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480V vs 460V

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R

retirede

Senior Member
Location
Illinois
Drod04Mustang said:
Ok Good Morning,


Have a problem. I got the motor rewound and new and the first time they turn it on it tripped the main breaker.

100 Amp Square D 480V 3 phase breaker

The readings are as follows.

496 Leg to leg
497 leg to leg
495 outside leg to outside leg

237.5 leg to neurtral
236 leg to neutral
237.3 leg to neutral

0.04 Ground to neutral

Gonna try to change out breaker first.
Click to expand...

Did it rotate at all? Can you uncouple the compressor? What make/model compressor?
 
D

Drod04Mustang

Member
Location
Avon Park Florida
Occupation
Industrial Electrician-Residential Electrician
retirede said:
Did it rotate at all? Can you uncouple the compressor? What make/model compressor?
Click to expand...
The first time they turned it on it tripped. Reset the breaker and then it turned on and function. The Air compressor guy said that it was possibly the breaker. He said that it was getting more voltage than it should. I check the other breakers in that same panel and all are around 495, 496.
 

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J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
zbang said:
Not to put too fine a point on it, but IMHO the AC guy is a bonehead. OTOH, the breaker could be the problem for other reasons.
Click to expand...
A breaker does not determine how much voltage is supplied to equipment. It sounds like your Air compressor guy is simply passing the problem to someone else.

Your breaker has a 100A nominal trip rating. It is built into a 225A breaker frame, so its instantaneous/magnetic trip point is roughly 2250A. It should be good for that motor nameplate with the maximum voltage at 380V 60Hz.
 
D

Drod04Mustang

Member
Location
Avon Park Florida
Occupation
Industrial Electrician-Residential Electrician
zbang said:
Has the breaker been tested or swapped with another? That's not clear to me from earlier postings.
Click to expand...
No that is the next step to change out the breaker. Dont have an extra one of those lying around. Ive checked all other breakers in that panel and they all are around 495, 496, 497. I assumed this was correct due to the fact that its 480V with 5%+- which would be from 454V-504V. I dont see anything wrong with it but I guess since they just spent 7 k on a new motor and install they want to have longevity out of it. Thanks for help.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Drod04Mustang said:
No that is the next step to change out the breaker. Dont have an extra one of those lying around. Ive checked all other breakers in that panel and they all are around 495, 496, 497. I assumed this was correct due to the fact that its 480V with 5%+- which would be from 454V-504V. I dont see anything wrong with it but I guess since they just spent 7 k on a new motor and install they want to have longevity out of it. Thanks for help.
Click to expand...
Motor probably will last longer if supply volts is 495 than if 465.
 
G

garbo

Senior Member
Voltage is in acceptable levels but would check it during a weekday when building has loads running and a few times at night & /or weekends. Have came across three phase voltages 509 to 507 volts late at night and weekend but very close to 480 to 483 volts with loads running. Like another post said would run the motor with coupling removed from compressor. Then see if it runs. Not sure but unable to remove motor from compressor ask the HAVC /Refrigeration tech if they can make the compressor run unloaded. Years ago I had a 400 HP trip out the circuit breaker on a centrifugal compressor. That type goes thru a gear box that increased speed to over 6,000 RPM. It was attempting to start while fully loaded. Best practice is always to see how easy a rewound or motor that was worked in ( new bearings ) turns before installing. I usually do not take voltage readings to neutral on three phase motors. Did you megger the motor?
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Motor is rated 46.8A, breaker is a Sq.D. KH3600 rated 100A, with the instantaneous trip setting adjustable from 500-1000A. What is it set at? Under the rules in article 430, you can set the mag trips at up to 1300% of the motor FLA if you can show that it nuisance trips below that. 1300% of 46.8A is approx. 600A. If it is a newer “Energy Efficient” motor, it can be set as high as 1700%, which would be approx. 800A.

Don’t jump to assuming that the breaker is “bad”, it was just likely not set correctly and when the motor was rewound, the magnetizing inrush is now a little higher. Details matter…
 
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