Mains Switch Over Relay

[aaron]

Hey There,

I was wondering, is it possible to power a DPDT relay coil off of mains, use the NO contacts that will close upon coil current flow, for mains output, and use a backup battery inverter supply on the NC contacts that will be activated upon power failure?

I've never done anything like this before, I don't want to try without checking with you guys in case there's issues with the idea, the first thing that comes to mind is feedback from when the contacts switch over that could possibly damage equipment?

What are your thoughts?

Thanks

[nihilism]

You can, if the relay is rated for mains on the coil.

A safer option, get a small plug pack and a relay with a coil voltage the same as the plug pack and connect the relay to the plug pack.

[han]

No problem, if the inverter and the relay have a correct rating. UPS for computer do similar things

[aaron]

Awesome. Thanks guys. I might do this at a later date. It is for a UPS type device, more of a custom one.

nihilism I'm not sure what you mean by a plug pack...?

[nihilism]

http://www.altronics.com.au/index.asp?area=prod&grp=268

[aaron]

Ah right...

Do you mean get a relay that's rated for say 240V 10A, but has a 6v coil or something, and run the coil off of the power plug pack?

Wouldn't it be easier to just run it directly off mains since you run mains into the NO contacts anyways? Or is there a reason?

Thanks

[dfmischler]

You will probably have a tough time getting the power switched over within one cycle without serious power disturbance using that technique.  It might help to synchronize the oscillator for your inverter to the mains waveform so that you can pick up in the right place when the mains power drops.

You might even find that the inductance of the relay coil prevents the relay from even "noticing" the power drop that fast, let alone the "dead time" between the NC contacts breaking and the NO contacts making.  Anybody got relay opening switching time numbers?  You might be able to find something that was designed just for this purpose if you look hard enough...

Do you mean get a relay that's rated for say 240V 10A, but has a 6v coil or something, and run the coil off of the power plug pack?

Wouldn't it be easier to just run it directly off mains since you run mains into the NO contacts anyways? Or is there a reason?

The hold time on the plug pack would almost certainly delay the switchover even further and so this is probably not what you want to do.

[fcb]

You can, if the relay is rated for mains on the coil.

+1

[SeanB]

If you intend to use a relay for switching mains you need double break contacts as well, so that a contact that welds shut will still be opened. Thus you need a 4 pole contactor, 2NC and 2NO AC1 rated contacts, with a mains powered coil, and a single auxillary contact block ( 5A AC3 or 28VDC 1A) to drive the inverter power enable. This will typically have a 3-4 cycle delay between mains fail and the main contacts opening, and another 3-4 cycles before the NC set close on power loss.Typically they use 3-10W of power to drive the coil, and will handle 16A max current, though higher current units are available with longer release times. You will also need a VDR across the mains coil and on input and outputs to reduce arcing of the contacts on operation. Common electrical industrial units, but get pricey as power goes up.

[dfmischler]

This is a pretty good introduction to UPS technologies.  Your design concept so far is of the offline type.  I have had good experience with the line interactive, ferro-resonant and online types at client sites.