Lighting contactor troubleshooting

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electricmanscott

electricmanscott

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Boston, MA
I have a lighting contactor that controls two 20amp 120v circuits of lighting. The coil is also 120v. There is a wall mount motion sensor that activates the contactor. I installed this about three years ago and there is an identical setup with two additional circuits. The coil had some sort of melt down two months ago and I replaced it. Everything else seemed fine. The coil has melted again and this time the circuit board on the accessory terminal block seems to be fried as well. I find no obvious causes for either of the problems. I need some suggestions as this kind of work is not my speciality. (I know wires are wires :D )
 
Re: Lighting contactor troubleshooting

What powers the coil? Is it a transistor circuit?

Edit: A photo of it would help.

[ July 13, 2005, 12:05 PM: Message edited by: physis ]
 
Re: Lighting contactor troubleshooting

Yes, a photo or make and model number would help too.
 
Re: Lighting contactor troubleshooting

Contactor coils usually only fail due to:
a) high voltage (i.e. 140V on a 120V coil)
b) low voltage (i.e. 100V on a 120V coil)
c) excessive sealing current

I am guessing (c) is your problem. If the contactor does not seat properly then it will draw more current than it is supposed to and eventually "melt". Some items that can cause this are: dust causing the armature to stick, rust or dirt on the magnet faces, or a broken shading coil which will cause chattering.
 
Re: Lighting contactor troubleshooting

Jim, you're suggesting that the coil will draw substancially more current depending on what the contactor plate (I forget the correct term for the thing) is doing. I'm not necessarilly disagreeing with you but I will say that I'm having a hard time believing this can lead to melting a coil.
 
Re: Lighting contactor troubleshooting

Is it possible that overloading the switched outputs could cause the thing to overheat? It looks like heat could kind of get trapped in it.

Have the loads changed?
 
Re: Lighting contactor troubleshooting

I think Jim has hit on it. The coil will draw minimum current with the armature properly seated on the core.
 
Re: Lighting contactor troubleshooting

I'm having a hard time with that. If relays do act that way I'm gonna have to learn about why.
 
Re: Lighting contactor troubleshooting

The "resistance" of the magnetic circuit is not at it's lowest value if there is an air gap in it's path. This increased resistance causes an increased current in the coil.
 
Re: Lighting contactor troubleshooting

the circuit board on the accessory terminal block seems to be fried as well
Click to expand...
If I can guess, there is some electronics on this board working with the the coil of the contactor. Could be signal conditioner, activation time delay, simple shunt resistor etc. I think the problem could be there.
 
Re: Lighting contactor troubleshooting

I have to agree there is probably some interferance with complete seating of the magnet armature. Sometimes the coil meltdown will coat the surfaces of the surrounding laminations and make the mechanism prone to sticking especially when hot. It probably fried the control accessory board from overcurrent/shortcircuit.

Was it open or closed when examined? If it was stuck open, there is the diagnosis. Closed, not so strong a case. I haven't seen a big problem with a small amount of rust, dust, buzzing of the contactor causing failure if the magnet could pull in completely (except for the rust or dust between the two magnet pieces). usually buzzes this material out in short order :D

I have run into a bad batch of contactors before. After the 5th or 6th one :mad:

Jim
 
Re: Lighting contactor troubleshooting

shading coil with a crack in it is my guess also. A contactor acts a bit like a motor as far as in rush current is concerned and the shading coil is designed to help the armature move as it is a sort of short cicuit in a loop around the magnet and for a split second increases the magnetic field of the stator or fixed portion of the device until the awmature plate "seals in". When fullt seated the coil draws very minmal current as it is no longer working to puuuulllll the armature in but rather to hold it in. Think about pulling an anchor up. When you start to pull it takes great effort. If you stop your pull it doesnt take as much. Almost the same principle. Hopes this helps
 
Re: Lighting contactor troubleshooting

The "shading coil" on a magnetic contactor is used to retard the rate of change of the magnetic field caused by AC. this retarding action allows the electro-magnet to stay on as the voltage crosses zero. Without it the magnet would drop in and out with a loud "60 cycle" chatter.

We used to call this item (the coil of copper in the magnet assembly) a flux sucking shunt.
 
Re: Lighting contactor troubleshooting

I've been unable to find any reasonably comprehensive info on relays. Can anyone recommend a decent site?
 
Re: Lighting contactor troubleshooting

Thanks Stew, but I don't want to buy them, I want to look into the magnetic resistance concept with the armature. I suppose it's possible but I wouldn't expect them to get into that. Would they?
 
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