# motor questions



## macmikeman (Jan 23, 2007)

Try megger testing the motor


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## garfield (Jul 30, 2009)

It megged 1000 volts to infinity.


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## macmikeman (Jan 23, 2007)

Something else you could try. Move conductors. ''A'' goes to ''B'', ''B'' goes to ''C'', ''C'' goes to ''A''. Rotation should stay the same.


This trick has balanced more than one motor.


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## micromind (Aug 11, 2007)

I would be leery of anything over about 0.5 with that current. Eventually, that disconnect will burn up. 1.5 volts is a lot. 

89 to 97 is sorta on the edge of being ok, but since the highest current is less than the nameplate current, I wouldn't worry much. 

The different wire sizes won't matter at 8'.


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## Cow (Jan 16, 2008)

I'm just starting to use FOP tests more. Since I've just started I'm still establishing a baseline of what's good, not so good, and downright bad. It seems the best way to establish a baseline is to go around any chance I get and test any and all the connections I can just to see where most equipment runs at. 

The last 30 amp 3 pole breaker I was testing due to intermittent tripping issues was running around 19 amps of load but had a little over 1 volt drop on the worst leg. Feeling the side of the breaker case, it was pretty warm. Replacing the breaker solved the problem.

I use ohms law to determine how much heat a given connection is putting out. 19 amps x 1 volt = 19 watts.

In your case, 90.2 amps x 1.5 volts = 135 watts. With that wattage, that connection has turned into a small heater. I would be replacing the disconnect guts at this point. One other thing worth mentioning because it's happened to me before, is to verify the disconnect is sized properly for the driven motor load according to the HP ratings on the door. I assume some folks only use the motor amps and think because it only draws 90-96 amps, a 100 amp disconnect is okay.


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## tates1882 (Sep 3, 2010)

What's your voltage at the source? And what is your voltage at the disconnect? The amp draw difference will be proportional to you what your voltage difference is.


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## telsa (May 22, 2015)

garfield said:


> Couple questions.
> 
> I did a voltage drop test on a disconnect today. It was .6 of an amp on 2 legs and 1.5 amps of drop on the third leg. I isolated the drop to the knife of the disconnect. I wiggled the knife and another leg was the same story but the first one got better. I know this means the disconnect is questionable but my question is how much drop is to much?
> 
> ...


1.5 V x 90 A = 135 Watts

0.6 V x 90 A = 54 Watts

0.6 V x 90 A = 54 Watts

&&&&&&&

Bad make-up is always suspect at motor terminations.

You haven't given any details about other critical elements:

Age
Heaters
Contactor
Conditions of service

Are you new at this?


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## ELECTRICK2 (Feb 21, 2015)

macmikeman said:


> Something else you could try. Move conductors. ''A'' goes to ''B'', ''B'' goes to ''C'', ''C'' goes to ''A''. Rotation should stay the same.
> 
> 
> This trick has balanced more than one motor.


I've done that many times. Usually to track down a problem. Not sure how it would balance a motor though.


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## telsa (May 22, 2015)

ELECTRICK2 said:


> I've done that many times. Usually to track down a problem. Not sure how it would balance a motor though.


It works by way of re-doing the connection.

Otherwise, there's no logical reason it should change the dynamic.

Assuming it's a fused disconnect, they have to be looked at too.

OP just left all of the critical details that would point the way -- _out _of his post.

For example: corrosive atmospheres...

Level of vibration...

On and on...:whistling2::whistling2::whistling2:


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## garfield (Jul 30, 2009)

telsa said:


> 1.5 V x 90 A = 135 Watts
> 
> 0.6 V x 90 A = 54 Watts
> 
> ...


Motor is fairly new. Good connections there. i put in a new contactor and overload but there's no issue with those. It's a concrete plant so everything is ugly. I have been doing this quite a while but haven't come across much with the fall of potential test. Usually we get called when it's failed completely. On the imbalance i knew it wouldn't burn up the motor but I didn't know the acceptable numbers exactly.


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## DriveGuru (Jul 29, 2012)

Please clarify your question, current doesn't get "dropped" (see kirchoffs law). Are you talking about the current being unbalanced? If you are it's more than likely due to a slight voltage unbalance, and is nothing to worry about. A voltage drop of 1.5v will mean very little to your motors operation. As far as your disconnect, an infrared camera would be the best thing to look at it with(an infrared thermometer in a pinch but not as accurate), I generally look for a temperature rise of greater than 20-30 deg. above ambient. If it's not too hot...it's a non-issue.


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