Your talking about the 'ride through" after a dip in power. How long do you want to keep the contactor closed after a dip? For short durations, a capacitor might be used. For longer durations, an UPS would work with an inverter.
This is an issue that seems to have made its way to the top of the concerns in the last couple years. Should be lots of other ideas.

Buzzp,

Sounds like an AC coil - would a capacitor help with that?

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If we learn from our mistakes,
I'm getting a great education!

Sorry I missed that but a rectifier with the cap may work. I am no coil designer to know what is really happening with an AC coil. I believe they are all rectified internally.

Personally, if this equipment is critical it should be ran with station service power (120VDC) rather than AC. Since this is the power that is on back-up with an UPS, why not use an inverter off from your back-up supply?  

 i appreciate your views buzz, but to our system it is strictly separated since our battery station is on marginal load.. and the load is more critical than this magnetic contactor coil.
  
 regarding with the voltage dip, we keep the contactor close approximately 50millisec up to 1 second or up to 3 sec. we tried a off-delay timer but still didnt worked.
 
 i think, as you pointed out, a capacitor will work. could you help me to roughly design this rectifier with capacitor, maybe you could guide me to come up a solution with such a calculation. i greatly appreciate it!
 
 buzz, how about a COIL - LATCHING RELAY? had you ever tried this device? how's the reliabilty withstanding on undervoltage/fluctuation and voltage dip?  
  

The cap idea is not real feasible with an AC coil. It could be done if you can switch out the rectifier when the coil needs to be held in place (if rectifier is simple diode then this will block flow to the coil).

I dont think a latching relay is going to help since the problem seems to be low voltage causing the contactor to drop out. If you look at the spec you will see the drop out voltage level of the contactor. The best you can hope to do here is replace it with one with a lower drop out voltage. Maybe I am misunderstanding your problem.

I guess I would be looking at replacing this contactor with one with a DC coil and use a cap to maintain the coil in the event of a voltage dip or other. This would eliminate the need to have a rectifier and some electronics to switch out the rectifier when the voltage dips (would need something to monitor voltage levels and energize a relay - not real expensive).

  
  buzz, im referring a "coil lock relay" product from www.pqsi.com but i never tried it before. you may check out also there "constant voltage transformer" and i beg your feedback about this product.

 by the way, my friend had offered me an idea of putting up of a capacitor in series with the coil. he said it would rule out   the voltage dip but he cant support it with rough calculation in designing the capacitor value and its circuitry. does his idea is feasible? right now im still on brain storming, figuring out how would this be!

 buzz, thanks for your support!

In series with the coil would not work. It would have to be in parallel with the coil and with a rectifier. This is essentially what the product you mention is going to do. If you read their data sheet, they say they do apply a DC voltage/current to the coil (yes even an AC coil) when it senses a voltage dip.

The only thing that bothers me is it would be nice to have a selectable ride through time. This device appears to hold it in indefinately at 25% or greater voltage.

On the surface, it looks like a good product but you never know until you try it. The ideas I have given you appear to be the way this product is designed.

Also, I noticed it only has "UR" (UL recognized) mark. This means there is something else that needs to be done to be a UL "listed" product. Usually this means branch fuses or something needs to be installed to be a fully UL recognized product. A UL listed product has no requirements for installation to be a safe product.

Hope this helps and I think I would at least try the device in the lab and see how it performs.

buzz, thank you for your suggestions. right now, we are still in deliberation with my colleague about the issue. i will inform you about the outcome of this matter.

If your drop is short (a few cycles), you might consider
using a resonant transformer to supply the contactor.
For longer periods (seconds), the D.C. coil would be more practical.  A large enough storage cap should be able to
hold it up through any reasonable dip.  Post the periods
you desire and the working voltages, and we can give you
some values.
<als>

Buzz
Interestingly there is a rectification in an AC relay
since the AC electromagnet works against a soft magnetic
moving part generating atracting force in either polarity.
This is why it hums at 120 Hz.

If the moving part would be a permanent magnet the
rectifying action wouldn't take place.
 

<nbucska@pcperipherals DOT com> subj: eng-tips
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