# 3 phase motor current readings



## erics37 (May 7, 2009)

farlsincharge said:


> I was taking some readings this morning on a 575v 100 hp motor. FLA 99.6.
> The readings on each leg were as follows 89.9, 85, 73.2.
> I don't work with motors all that much, does this sound reasonable to you?


The third leg seems a little low. There's no motor not worth megging :thumbup:


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## JRaef (Mar 23, 2009)

Before you get too excited, compare that to the motor terminal voltages. A very small voltage imbalance creates a very large current imbalance. If the voltages are unbalanced, that explains the current imbalance but means the solution is different.


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## kaboler (Dec 1, 2010)

I went to a service call and my boss asked me to measure the voltages at the panel, as in, red to ground, black to ground, blue to ground, and also red to black, red to blue, and black to blue.

I also checked the voltages at the motor. And I checked the current. Now, I don't know what the problems would be, save a loose conductor somewhere, but if you guys could expand on this thread more, it'd be an interesting read.


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## nitro71 (Sep 17, 2009)

Could always call the manufacturer and ask them.


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## mattsilkwood (Sep 21, 2008)

I would start with the voltages also.


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## oldtimer (Jun 10, 2010)

kaboler said:


> I went to a service call and my boss asked me to measure the voltages at the panel, as in, red to ground, black to ground, blue to ground, and also red to black, red to blue, and black to blue.
> 
> I also checked the voltages at the motor. And I checked the current. Now, I don't know what the problems would be, save a loose conductor somewhere, but if you guys could expand on this thread more, it'd be an interesting read.


 What was the service call for??? 

I really think you are full of, what makes grass grow in Texas.:laughing:


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## mattsilkwood (Sep 21, 2008)

oldtimer said:


> I really think you are full of, what makes grass grow in Texas.:laughing:


 Mexicans???:laughing:


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## farlsincharge (Dec 31, 2010)

Thanks for the new sig material Matt.


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## micromind (Aug 11, 2007)

A couple of things come to mind;

1) Are you sure the meter clamp was completely closed around the conductor on phase C? If there's the slightest gap, it'll read low. 

2) As noted, a small voltage difference will cause a large current difference. Especially if the motor is delta wound; most motors above 15HP are. 

3) The current difference is about 17%. That's slightly high, but no current is above the FLA. I wouldn't worry much about it.


What type of motor is it? In my experience, an basic general-purpose motor is likely to tolerate voltage imbalance better than other types. Submersible well pump motors and hermetic refrigeration compressors are almost impossible to balance current on. 

This may be difficult on a large motor, but if it becomes a problem, try rolling the phases. For example, if you're using brown-orange-yellow for A-B-C, put brown on B, orange on C, and yellow on A. The rotation is the same, but often you can get better current balance this way. Especially with one of those high-strung motors that doesn't balance well. 

Rob


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## Hugo (Feb 21, 2011)

mattsilkwood said:


> Mexicans???:laughing:


 and farlsincharge 
who care's whos is where as long as they know what there doing SO WHO GIVES A


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## mattsilkwood (Sep 21, 2008)

Hugo said:


> and farlsincharge
> who care's whos is where as long as they know what there doing SO WHO GIVES A


 What in god's name are you talking about?


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## Jlarson (Jun 28, 2009)

Micromind threw out some very good info. Although I would still look into things a little better. Could have a loose connection or something else that will get worse over time. 




farlsincharge said:


> Thanks for the new sig material Matt.


Awesome :laughing:


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## wil335 (Feb 15, 2011)

While the motor is running, check the voltage between the motor circuit breaker/line side of the contactor and the load side of the contactor. If you have a noticeable difference, you could have a bad connection or bad contacts in the contactor.


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