3 Phase Motor Blowing Fuse

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GoldDigger said:
Since the OP says that it pumped water for awhile (but not whether it was at full flow) the "locked" rotor is probably just a lot of extra drag.

What is confusing me is that after one fuse blows the current in the remaining two fuses is not high enough to blow one of them (and the pump continues to move water?).
My brain is not coming up with anything that would cause a momentary or short term overcurrent but allow the pump to continue running for minutes until the overload trips.
Maybe something about the torque/thrust that is reduced when the first fuse blows, reducing the resistance to rotation, hence reducing the current seen by the remaining pair of fuses?
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Possible scenario:
Fuse blows, motor single phases, torque drops to roughly 1/2 of normal, which drops flow and load on the motor so it has less up thrust against the thrust bearing problem. So it continues to run and move SOME water, but not what it's supposed to be pumping. The reduced load allows the other fuses to hold in, but the current imbalance is being picked up by the Overload Relay (luckily).

This is why I hate fuses on centrifugal pump controls... If you use them, you MUST use a good phase loss protection scheme. I prefer using Solid State OL relays now, because most of them do current based phase loss protection, which is better than voltage base line monitor relays. A typical SSOL would take the motor off line within 3 seconds of having no current on one phase.
 
Jraef said:
Possible scenario:
Fuse blows, motor single phases, torque drops to roughly 1/2 of normal, which drops flow and load on the motor so it has less up thrust against the thrust bearing problem. So it continues to run and move SOME water, but not what it's supposed to be pumping. The reduced load allows the other fuses to hold in, but the current imbalance is being picked up by the Overload Relay (luckily).

This is why I hate fuses on centrifugal pump controls... If you use them, you MUST use a good phase loss protection scheme. I prefer using Solid State OL relays now, because most of them do current based phase loss protection, which is better than voltage base line monitor relays. A typical SSOL would take the motor off line within 3 seconds of having no current on one phase.
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SSOL? Are your referring to something like a TesysT? I spent a full day configuring software and on the phone with Schneider working out communications with one of those. Once I got SoMove up and running I kind of like them.

We ended up trying it one more time......... Blew all 3 fuses and all windings are directly shorted to ground. The guys are going to take it apart and look for damage that could have occurred during the bearing installation. I am betting they skinned some of the varnish of the magnet wire and its hitting the case.

Thank you everyone with your comments.
 
kwired said:
Tripping overload after running on just two lines would be expected, so you need to narrow down why it is blowing fuses.

One possibility - metal parts/tools got left in the motor when it was being worked on and were pulled into the coils when they were energized causing a turn to turn fault or even turn to ground fault.
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That theory looking a little more promising.
 
kwired said:
That theory looking a little more promising.
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So promising that it has almost been confirmed. Good enough for government work. :)

We ended up trying it one more time......... Blew all 3 fuses and all windings are directly shorted to ground.
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Keep starting it enough times and any intermittent fault is likely to become catastrophic instead. Hopefully the motor can still be rewound or otherwise rescued. If not, hope it is a standard size to replace.
 
Saturn_Europa said:
iwire called it. Spring washer was jammed into the stator windings.

Thank you everyone for your posts
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This thread is a week old, but I would have drug out a thermal camera at some point. Now that you know the cause, would that have helped?
 
11bgrunt said:
This thread is a week old, but I would have drug out a thermal camera at some point. Now that you know the cause, would that have helped?
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Don't sound like it ever ran long enough to get thermal images that would pinpoint this particular problem, unless you could get an image of the interior of the motor - but if you were able to do that you would probably notice the foreign part in the wrong place without needing thermal properties of the camera.
 
11bgrunt said:
This thread is a week old, but I would have drug out a thermal camera at some point. Now that you know the cause, would that have helped?
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I work with an IR cam pretty often and I doubt it would have helped find and intermittent short caused by loose parts.
 
Meggered the windings on the armature

Meggered the windings on the armature

Saturn_Europa said:
I have a 15 hp, 3 phase, 480 v, 17 FLA, motor, running a multistage centrifugal pump. It was taken out of service today so our mechanics could replace the upper and lower bearings.

After re-installation it has been non-functional. The motor will come up to speed, run for a few minutes, pump water then trip the overloads. It has been blowing a single fuse, single phasing then dropping out on overloads. The fuses are LPJ-30SP , it was fully functional before the mechanics pulled it. It just had bad bearings.

The first time it did this it blew the B phase fuse. The second time it blew the C phase fuse.

I megged out the wire going to the motor from the motor starter and it was good. I megged the windings to ground and they are good. I ohmed out the windings and have roughly 0.9 ohms on each winding. There is no problem with the motor that I can detect.

The pump and motor spin freely by turning the fan on the top of the motor.

I am leaning to a phase to phase short caused by damage during bearing replacement. But I want to make sure I have thought about everything before I tell the guys they have to pull it again in 90 degree heat. Its not a fun job.

But I think I have checked everything, I can with the meters that I have. The wires are good going to the motor. Windings arent shorted or open. 480/277 V on the top of the motor starter before it I start the motor. Everything looks ok.

Any ideas?
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You have to remove the armature to fully megger the motor windings there are separate windings on the armature that cannot be measured unless you remove it.
 
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