Magneticlly lached contactors 1

[akkev]

Please forgive me if this is a dumb question. But the dumbest 1 is the 1 unasked. My employer is trying to tell me that you can use ac power with out a rectifyer on the coil side of a contactor to latch it in. It has allways been my understanding that this could only be done with dc power. Otherwise you would get chatering (the coil pulsing to the frq. of 60 hz) and that if ac was used you would have to use a rectifier to rectify ac - dc. I asked how this could be true and how is it acoplished. The answer was EMF I then asked what? They didnt know what the answer to that. I can only assume that they mean Elactro Magnetic Frequency. I was told they would look in to it. As a Tech. not an engineer these are things I would like to educate my self on to better my self as an employee so to performe my job more safely and productively. Thanks for your time

 

[aolalde]

AC or DC can be used. The force is developed to reach the minimum reluctance of the magnetic circuit.
A high magnetic conductivity ( high permeability) frame has a mobile element with an air-gap. When a magnetic field (AC or DC) is imposed to the circuit a force to close the air gap is developed because air has low permeability or high reluctance, until the minimum reluctance is reached (zero air gap).Note that the force do not change direction when the polarity of the AC magnetic field reverses polarity.
Certainly an AC flux has zero crossings were the flux is null and this could cause vibration, for that reason a shade coil (short circuited loop) is added to get a second flux out of phase.

 

[davidbeach]

There are AC coils and there are DC coils, using AC on a DC coil could well result in chattering, but an AC coil will seal in, no problem.

 

[swn1]

AC coils on many (most? all?) contactors have shading coils. That's a shorted turn around part of the pole face that causes the flux through that fraction of the face to be out-of-phase with the rest of the flux, helping to reduce chatter.

Works, but DC works better. The AC stuff is a little more reliable, but the technology evolved in the absence of inexpensive and reliable silicon rectifiers. New stuff is all DC.

 

[instrument1717]

Suggession : Never Replace a DC coil ( or where DC coil is to be put ) with an AC coil of same voltage rating , It may malfunction /burn.

 

[rbulsara]

I am with davidbeach. A properly selected and applies (proper voltage) AC coil do not chatter. Only cheap, misapplied and/or poorly maintained contactors/relay chatter.

 

[eglaude]

This is perfect timing for a strange event that just occured to me with regards to AC coil contactors. Has this ever happened to anybody out there.

A standard contactor with a 110 Vac coil was chattering and was "humming" terribly. Suspecting the shading coil had failed somehow, we changed the whole contactor with a new replacement and the same chattering occured. Once the electrician removed it from the steel backplane of the enclosure, and moved gradually away from the steel, the chattering stopped and the contactor was experiencing less line hum. Moving it back to the steel gradually resulted in the chatteing and humming again. Any idea what could cause the magnetic flux to change, causing the contactor to chatter?

Thanks for you insight.

E

 

[jraef]

eleceric,
Seeing that you are relatively new, we'll give you a "pass" on this but for future reference it's considered inappropriate to post a new issue into an existing thread. Don't worry about it this time but in the future if you have a similar issue, you can make a reference to an existing thread by copying and pasting the "thread number" into your new thread, like this:
"After reading thread237-172621 I was reminded of a similar problem I am having." That way IF someone feels it necessary to read the background issue, they can click the link and then come back. By "hijacking" an existing thread, you run the risk of being ignored.

As regards to your question, either your back panel has become magnetized and/or you have current flowing in it, possibly a ground loop, which is creating an EM field emanating from it which is interacting with the field in your contactor armature. The easiest thing to do first is to double check all of your ground connections, i.e. remove them, clean, scratch off paint, corrosion etc., then reinstall them. If it doesn't go away, either switch with a new steel back panel or to aluminum.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[eglaude]

jraef,

Thanks for your comments. I appreciate the advice and I will post correctly in the future.

Thanks again.

eleceric