# Three phase motor tripping overload



## JakeTheSnake (Jan 4, 2013)

Hey guys I'm having an issue with a lift station pump.

The motor is 3 phase 600vac and the FLC is 15 amps. The motor can spin freely by hand but when it is turned on it rotates about an inch then stops and just hums. The amperage goes up to about 50 amps within seconds and then the overload trips. I thought it may be single phasing due to a bad starter but the previous pump was working off the same starting and, despite having worn bearings, it worked.

Also, if the motor is single phasing. What kind of amperage reading would I be getting? 

Any input would be greatly appreciated. These things only happen on Fridays.


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## Hippie (May 12, 2011)

2 legs will have abnormally high amps and the other will have 0. Check your connections and also starter contacts, a bad set of contacts will cause a phase loss. From your description I would say its single phasing most likely


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## Big John (May 23, 2010)

Sounds like single phase or very seriously misaligned rotor. I'd disconnect the motor and see what voltage you have present at the peckerhead.


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## Wirenuting (Sep 12, 2010)

What size motor, is it new?
It sounds like a bad connection or a set of contacts blew in the starter.
Post a pic of the starter and motor data plate..

fwiw:
10 HP @ 460v = 14 amp motor,
30 breaker
Size 1 starter
3/4 conduit
#12
16.1 heaters

Data plate trumps what I just listed.

Oh ya, welcome to ET, we love pictures.
Drop a dime on the introduction thread page.


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## JRaef (Mar 23, 2009)

Wirenuting said:


> What size motor, is it new?
> It sounds like a bad connection or a set of contacts blew in the starter.
> Post a pic of the starter and motor data plate..
> 
> ...


Notice he is in Canada. 600V, FLA = HP, so he has a 15HP motor.


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## JRaef (Mar 23, 2009)

JakeTheSnake said:


> The motor can spin freely by hand but when it is turned on *it rotates about an inch then stops and just hums*.


Classic single phasing. With only 2 of the 3 phases getting to the motor, there is no relative rotation of the magnetic fields, so wherever the rotor is at the moment you energize it, the 2 remaining phases create a stationary magnetic field and the rotor just aligns itself to it, that's likely the _"rotates about one inch"_ thing you see. So you either have a blown fuse in the power circuit, a burned contact in the starter, or bad connection somewhere in the power circuit.

But there are other possibilities as well!

Since you said it's a lift pump, if it is vertical, it may also be a bad thrust bearing. Vertical motors lift up a little when energized, so they have a special bearing at the top called a "thrust bearing" that allows a little bit of lifting to occur, but then allows it to spin against it. If that thrust bearing breaks and jams, so does the rotor. But when not energized, the shaft is dropped down and rotates freely.

Also because it is a lift pump, it may have what's called a "backspin ratchet" assembly on it that prevents the "stuff" wanting to flow back down hill from allowing the motor to spin backward. If you spin it by hand in the correct direction, it spins fine. But if you have the motor leads connected so that it spins the wrong direction, it turns until the ratchet catches it, maybe one inch at best. This happens a lot when people replace motors. They mark the leads on the old motor and connect the new one exactly the same way, expecting that they don't need to re-check rotation. But that's wrong, two different motors may or may not spin the same direction when connected the same, it's just the nature of the beast. You have to check each time.


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## Wirenuting (Sep 12, 2010)

JRaef said:


> Notice he is in Canada. 600V, FLA = HP, so he has a 15HP motor.


I saw that, but since he didn't give HP I shot from the hip with an example. 
Like I wrote, data plate trumps what I listed.


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## sparky970 (Mar 19, 2008)

JRaef said:


> Notice he is in Canada. 600V, FLA = HP, so he has a 15HP motor.


15hp motor should be on a size 2 starter


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## Glen Walker (Dec 30, 2009)

If this is a turbine pump you may want to check the rotation. Turbine pumps have a brake mechanism built in to prevent spinning the impellars off by reversing the rotation. If it is reversed the motor will turn less than a quarter turn before the brake mechanism engages.


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## Fixastang (Sep 4, 2012)

JakeTheSnake said:


> Hey guys I'm having an issue with a lift station pump.
> 
> The motor is 3 phase 600vac and the FLC is 15 amps. The motor can spin freely by hand but when it is turned on it rotates about an inch then stops and just hums. The amperage goes up to about 50 amps within seconds and then the overload trips. I thought it may be single phasing due to a bad starter but the previous pump was working off the same starting and, despite having worn bearings, it worked.
> 
> ...


We had a similar issue where I work. It turned out to be a bad leg on a remote CB was causing the motor to single phase. We replaced the CB with a new one and everything is working fine.


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