24 V control for mechanically held AC lighting contactor.

[antenna2001]

I have installed a Square-D, class 8903, Type LX, mechanically held lighting contactor. A Methasis panel supplies one control signal. The problem is that this contactor requires two signals, one to latch and one to unlatch. I have been unsuccessfully looking for a multiplexer that can route the signal, alternatively, to the latch and to the unlatch terminals.

I could design a multiplexer but that would require several timers and contacts. The cost would be prohibitive.

Does anyone know of someone that manufactures such a device?

The contactor's coil is rated at 120 V, 60 Hz, and the signal from the Methasys panel is 24 VAC.

Thanks

 

[Skogsgurra]

If there are spare auxiliary contacts available on the contactor, the contactor and a simple relay can do the job.

Let the control signal pull in the relay. Relay activates ON coil via a NC contact of the contactor. As soon as the contactor has pulled in, the NC opens. When the control signal goes to zero, the relay drops and activates the OFF coil via a NO contact. Contactor drops and the NO opens. This usually requires late action of the auxiliary contacts.

Some makes of latching relays have auxiliary contacts for this purpose built in. All you need then is a relay to make two active signals out of the single one.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

There are also alternator relays available for this purpose.
There is a small cog wheel inside the relay and each pulse on the relay coil advances the cog wheel. each step of the cog closes all the open contacts and opens all the closed contacts.
Also look for "Pump Alternator Panels". These panels control two pumps from one signal. There is a standard circuit with relays which you could use for your application. You may be able to adapt the circuit to work with the existing auxilliary contacts as Gunnar suggests. If you don't have the contacts to use Gunnars method, try an alternator circuit.
Hint, In many alternator circuits, the main motor contactors act as relays in the alternator part of the circuit. For a bare alternator circuit, replace the motor starting contactors with small relays.
respectfully
respectdully

 

[antenna2001]

Thanks to both skogsgurra and waross. I'll study the alternatives you give and make a decision.

 

[MikeHalloran]

A long time ago I tried to debug a rinse hoist control (lift / travel/ drop/ dwell/ rise/ dwell/ travel/ drop) that was implemented with latching contactors.

I failed. So did the six guys who debugged it before me, and left tracks of their various modifications and improvements...

... all of which worked just fine (I rewired it to test every damn scheme), as did the original design...

... until there was a power glitch while the load was down in the tank. Then the damn thing went crazy and tried to move the load with the load in the tank, and tipped the tank over, again.

Since that adventure, I have pretty much declined to work on control panels, and when pressed for a reason, said something to the effect that latching contactors are the work of the Devil himself.



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

Mike... I could not agree more. If you are into needing latching relays you should be using a PLC or some other logic machine.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I've been there also.
I consider latching relays to be a necessary evil when a machine must remember the position it was in when the power failed and restart from that position.
I much prefer stepping switches.
I have found that problems are usually not with the latching relays themselves, but a conventional relay de-energised when the power returns when it should have been energised.
My most memorable latching bug was a PLC that used a latching function so that the operators could not "short cycle" a safety purge time by cycling the power. There was a branch in the programing logic that was entered very infrequently. The PLC was in Toronto and I was in Vancouver when we discovered that if there was a power failure while the logic was in that branch, the PLC would lock-up permanently.
Yards of brown wrapping paper on the kitchen table and a very long phone call later, I gave them a "Kludge" that got the process back on-line until I got back across the country and re-wrote the program.
My bad, it was my mistake in the original latching application.
I appologised for my mistake and didn't charge for my time to trouble-shoot and reprogram the PLC.
The customer liked my style enough to give me two subsequent, lucrative projects. I haven't had to use Boolean since.
But, latching relays and functions, challenging, relays or PLCs.
respectfully

 

[DaveScott]

I have managed to avoid latching functions (L) and (U) in my PLC programs for all of my working career, by using holding contacts on standard coils. Just my style and preference, I suppose. Whether it's better or not, I couldn't say. I just don't trust my coding discipline any other way.
I am probably mis-understanding the original poster's intentions, but I am having trouble with the concept of the mechanical latching contactors with a single signal. I presume that the antenna2001 wants the lights to relight if they were on just before a power failure, or stay off if they were off just before the power failure. But the single signal to the contactor may be temporarily off during power re-application (glitch), in which case the contactor will open.
In this case, wouldn't the same result be achieved if it was a standard contactor with a signal which is held ON/OFF, say by a selector switch?