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Old 04-19-2013, 06:59 PM   #1
D.T. OP
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Question Electric motor fail??

Have a weird problem at work with a 3 phase 13.2kW motor. It blows fuses but the overload doesn't trip.

We checked the windings for a ground and it's not grounded. Brought it to a motor place and they had a "good" and "bad" tester for it and they say it's fine. I think they used a megohmmeter.

Some dude says it's the bearings. So I just replaced them. The front one looked like it overheated and had a bit of drag.

Any ideers? We haven't tried it yet. 480VAC @ 22 amps at nameplate.

Oh, and we tried it without the leads hooked up to the motor so it isn't the leadwires.
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Old 04-19-2013, 07:07 PM   #2
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you seem to be totally focused on the motor...sure the controler is working?
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Old 04-19-2013, 07:14 PM   #3
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What size are the fuses
Are they time delay
What type of starter
What voltage
What load are you starting?

Motors can take 3-5 times their running current to start.

the overloads have a time delay inherent to them.

fuses protect wire ol is suppose to protect motor if sized correctly.


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Old 04-19-2013, 07:20 PM   #4
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fuses protect wire ol is suppose to protect motor if sized correctly.
This.

Don't know what the installation is, but vibration can cause the feeders to chafe and only short periodically...they then blow themselves clear and wait for the next time.
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Old 04-19-2013, 07:21 PM   #5
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Did you try the motor un-coupled?
What is the service it is in, pump, fan?
What brand controller/starter is it?
Motor control and automation is my forte
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Old 04-19-2013, 08:37 PM   #6
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Fuses were 40A, I think there is 25A in there now. I think they ran out of 40A's.

OL is set for 15 amps. Standard contactor and OL.

Motor for a hot oil pump. 365 degrees hot oil.

Schematic says 35A fuses with 10-16A OL setting. 12.7 A is standard current draw.
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D.T. screwed with this post 04-19-2013 at 08:43 PM
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Old 04-19-2013, 10:42 PM   #7
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I am assuming the conductors are #10 or larger(depending on the distance from the contactor), so 40A would be proper protection. 15A O/L on a 12.7 FLRA is around 125%, which should be correct as well. Blowing fuses on startup would indicate that there is a short somewhere(fuses can take a limited amount i of inrush). You could try engaging the contactor with the leads disconnected at the motor. This will let you know if the cable is good up to that point. It is a good idea to meggar the cable before it is re-energized. Also check the contactor for any carbon tracking and that it will move in and out freely. Check the resistance of the motor windings, making sure they are balanced. There is also the posibility that the control transformer is shorted (480-120VAC most likely). Try to eliminate each component until you find the culprit.
I hope this helps and good luck...if it is really important, I'm not afraid of flying
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Old 04-20-2013, 12:47 AM   #8
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Start capacitor is gone ?.

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Old 04-20-2013, 01:03 AM   #9
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Start capacitor is gone ?.

Pete
3 phase motors don't use caps.
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Old 04-20-2013, 05:46 AM   #10
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Well, of course it's do or die. We have been trying to get them to buy us a spare pump for a while. But they have to make our lives a PITA. Boss wants to get a good megger now though.

Tried it with the motor disconnected and no problems with the fuses. It isn't VFD controlled. Learned recently that they can generate voltage and destroy bearings, so our 125 hp motor has a grounding brush on the shaft. I tested the windings at .8 ohms across all 3 phases. Boss got around 1.2 ohms. Could be the contactor. Lead wires are 10 gauge I'm pretty sure of.

Just had to replace a 100A circuit breaker because it was tripping. Internally it got fried. This was for another piece of equipment.

Electrical gremlins have been kicking my ass lately.
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Old 04-20-2013, 05:54 AM   #11
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Have you tried replacing the fuses with buss bars?

Try it and let us know what happens...

















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Old 04-20-2013, 06:12 AM   #12
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Are the time delay fuses? If the are fast blow fuses to protect electronics they won't hold for starting current.

Post the exact fuse # so we can look it up.

A good meger will cost as much as that motor if it is a standard one.

I have literary hundreds of AC drives from 1/4 to 800hp. in our plant and haven't had any bearing failures we do use good VFD Faraday type shielded cable on them.

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Old 04-20-2013, 08:37 AM   #13
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Have you measured voltage on both sides of the contactor at each leg?
Have you measured amperage on the load side of the contactor at each leg?
Are you blowing all the fuses?

It really sounds like a low voltage/high amperage on a single leg caused by a bad contact or corroded terminal on the contactor.
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Old 04-20-2013, 10:27 AM   #14
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cut to the chase- pay an electrician to look at it. an industrial setting is a poor choice for a home hobbist to learn industrial electrics
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Old 04-20-2013, 10:55 AM   #15
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There are some good suggestions here but some by people who don't know a damn thing about 3 phase and motors

1. are all the fuses blowing, if not is it the same ones

2. are they time delay fuses (the 25a ones probably won't work)

3. have you checked the contactor? if it cycles alot there is a good chance the contacts are burnt.

4. do the fuses blow instantly? if not try checking the current on each leg.


In my years working on this type of thing, shorting windings are hard to tell, current readings tell more than resistance (unless it's shorting to Ground)

doing a quick look the 40 amp fuses may be a little small for the motor.

try just the motor without the load hooked up, the pump may be bad.
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