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Old 4th October 2010, 23:07   #1
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Default Technical query : How to find out the rating of a 3 Phase AC motor??

Hello

I have a work related query and google could not help me find an answer, so I thought I take the help of fellow TBhpians..

We have an old 3 phase A.C. Motor at work which does not have any model no./information plate on it. We dont know what is the rating of the motor!!

Our electricians best guess is that it is between 60 hp(45 kw) to 90 hp(67.5 kw). Due to the age of the motor (15 yrs+), he is unable to help us. The motor is in working condition.

Is there any way we can find out what is the rating of the motor in kw/hp?
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Old 4th October 2010, 23:42   #2
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ABhi - I'm no expert here, but I can suggest a simple method, but again don't ask me how to extend further.

Switch off all the appliances & run only the motor; note down the time & allow it to run such that it consumes exactly one unit. One unit is nothing but a 60W bulb glowing for 60 minutes. Do the math here or wait for some gurus to throw some more light on this.

This is the best suggestion I can provide you with my limited knowledge, sorry.
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Old 4th October 2010, 23:46   #3
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If the motor is connected to some equipment, check the power requirements / specifications of that (driven) equipment!!!!

There would be methods which require using only this motor as a load through a powe meter (electricity meter) and making calculations based on power consumed. But that would be extremely inaccurate.
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Old 5th October 2010, 16:10   #4
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There is no info on the equipment / motor which I can use...

Can I ascertain the motor rating from idling load? (if motor is disconnected from the machine)
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Old 5th October 2010, 17:05   #5
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Quote:
Originally Posted by AbhiJ View Post
There is no info on the equipment / motor which I can use... Can I ascertain the motor rating from idling load? (if motor is disconnected from the machine)
Hi AbhiJ... No, you can't use idling load to determine its fully-rated power. At idle there would be very little "slip" & the motor would settle down at its "no-load speed" - even if you measure it, the current draw would be very small.

You could, however, measure its full-load current-draw & figure out its rated power (not max). Is this motor a squirrel-cage motor ? Maybe you could post a pic, chances are that somebody would recognize it & post the specs.
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Old 6th October 2010, 00:27   #6
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It is a squirrel cage motor...

Will be posting pictures tomorrow...
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