Electrical troubleshooting question

I know this is a bit off topic, but there are many electrical experts here, so here goes.
I have a machine at work that is a standard 3 phase, 480v. They called me out to the machine today and somehow all the motors are trying to run backwards, like a phase has been switch or something. The odd thing is that it's all the motors on the machine. Something like 5 motors. Checking voltage between each of the 3 legs, gives the standard 480, except for between 1 and 3 that reads 0. Now periodically, the machine, will all of a sudden start running correctly and may do so for several days, until this happens again.

I checked contactors all day, with no luck. Any other ideas?

Sounds like you are intermittently loosing 1 phase.
Check for voltage across a fuse or contactor. For example, if a fuse is good, it would read just 1 or 2 volts. If it's open, it will read 480.
Do the same thing across each contact on the contactor. Don't bother with the center phase - it's 1 or 3 that's going open.

You have an intermittent somewhere. The only way to find it is when it's in the failure mode. With power off, check for loose connections at contactors and plugs.

I would replace all 3 of the incoming 480v fuses. A lot of fuses have the element soldered to the inside of the end caps. On rare occasions the solder connection will crack and you will lose conductivity through it. The cracked solder connection will have a certain amount of resistance just like a bad wiring connection. Under load, heat will be generated at the bad connection which can cause the fuse element to pull away from the end cap. Once connection is broken the element cools down and conductivity is re-established. It's happened to me twice. Motor on the machine runs forward at the beginning of the shift, shut it off and restart it and it runs backwards. Just something for you to check.

Power down the feed before checking for this fault.
If it fails open under load it could cause an arc that can have devastating results if it flashes to ground. You don't want to be standing in front of it without full arc flash protective gear.
You can do the same tests, fuse and contact resistance with power off. If you are not a qualified electrician with the proper tools and PPE find one.

Brian.

thanks for the replies,


Keith, I did check across the fuses, no luck

Vette, The incoming voltage comes through a breaker that is not tripping. I've removed all wires from it and checked continuity across it, it checks fine.
Sliver, I am qualified to work on the equipment.

I would start looking for ground faults. Get your megger and have at it.

Patriot,

Now periodically, the machine, will all of a sudden start running correctly and may do so for several days, until this happens again.

Click to expand...

The above means that it is an intermittant problem, the hardest type to find.

1. It has to be something that effects all 5 of the motors.
2. It has to be something that can come and go randomly.
3. You have ruled out the fuses. (You are sure, aren't you?)

The only scenario left that make sense to me is that the main breaker is loosing one phase at times. Just because it doesn't trip does NOT rule out one of the internal contacts in the circuit breaker working intemittantly, at time connecting and at times not conecting.

Once the main 2-pole breaker on the power pole feeding my house failed on one leg, with the other pole still good. It did not trip. It caused a lot of problems, with 240 volt loads trying to run on 120 volts. I turned the power off and checked it with an ohmmeter. It checked good for continuity on both poles. With a load, it sometimes worked on both poles, sometimes not.

Lancie,


I agree, the most logical explanation is that it is the feeder for the rest of the panel that is sometimes failing. Now, to be honest, I did not replace it, however, I checked it many times over and it always shows good checking continuity(with power off).
I could replace it, but this has the ominous feeling of throwing parts at the machine. NOt that I'm above that, it's just that 99% of the time, that doesn't work.(at least for me!!)
However, I will do it Monday morning unless a better idea comes along.

What about the REVERSE PHASE RELAY!! If your motors are running backwards or single phasing either the controller is not equipped with a proper power/phase monator or it's wired incorrectly.

well seems to me if when this intermittent problem arises and you have 0 volts between legs 1 and 3 or whatever, are you checking it at the feeders coming in or where, i mean if its at the feed for the main breaker then you may want to check further upline, what i,m saying is pinpoint where the phase loss starts. usually tho when a phase loss occurs it doesnt cause motors to run backwards but rather and over load condition.

Is the 480 (277) coming from a delta or wye secondary?

I agree with the PPE post, you are dealing with a strange one here, but I think you will only find the problem with the power on. Trace it back all the way to the feeder until you find your missing phase.

He said they were "trying" to run backwards. I assumed they're sitting still and shaking.
Here's a scenario. One contact is bad in the contactor. One time, it flashes and welds - good connection. Denergize breaks the weld, exactly what a contactor is designed to do. Next time, the contact arcs and blows the metal out - no connection.

This weld/blowout scenario is what's happening, but where? Inside contactor, inside fuse, inside a screw lug, etc. You're not going to find this with a voltmeter. The tools needed here are a flashlight and eyeballs.

One thing is for sure. This intermittent will go solid soon, after the defective part finishes burning itself up.

Consider a Phase Failure Relay

This type of relay is designed to prevent damage to equipment and to operators.
Is it just one machine or is it your whole plant?

Last March our Hydro station lost a phase and shut us down for the day. After that day we still seen the effects of the failure for two weeks. We had coils blown all over, and like I said it took a long time to test the whole facility.

John

Whenever I have seen this type of event it was a bad contact.Like everyone else has sadi look at what is common to all of the circuits. With powers on, look for a voltage drop on the contacts.
Generaly, what ever "moves" in an electrical circuit is the part that will give you problems first.

If the machine is running or can run when you get back on Monday, check each phase to ground. Then when things go wrong check the same points to the same ground reference and see what you get. Where ever you get a voltage drop to ground the circuit upsteam of that point is your problem.

What you are looking for is a big difference in Voltage. It could be some part of your circuit is acting as a current shunt but you said 5 motors at 480 volts and if you where shunting current in something other than a fuse it would get quite toasty.

Recently a customer of ours ran into a similar problem. The cause ended up being thermal expansion/contracting of the wiring that loosened lugs on a contactor. The early signs are loose terminal connections and oxidization on the wire and terminal. This was on a 400 amp contactor (soft start bypass). The customer has since changed his PM program to be more frequent and we advised geting a thermal camera for spot checking near the end of the shift while the machine is running. The weakest link in a power system are the wire connections.