Indicating fuseholder conducting current?

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sii

Senior Member
Location
Nebraska
I recently found something while troubleshooting that I am now having a (putting it politely) heated conversation with a co-worker about.

Equipment: 1.5 hp, 3 phase, 480 volt electric motor, line started, connected to a gearbox, further connected to a fairly constant load. The starter is powered directly by a momentary pushbutton used to jog a small part of the machine into place. Starter is fed by a Littlefuse 3-pole indicating fuse holder. The load on this motor is rarely above nominal current.

The operators complained that the motor would occasionally not respond to the pushbutton but it seemed random and might only last for one or two pushes of the button. It would then work properly for some time. One of the guys on another shift then mentioned that he had changed one of the fuses out a couple of times and that seemed to make the problem go away for awhile. This had been going on for some time before I had the opportunity to take a look at it.

The first time I was called for this I found that one of the poles of the fuse holder was lit indicating a blown fuse. However, the light did not look "right" to me, it seemed to be dimmer than I would expect. The really odd thing is that the motor was still operating normally, even though the fuse was showing blown. I shut the machine down and traced the circuit, finding a very old wirenut where the plastic cap had vibrated off exposing the coils in a conduit body:Dthat was very moist and rusty inside.

I replaced the conductors and cover on the conduit body (eliminating the splice) and have had no problems since. My theory is that the exposed wirenut was occasionally shorting to ground, causing the fuse to blow and that the motor was still drawing power through the indicator of the fuseholder. I wish I had experimented with this a little more before I repaired it but I needed to get the machine running again.

My co-worker says I'm full of @#$%. We don't see eye to eye on much, mainly due to, again putting it politely, the amount of motivation he shows and I really want some backup on this. Can anyone provide some insight on this? Am I crazy?
 
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ray cyr

Senior Member
Location
Yakima, Wash.
My guess is that the fuse blew after the motor was at full operating RPM and the motor then kept running in "single phase." I agree with your co-worker about the motor not drawing current through the indicator light. Think of it this way, if the motor circuit had blown all 3 fuses would it keep the motor running? It better not!! Otherwise, what would be the purpose of having these fuses?
 

sii

Senior Member
Location
Nebraska
Did you do any testing of anything? Megger test at least? What were the results?
Nope. It's only about 25 feet to the motor so my first thought was to open and inspect all access points which is when I found the wirenut. Fastest course of action was to replace conductors and get running again. It was quite obvious there had been some arcing in the conduit body.

My guess is that the fuse blew after the motor was at full operating RPM and the motor then kept running in "single phase." I agree with your co-worker about the motor not drawing current through the indicator light. Think of it this way, if the motor circuit had blown all 3 fuses would it keep the motor running? It better not!! Otherwise, what would be the purpose of having these fuses?
My thoughts too but it started repeatedly with the blown fuse.
 
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gar

Senior Member
100828-1923 EST

If the source is a Y supply, the motor is Y connected, and there is a neutral, then if the motor is not too heavily loaded, then it is working on two phases.

If the load is not too high and there is some crude rotating magnetic field, then an AC induction or synchronous motor will start. Two phases are sufficient to do this. Basically single phase motors are really some sort of a two phase motor for at least the starting period.

There was was an induced voltage from the motor phase that was open and that is why the fuse holder light was dim.

.
 

mcclary's electrical

Senior Member
Location
VA
100828-1923 EST

If the source is a Y supply, the motor is Y connected, and there is a neutral, then if the motor is not too heavily loaded, then it is working on two phases.

If the load is not too high and there is some crude rotating magnetic field, then an AC induction or synchronous motor will start. Two phases are sufficient to do this. Basically single phase motors are really some sort of a two phase motor for at least the starting period.

There was was an induced voltage from the motor phase that was open and that is why the fuse holder light was dim.

.



I agree with all the above. I've seen entire production facilities running on a lost phase. They could kinda tell the motors had less power, and the lights were a little dim, but they didn't realize they had lost a phase. Everything was still running, which was inducing voltage onto the lost phase
 

millelec

Member
Location
New Jersey
I agree with all the above. I've seen entire production facilities running on a lost phase. They could kinda tell the motors had less power, and the lights were a little dim, but they didn't realize they had lost a phase. Everything was still running, which was inducing voltage onto the lost phase
have also seen 3 phase systems running w/a lost phase. we've installed overload relays that will trip out motor contactors when the overload senses a phase loss/phase imbalance or phase reversal. we have some 35 year old A/C chillers from Trane that use hermetic (canned) 480V 3 phase motors to drive refrigerant pumps. price to replace motor is $15,000 since we upgraded. older motors were $25K-30K (if you could find them)
 
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