Relays in Parallel

[kittysmitty]

Working on a low cost transfer switch between my gas generator and inverter for my off grid cabin. The DPDT relay I have is rated at 10 amps, not quite enough for my 1500 watt inverter. Can I put to of these relays in parallel to double the rating to 20 amps? Thanks

[madlabs]

Well, it will work. It's not reccomended though. The problem is that the relay contacts won't touch at the same time. So the fastest contact will wear faster. If the device being controlled has a sizeable start up surge, the problem will be a problem faster. I have to admit I have done it in the past, but that don;t make it a good idea.

20 amp relays are still really cheap, especially from a surplus joint. I'd go that route.

Jonathan

[ghurd]

I agree.
Also, something with contacts rated for some HP tend to take more abuse.
G-

[Madscientist267]

A bit of a brain fart, and may turn out to be worth even less, but came to mind anyway...

Depending on what your coil voltage needs to be, the 'standard' automotive relay can handle some significant juice, although I don't know how well it would deal with the higher voltages at the contacts...

Common current capacities are 30 and 40 amps.

Granted, they're SPST, so you'd have to run sets of them in tandem to switch both sides of the line. But they're cheap... like 5 bucks.

Eh just a thought; sue me, I'm tired today. 

Steve

[joestue]

there's no guarantee the relays will share the current equally.

[rossw]

Working on a low cost transfer switch between my gas generator and inverter for my off grid cabin. The DPDT relay I have is rated at 10 amps, not quite enough for my 1500 watt inverter. Can I put to of these relays in parallel to double the rating to 20 amps? Thanks

Seriously... and I mean it, as someone who has built, bought and used a good many of these over the years - do it *RIGHT* or *DON'T DO IT AT ALL*

The safest way is to use two, mechanically interlocked relays. Usually, its done with normally-open contactors. The coil of one is through an interlock contact of the other so it can't energise when the opposite contactor is on, and the mechanical interlock prevents problems if/when a contactor sticks or has its contacts welded.

Light duty relays are dangerous in this application because it's all waaay too easy for something to flash over and connect your genset to the grid - when you think it's off - and kill or injure someone.

[richard]

   Ford starder Relay 600+ amps.    richard

[Madscientist267]

Not trying to start a wizzing contest or anything, but -

flash over and connect your genset to the grid

gas generator and inverter for my off grid cabin

No grid. Worst case here is a serious smoke fest, with the potential for maybe some magic fire.

In and of itself, not really likely to kill anyone.

Inverter to genset is most likely going to just cook the inverter and irritate the generator while it's doing it.

You do have a point, however. Good practice is good practice. Not a good idea to cut corners, particularly as the power levels go up! 

Steve

[rossw]

Not trying to start a wizzing contest or anything, but -

flash over and connect your genset to the grid

gas generator and inverter for my off grid cabin

No grid.

yeah, my bad, good catch. I meant the "other supply". Can happen either way - if a changeover relay fails (I've seen it happen a few times), or worse, you use a make-before-break contact arrangement, you can get power feeding the side you "think" is dead. "I'm on generator, I can pull these mains wires out of the inverter now to relocate it...." and bzzzt, you're toast.

Worst case here is a serious smoke fest, with the potential for maybe some magic fire.

In and of itself, not really likely to kill anyone.

Yourself, when you "think" it's safe?

You do have a point, however. Good practice is good practice. Not a good idea to cut corners, particularly as the power levels go up! 

Yup. The other thing  I meant to say - if one were to use (lets say, as implied before) *TWO* double-pole changeover relays in parallel - whichever way you connect them, if one of them burns out a coil or gets stuck in one position - as soon as the OTHER one operates, you will back-feed through the relays and connect generator and inverter (or generator and grid, or inverter and grid, whichever is applicable) directly together.

Not necessarily a problem - but if things are out of phase, it's a very impressive kaboom.

Which leads on to the final point - a single changeover relay, even a break-before-make, will operate quite quickly. If your two supplies are not in phase, you can (will) get very nasty spikes that can (frequently will) kill most electronics. Imagine if the inverter is at its maximum positive part of the cycle, and the genset is just comming up to its maximum negative part of the cycle, your output will "almost instantly" go from +340V to -234V (240V supply) (+155 to -155 on a 110V supply). This "DV/DT" is bad for most electronics, and puts a lot of force on electric motors (and makes them draw a LOT of current while they adjust to the new phase)

[Madscientist267]

I've considered the phase alignment thing before, and even apply caution when this isn't a factor.

Many people, upon realizing they've unplugged the wrong cord from a power strip (for example), will plug it back in as soon as they realize its the wrong one, which can be rather quick sometimes. As if getting it plugged back in immediately would 'abort' the effects of  losing power to whatever it was that lost it.

Little do they know that even that alone can cause damage, depending on what lost juice. Unless its an incandescent light bulb or other purely resistive load, there's a good chance that it underwent some stress in the process.

For this reason, I've gotten into the habit of holding the plug just 'outside' the outlet for a few seconds (in case I took my source of light with it ) and regardless of what died from the disconnect, if it was the wrong device, it comes up from scratch rather than potentially damaging it.

Before I got religious about UPS units, I built several 'dropout' switches designed as electrically latching so that if a glitch was long enough to be a problem for a piece of equipment, it let the relay drop out, and the load completely lost power until manually reset.

Taking this idea further, when I first looked into using a generator with a changeover, I came up with an idea of a worm driven (via a motor) actuator that would take a few seconds to change over, eliminating changeover glitches, sticking contacts, flashover, and dv/dt issues from phase sync as well.

Never built the thing, haven't had the chance yet, but I'm throwing it out here because someone can probably build it sooner than I can.

Steve

[bob g]

do an ebay search for a "reversing contactor"

basically they are two relays, mounted side by side, generally 3 phase so they
have three sets of contacts on each relay (use what you like) the two are mechanically
interlocked as well as electrically interlocked.

this means that you cannot have both locked on at once, one has to disconnect before the other connects.

quality reversing contactors surplus are relatively  inexpensive compared to new, and they are generally all rebuildable with small parts kits making them a long term solution to most problems.

they come in a variety of sizes, 00,0,1,2, 3,4, and 5 and with an assortment of coils
voltages (coils are also replaceable) typical are 24vac, 120, 240 and a few other voltages like 208, 277 and higher vac, and there are also ones setup for dc coil operation.

there is also dpdt relays that would work for what you are after, again ebay is a good source, i bought a crate of 50 units for about 3 bucks each iirc that have contacts rated at 30amps @24vdc, 30amps @ 120/240vac. they are very nice units suitable for what you have in mind however they are not interlocked so while is unlikely that they could connect both at the same time, it is possible to have one set hang up i suppose but likely a rare event.

bob g