motor short

[dshelley]

I am running a 50 hp motor 460v 1800 rpm it keeps tripping a 80 amp breaker before the starter about every 4 hrs is the motor possibly bad?

 

[mcclary's electrical]

I am running a 50 hp motor 460v 1800 rpm it keeps tripping a 80 amp breaker before the starter about every 4 hrs is the motor possibly bad?

How many amps is the motor using?

How many amps is the motor rated for?

Is it mechanically being overloaded or overworked?

Have you megged the motor?

Have you megged the wires?

 

[iwire]

Assuming the motor has proper overload protection I think we can rule out anything but a line to line or line to ground fault of some type.

Bring out the mega.

 

[dshelley]

ive done all of the above

 

[mcclary's electrical]

ive done all of the above

If you've done all the above, then I would bet bad breaker.

an 80 amp breaker for a 50 hp 460 volt is WAY SMALL. I would put in at least 100 or larger depending on a few circumstances, of course.

 

[david luchini]

an 80 amp breaker for a 50 hp 460 volt is WAY SMALL.

I'd agree that the 80A breaker is too small. Replace it with a 150A c/b and see if the problem persists.

I'd guess that your loading an 80% rated 80A (64A) circuit breaker near or above the 80% level with a continuous load (50HP-460V = 65A FLA by T430.250.) So after several hours of continuous load, it trips.

 

[Jraef]

I'd agree that the 80A breaker is too small. Replace it with a 150A c/b and see if the problem persists.

I'd guess that your loading an 80% rated 80A (64A) circuit breaker near or above the 80% level with a continuous load (50HP-460V = 65A FLA by T430.250.) So after several hours of continuous load, it trips.

Unless it is an IT-only (Instantaneous Trip) breaker (a.k.a. MCP) that has no thermal trips, which is what is mostly used in combination starters...

dshelley,
You need to look more closely at that "80A breaker" to determine what kind it is, post a part number if you aren't sure and we'll help. If it is an IT-only breaker, then that's a completely different issue in terms of what could make it trip. If it's a Thermal-Mag (TM) breaker, then the above issue is probably the cause.

 

[mbeatty]

I would also like to know if the motor runs continuously for 4 hours or is stopping and starting.
Regards,
Mark

 

[zog]

ive done all of the above

What were the results?

 

[hurk27]

I had one that was driving me nuts, turned out to be a bad contact that would heat up and then loose connection, this would single phase the motor and trip the breaker, but it should have tripped the over loads, found they were two sizes to high.

but any bad or loose connection in the circuit can cause this.

 

[nollij]

A neighbor of ours up north had a problem with one of their large motors tripping out. Luckily, they had the motor metered and had an event report (although it would have been preferable to have a higher sampling rate).

Anyway, the coupling between the motor and pump was not properly made as a result of improper/inexact motor mounting. This caused a high torque and a resulting high current when the vibrations shifted the motor enough.

Check the electrical side of it: megger cables, check breaker type, etc. but also keep in mind the mechanical aspects of the motor that could potentially cause tripping.

One thing we have done recently with a spare is running the motor that is tripping with the spare's bucket. This will pinpoint whether the problem lies in the starter or the motor.

 

[weressl]

Unless it is an IT-only (Instantaneous Trip) breaker (a.k.a. MCP) that has no thermal trips, which is what is mostly used in combination starters...

dshelley,
You need to look more closely at that "80A breaker" to determine what kind it is, post a part number if you aren't sure and we'll help. If it is an IT-only breaker, then that's a completely different issue in terms of what could make it trip. If it's a Thermal-Mag (TM) breaker, then the above issue is probably the cause.

It is likely not an MCP as their continuous ratings, which are printed on the handle/toggle is either 70A or 100A. A 50HP starter would require a 100A continuous rated MCP with a recommended adjustable trip rating of
300-1000A.

So an 80A T&M CB would definetly be under-rated.

 

[Jraef]

It is likely not an MCP as their continuous ratings, which are printed on the handle/toggle is either 70A or 100A. A 50HP starter would require a 100A continuous rated MCP with a recommended adjustable trip rating of
300-1000A.

So an 80A T&M CB would definetly be under-rated.

You're right, IT breakers in that range either come in 50, 70, or 100A depending on mfr. Didn't think of that.

But Eaton is now selling a Motor Protection Circuit Breaker (MPCB) that has an adjustable magnetic AND adjustable thermal trip elements that is essentially like a Manual Motor Starter (something they absorbed when they bought Moeller) which can be used with a contactor to make a complete combo starter. Their 80A unit would be appropriately sized for a 50HP motor actually.

Still comes down to the OP providing more info and he seems to have checked out.

 

[zog]

Still comes down to the OP providing more info and he seems to have checked out.

Help us help you Mr OP.