48 OCCASIONAL EXCESS CURRENT
Polyphase motors having outputs not exceeding 500 horsepower (according to this part) and rated voltages not exceeding 1kV shall be capable of withstanding a current equal to 1.5 times the full load rated current for not less than two minutes when the motor is initially at normal operating temperature.
12.49 STALL TIME
Polyphase motors having outputs not exceeding 500 horsepower and rated voltage not exceeding 1kV shall be capable of withstanding locked-rotor current for not less than 12 seconds when the motor is initially at normal operating temperatures


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[thekman]

1.5x for < 2min...is that on a curve by chance, or just a statement they have? I appreciate the help!!

 

[LionelHutz]

NEMA MG-1 just gives times at certain current levels.

The typical overload curve is created assuming the motor heating will follow an I^2 x t curve.

The statement was 150% for at least 2 minutes, not 150% for less than 2 minutes.

 

[jraef]

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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