What is this contact for on Allen Bradley IEC contactor coil?

[ElectricalMaintenance]

I had a contactor fail and I suspect it was chattering. I took it apart to try to figure out what went wrong. I noticed that the coil has a small set of contacts underneath it that short out part of the coil (with contacts closed, I read 3 ohms across A1 and A2. With it open I get about 176 ohms). This contact was stuck open and I think that's why it was chattering. Does anyone know what the contact is for? There doesn't appear to be any mechanism that opens it so I don't get why its there at all.

Below is the coil with the Allen Bradley part number (TE714M) and a close-up of the contacts on the right showing pitting after only a few hours of running.

 

[retirede]

My WAG is that it functions similar to the centrifugal switch in a single phase motor. Once the contactor pulls in, the contact opens to deenergize the part of the winding that’s used only to accelerate the armature initially.

 

[synchro]

My WAG is that it functions similar to the centrifugal switch in a single phase motor. Once the contactor pulls in, the contact opens to deenergize the part of the winding that’s used only to accelerate the armature initially.

I agree with that analogy. The contact that the OP mentions is likely for connecting a pull-in winding initially to create a more intense magnetic field in the gap between the armature and the core so that there's enough force on the armature to move it and close this gap. Then the pull-in winding can be disconnected because the lower current in the holding winding is enough to maintain a suffficient force when the gap is closed to keep it closed.
It sounds like in the OP's case the pull-in winding by itself provided enough force on the armature to intermittently and partially close the contacts in the contactor, which then caused the observed pitting.

 

[synchro]

... It sounds like in the OP's case the pull-in winding by itself provided enough force on the armature to intermittently and partially close the contacts in the contactor, which then caused the observed pitting.

I should have said that it was the holding winding instead of the pull-in winding that was partially closing the contacts.

 

[retirede]

I should have said that it was the holding winding instead of the pull-in winding that was partially closing the contacts.

I was thinking that possibly some mechanical contamination prevented the armature from closing the magnetic gap completely, but it moved enough to open the contacts. This would cause a chatter and pitting of the contacts.

 

[tom baker]

Those IEC contractors are very compact with small parts. I took apart an IEC overload to look at the differential trip mechanism, it was not possible to open it without breaking the overload

 

[GoldDigger]

If you do not see any mechanical link to this set of contacts from the armature, perhaps it is directly affected by the magnetic field of the coil? If so, it might allow very high current through the smaller section of the coil until the magnetic field has built up against the inductance of the coil (designed for DC)?
Or might these contacts be subject to the "stray" magnetic field that is found between the armature and the coil core until the magnetic circuit is closed by the armature completing its travel?

 

[hbiss]

What is that gray "I" beam shaped piece just to the right of the red circle in the right picture? Looks like linkage that slides in and out?

-Hal

 

[ElectricalMaintenance]

What is that gray "I" beam shaped piece just to the right of the red circle in the right picture? Looks like linkage that slides in and out?

-Hal

Yes that is connected to the contacts similar to the part on a PB that pushes the contacts open. There is nothing mechanical to push on that part.

 

[ElectricalMaintenance]

Those IEC contractors are very compact with small parts. I took apart an IEC overload to look at the differential trip mechanism, it was not possible to open it without breaking the overload

I only opened it after it failed to investigate. This one it is possible to open without breaking and the coil is intended to be a replaceable part (Allen Bradley sells them separately).

 

[ElectricalMaintenance]

Thanks for the responses. I did a little more digging/destroying and noticed there are 2 sets of windings on this coil. One very small gauge and one larger gauge are in parallel across A1 and A2, but the larger gauge coil goes through the mystery contacts with one extra turn going around before going back to A1. To clarify, it goes from A1, one turn around to the contacts, out the other side of the contacts and then around many turns to A2.
Some kind of current limiting circuit looks likely, but I still can't tell how it opens the contacts. Everything is plastic except for the brass and copper contacts and wires. Nothing is attracted to a magnet. Very strange.

 

[hbiss]

Yes that is connected to the contacts similar to the part on a PB that pushes the contacts open. There is nothing mechanical to push on that part.

Has to be somehow.

-Hal