# Rule of thumb?



## industrial951 (Jan 29, 2011)

Hey gotta quick question, is there a rule of thumb on measuring motor winding resistance? Do you guys always refer to manufacture specs or can you automatically tell by the size of the motor your measuring what type of reading your going to get. In example what would be the winding resistance for a 240volt single phase, 5hp motor? Thx for any replies


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## McClary’s Electrical (Feb 21, 2009)

industrial951 said:


> Hey gotta quick question, is there a rule of thumb on measuring motor winding resistance? Do you guys always refer to manufacture specs or can you automatically tell by the size of the motor your measuring what type of reading your going to get. In example what would be the winding resistance for a 240volt single phase, 5hp motor? Thx for any replies


 

The only rule of thumb is , the bigger the motor, the less the resistance. I've seen from 1/2 ohm to 900.

I would expect a 240 volt motor 5 hp to be around 17-25 ohms, but that could really vary from man. to man.


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## Dennis Alwon (May 9, 2009)

I would look at the amperage on the nameplate and divide that into the voltage.

Say 28 amp motor at 240V V= I x R or R= V/I

240/28 = 8.58 ohms

I would probably call the manufacturer


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## nitro71 (Sep 17, 2009)

industrial951 said:


> Hey gotta quick question, is there a rule of thumb on measuring motor winding resistance? Do you guys always refer to manufacture specs or can you automatically tell by the size of the motor your measuring what type of reading your going to get. In example what would be the winding resistance for a 240volt single phase, 5hp motor? Thx for any replies


I think you're gonna need manufacturer specs. Check amperage on the motor. If it's high check make up in the motor. Then I'd take the load off it and check amps again. Megger couldn't hurt either.


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## MDShunk (Jan 7, 2007)

I never concerned myself much with the actual number, and more with how close each phase was to each other. Nothing to ground, naturally.


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## industrial951 (Jan 29, 2011)

Thx guys for the quick and informative replies.


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## oliquir (Jan 13, 2011)

Dennis Alwon said:


> I would look at the amperage on the nameplate and divide that into the voltage.
> 
> Say 28 amp motor at 240V V= I x R or R= V/I
> 
> ...


no that is not true since a motor is an inductive load (impedance), resistance will be a lot lower than that


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## Dennis Alwon (May 9, 2009)

oliquir said:


> no that is not true since a motor is an inductive load (impedance), resistance will be a lot lower than that


Sure it will be lower but how much. He wanted a rule of thumb-- other than calling the factory that was the best I could come up with.


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## Jmohl (Apr 26, 2011)

Rule of thumb: From the Old English Common Law, A man was legally allowed to beat his wife for disobedience provided the branch he used was no larger in diameter than his thumb.....:thumbup:


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## Lone Crapshooter (Nov 8, 2008)

The winding DC resistance is not important. What is important is that all 3 resistances be the same. I can tell you that the larger the horsepower of the motor the smaller the DC resistance of the motor. It is not uncommon for a large motor to have less than .5 ohms DC resistance.


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## Wireman191 (Aug 28, 2011)

Dennis Alwon said:


> I would look at the amperage on the nameplate and divide that into the voltage.
> 
> Say 28 amp motor at 240V V= I x R or R= V/I
> 
> ...


 Why didn't I think of that! :laughing: So simple! Thats why i love this site!:thumbsup:


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## RIVETER (Sep 26, 2009)

industrial951 said:


> Hey gotta quick question, is there a rule of thumb on measuring motor winding resistance? Do you guys always refer to manufacture specs or can you automatically tell by the size of the motor your measuring what type of reading your going to get. In example what would be the winding resistance for a 240volt single phase, 5hp motor? Thx for any replies


That may be a fair question but I have been in this business for almost 40 years and have never had to be concerned about that. Motors usually run, or not. If they overheat due to bearing wear they will trip the overloads if installed. If they are not properly protected they will just melt the insulating material and eventually short out.


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## Lone Crapshooter (Nov 8, 2008)

What you are saying is true however checking the winding resistance is part of motor troubleshooting. When I am ask to troubleshoot a motor that has shut down for some reason I will meg then check the winding resistance. If it has line fuses in the starter I will check the line fuses. Also I will always reset the overloads individually then go from there.

LC


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