Hi Martin, it sounds like you are describing a "Start" circuit.  Push the
start button a small relay pulls in... A contact on the small relay then
shorts out the Start button... This provides power to a bigger contactor.

If this is so could your problem be vibration? If the smaller "latching"
relay is being vibrated "open" it could cause the contactor to drop out.

I saw this problem years ago on milling machines. The smaller sealing relays
were being vibrated open when doing aggresive cuts on steel. Things worked
fine milling air. Things worked fine milling plastic. Things worked fine
milling some steel. But, try to make an aggressive cut on heavy steel and
the spindle motor would just shut itself off. We solved the problem by
moving where the small relays were mounted.

Now that I read your post again I see that you are having problems getting
the circuit to latch in the first place. Maybe the small relay has dirty
contacts. Check the wiring for loose wires too.

Hope this is some help

-- Mark

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
> Of Martin K
> Sent: Wednesday, August 09, 2006 11:07 AM
> To: piclist
> Subject: [EE] contactor dropping out
>
>
> I have a problem with a contactor (relay) dropping out in an industrial
> control system. The contactors have 24VAC coils. I have hooked an
> oscilloscope to the 24V line and it appears to not have an issue with
> sagging, but it's hard to tell even on an oscilloscope when the event
> could be happening for just a few mS.
> Is it common for contactors to wear out in a method that prevents them
> from staying on, or should I be looking closer at the circuit that
> controls the coil?
> The contactor is supposed to be latched with another smaller relay that
> is fed by a momentary switch. After several attempts at getting it to
> stay on, it will eventually stay on for an extended amount of time
> (presumably when something warms up?) and perhaps drop out at an
> inopportune moment.
> I know I haven't provided a lot of info but hopefully someone has had a
> similar issue - I don't have much experience with this.
> Thanks,
> Martin K
>
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