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Old 05-11-2008, 01:31 PM   #1
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Default 480 motor help please.

hi all, i am trouble shooting a motor at a water slide park. This is a 480v, 3phase, 100 amp, with a combo disconnect starter with overloads and push start, stop.
The story i am told is: this motor and others, were sent out and tested, then re installed.
All the other motors work fine (4). This one motor is tripping out the main c.b. instantlyt. no sparks, no noise. (125 amp cb). Isolated the motor and the contactor works fine many times without the c.b. tripping. I checked the resistance from each leg to ground and there is infinate ohms. (on motor). The shaft on the motor spins by hand freely.

Is it possible that if one of the overloads was bad, it would trip the c.b.?
Also, I thought i had a bad c.b. , so i replaced with a new one.

I just want to look at all options before i pull the motor and send it in to be checked out again. Any ideas anyone?

kris.

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Old 05-11-2008, 02:45 PM   #2
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Quote:
Originally Posted by gardner View Post
hi all, i am trouble shooting a motor at a water slide park. This is a 480v, 3phase, 100 amp, with a combo disconnect starter with overloads and push start, stop.
The story i am told is: this motor and others, were sent out and tested, then re installed.
All the other motors work fine (4). This one motor is tripping out the main c.b. instantlyt. no sparks, no noise. (125 amp cb). Isolated the motor and the contactor works fine many times without the c.b. tripping. I checked the resistance from each leg to ground and there is infinate ohms. (on motor). The shaft on the motor spins by hand freely.

Is it possible that if one of the overloads was bad, it would trip the c.b.?
Also, I thought i had a bad c.b. , so i replaced with a new one.

I just want to look at all options before i pull the motor and send it in to be checked out again. Any ideas anyone?

kris.

Are you familiar with the word "megger"????

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Old 05-11-2008, 04:50 PM   #3
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hi all, i am trouble shooting a motor at a water slide park. This is a 480v, 3phase, 100 amp, with a combo disconnect starter with overloads and push start,
kris.
do you mean 100 HP.

I would hope so becuase if you really mean 100 amp, a 125 amp breaker is way too small.

actually. come to think about it, that breaker is too way too small for a 100 hp motor as well. NEC lists it at 124 FLA.

I presume this was running prior to this? Using this same gear?

In rush current is going to be huge. Any chance this is a wye-delta start motor? or some other type of soft start (although that would be fairly obvious so I doubt it)

what size motor is this really? I'm rambling on and I suspect it is neither 100 HP or 100 amp rated.
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Old 05-11-2008, 08:26 PM   #4
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Is the circuit breaker the right size for the motor? A 100 hp, 3 phase, 480 V motor would use 175 A time delay fuses or a 250A mag trip circuit breaker. An undersized breaker would trip when you start the motor due to the in-rush current exceeding the breaker rating. You said this motor was in service in this same circuit before and sent out to be tested so it's likely that things are sized correctly.

If the breaker is sized correctly, look for a ground fault or short circuit either in the motor leads or the motor itself. A megohm meter for insulation testing or "megger" will help you determine that. First lock out the circuit you are working on, and then following the manufacturers instructions for your tester, start by megging the motor leads from the bottom of the starter. If the readings are bad, disconnect the motor from the circuit and try again. This will tell you if the motor or the wires feeding the motor are bad.

If the megger doesn't show a bad motor or t-leads, look at the control circuit. This can be a little trickier, but a megger and some patience will help you find the problem. Its possible the controls are not wired correctly. For instance, if there is no control voltage fuse installed and there is a grounded wire in the start stop circuit, that would cause the breaker to trip.

Bottom line is, there are many things that could cause the breaker to trip and I've just touched on a few of them. Usually its something simple, but sometimes it can be hard to find. Use the right test equipment, take your time and good luck. And remember, safety first!
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Old 05-11-2008, 11:23 PM   #5
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actually I come up with more like a 200 amp fuse and a 300 amp breaker. or actually up to a 250 amp fuse or a 350 amp breaker with the exceptions.


OP states this is tripping a 125 amp breaker.

that is why I asked for OP to make surre of the motor size stated in the original post.
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Old 05-12-2008, 12:45 AM   #6
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This likely wouldn't apply to a same-motor changeout, but if the C/B in question is a motor circuit protector (MCP), and it's new, the trip setting is likely too low. They usually come from the factory on the lowest setting, and must be turned up in order to start almost any motor. It's also possible that it was turned down, either by a person with little knowledge, or possibly as a prank.

Generally speaking, if a 125 amp 480 volt breaker trips on a ground fault, or a phase-to-phase fault, it's pretty loud.

It's also possible that the breaker has a shunt-trip coil that's wired through a contact on the starter and the trip circuit is energized but the coil isn't until the starter pulls in.

Rob

Last edited by micromind; 05-12-2008 at 12:54 AM.
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Old 05-12-2008, 01:10 AM   #7
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Thanks guys for all your input.
the motor in question drew 68 amps at f.l.a. . The motor was painted over and a fla rating is not available. the testing facility did not put the old label back on. I used a megger for line to ground on each leg. for leg to leg i got very high resistance almost a short. That being said, i need more information on how to use a megger.
I will check the low voltage controol circuit and overloads for shorts. There is a possibility that the motor replaced was not the same motor that was there origionally. I am not sure of the h.p. rating of the motor. This motor did work fine until it was checked out.
It also had a binding problem with the impeller when it was replaced. Could this have damaged the motor when they tryed to start it. They did fix the binding problem. after that , they said that the motor is running fine. Then the next day, the circuit breaker tripping problem came back and still exhists.

still scratching my head,
krisf
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Old 05-12-2008, 04:44 AM   #8
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I'll also say cb trip setting needs adjusted.
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Old 05-12-2008, 05:17 AM   #9
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Originally Posted by gardner View Post
Thanks guys for all your input.
the motor in question drew 68 amps at f.l.a. . The motor was painted over and a fla rating is not available. the testing facility did not put the old label back on. I used a megger for line to ground on each leg. for leg to leg i got very high resistance almost a short. That being said, i need more information on how to use a megger.
I will check the low voltage controol circuit and overloads for shorts. There is a possibility that the motor replaced was not the same motor that was there origionally. I am not sure of the h.p. rating of the motor. This motor did work fine until it was checked out.
It also had a binding problem with the impeller when it was replaced. Could this have damaged the motor when they tryed to start it. They did fix the binding problem. after that , they said that the motor is running fine. Then the next day, the circuit breaker tripping problem came back and still exhists.

still scratching my head,
krisf
On a go/no go test with the megger, you want to see very high resistance phase to ground, and little or no resistance phase to phase, as you will be reading through the motor windings. Sounds like your motor leads and motor are okay, if I understood your megger findings correctly.

What is your control voltage? Is it fused?

68 A at 480V sounds like a 50 HP motor, in which case a 125A mag trip breaker is fine. Make sure the trip adjustment is set correctly.

What type of motor is it? How many leads does it have? Are they connected correctly?

What load does it drive? Was the "binding problem" with a pump the motor drives? Is it possible to disconnect the motor from the load and try to start it that way?

Sounds like the motor may be suspect, or not even the right size. I would contact the motor shop that worked on the motor. They should know the motor data.
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Old 05-12-2008, 05:44 AM   #10
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Originally Posted by Mountain Electrician View Post
What is your control voltage? Is it fused?

68 A at 480V sounds like a 50 HP motor, in which case a 125A mag trip breaker is fine. Make sure the trip adjustment is set correctly.

What type of motor is it? How many leads does it have? Are they connected correctly?

What load does it drive? Was the "binding problem" with a pump the motor drives? Is it possible to disconnect the motor from the load and try to start it that way?

Sounds like the motor may be suspect, or not even the right size. I would contact the motor shop that worked on the motor. They should know the motor data.


Exactly my next choice.
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Old 05-12-2008, 07:07 AM   #11
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actually I come up with more like a 200 amp fuse and a 300 amp breaker. or actually up to a 250 amp fuse or a 350 amp breaker with the exceptions.

I like to use this calculator, its like the slide rules you get from your Square D distributor. Its based on the 2005 NEC which we are still using here in Maine. (Until July)

http://www.squared.com/us/squared/co...mtrdatacal.htm

Its pretty handy. That 250A breaker rating I came up with was actually from the IEC motor calculator. According to the NEMA calculator, 200 A thermal breaker is good for a 100 HP motor.
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Old 05-12-2008, 10:43 AM   #12
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I like to use this calculator, its like the slide rules you get from your Square D distributor. Its based on the 2005 NEC which we are still using here in Maine. (Until July)

http://www.squared.com/us/squared/co...mtrdatacal.htm

Its pretty handy. That 250A breaker rating I came up with was actually from the IEC motor calculator. According to the NEMA calculator, 200 A thermal breaker is good for a 100 HP motor.
Yes, they do make things more simple. I was merely grabbing from the NEC.

175% FLA for time delay fuses and 250% FLA for inverse time breakers with the exceptions of up to 225% for the fuses and 300% for the breaker for over 100 FLA.



NEC motor rating of 124 FLA for a 100hp motor or the possibility of 100FLA from the poster, both of which are obviously wrong so it was all an effort in futility.

so based on that 100 hp motor (124 fla) you can use up to a 372 amp breaker and 279 on the fuses. Preferred would be kept down to 200 amp on the fuses and 300 on the breaker.

calcs could be wrong but that is what I come up with.

Thanks for that calculator. My numbers were from the '05 as well. I do wonder why the difference though. My allowances were from 430.52 and came up with the numbers I did.
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Old 05-12-2008, 02:23 PM   #13
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Checking 430.52, I see nothing at all wrong with your numbers. It is 225% for a fuse, and 300% for a breaker if the FLA is over 100a, but I think the difference is that is a maximum rating, not a minimum.

430.52 (B) states the branch-circuit short circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. So if the 200A breaker will carry the load, its okay to use. There would be no code violation with a 300 A breaker, just some overkill.
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Old 05-12-2008, 02:39 PM   #14
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I am sure i may come up little diffrent numbers as well

this number is based on 100 HP @ 480V .,,

• FLA 132
• TDF 230A [ avce 175% over ]
• TCB 330A [ avce 250% over ]

and i down step to 200 amp TD fuse or 300 amp breaker per 430.52{B}


but if this person talking about 50 HP @ 480V.,,

• FLA 69.0
• TDF 120
• TCB 173

ditto again can use 100 amp fuse or 150 amp breaker.

hope that number come up ok with this one.

Merci,Marc
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Old 05-14-2008, 03:50 PM   #15
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Have you contacted the motor shop about this. There is no telling what they did at the shop. They may have rewound it and changed the internal connections. If all they did was test it then it is not a motor problem. I still would contact them and tell them what you have told us.
If they are a good reputable shop, they will either walk you through the troubleshooting or send someone out to check it.

Ps...how many lead motor is this? and what type of starting are you doing. I doubt a 100 Hp motor is on a "across line contactor".
If this is a pump motor, back off on the suction end before trying to start. There should be a valve.

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Old 05-14-2008, 10:09 PM   #16
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Thanks for all your input,

The broblem still exhists. This is a three lead motor. The motor was sent out to a different motor company this time. I talked to the technician and told him my situation. He thinks that this motor is between a 50 h.p. and a 75 h.p. He said he will do the calculations to find out. He also said that he started the motor and it is fine. He said pulling 38 amps without a load. connected to a 100 amp c.b. This motor starter has a 480/120 controol transformer for stop/start . motor coil is same.
I think there is a possibility that the contactor is somehow single phasing. I am going to take contactor and heaters in for checking. I am also going to install this contactor in to a startor that i know works.
Also, The motor company did three different tests on this motor. Some i havent heard of.
I will keep you all up to date.
kris
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Old 05-15-2008, 05:29 AM   #17
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are you sure that pump interference problem was fixed?

If you can, when you get it put back together, leave the motor disconnected from whatever it drives and start it that way.

back to the megger;

did you meg it at the motor or at the starter? You could have bad wire from the starter to the motor.
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Old 05-15-2008, 05:33 AM   #18
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Thanks for all your input,

This is a three lead motor. kris

That's odd, it should have 9 or 12 leads.
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Old 05-15-2008, 12:43 PM   #19
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That's odd, it should have 9 or 12 leads.

The motor would only have 9 or 12 leads if it is dual voltage, or is wound for Wye/Delta starting. Since it only has 3 it is obviously a single voltage motor with out Wye/Delta start windings.

As far as the pump interference issue, if it is a mechanical issue it should be tripping the motor overloads and not the circuit breaker.

To the Megger test, check the motor windings and the leads between the starter and the motor. On the motor you should have very high or infinite resistance between each motor lead and ground, and very low resistance between each of the 3 motor leads. On the leads going back to the starter you should have very high or infinite resistance to ground AND between the leads.

GOOD LUCK!
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Old 05-15-2008, 03:10 PM   #20
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The trip setting or the size of the CB is the problem.

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