I am working on building a home automation system using an ethernet based connection. I am controlling my appliances via relays and i needed your advice regarding selecting a relay for controlling my 2 Ton air conditioner.
I have found this relay which seems to have a high current capability of 30 A http://www.seeedstudio.com/depot/Grove-SPDT-Relay30A-p-1473.html
Will this be good enough for my 2 Ton AC . Any other alternative measures that can be suggested for switching my AC would also be appreciated.
The relay can probably handle it just fine, but I worry about the connections for the wires. Usually things that carry 30 Amps, which is a LOT of power, have big connnections. A little heating there will cause arcing, which will cause more heating until the connection burns up.
Take a close look at SSR (solid state relay), these can be controlled directly by the arduino and isolate the control from the SSR pretty effectively. They last a long time; I've never had a failure. They have big connectors. They don't make any noise.
ost large 220v AC units use "contactors" to do the power switching.
That's totally true, and I'm starting to wonder why. I used to have a bunch of contactors around the house. The water heater, well pump, pool control, both AC units, and I've been replacing them with SSRs as they start to have trouble. The well pump was a real problem since it was outside near the ground in an enclosure and ants would get caught between the contacts as they looked for food. Then other ants would smell the roasted one and come to investigate which roasted more ants .Bug poison would help, but the rain (rare) would wash it away and back came the ants.
After a short while, the acid from the roasted ants would corrode the contacts and I'd have to clean them to make the pump work. Then after about three times doing that, I'd have to replace the contactor, which is a high dollar item at the pool pump repair store. Now, with an SSR in place of the contactor, I haven't even seen a sign of a problem. Had to use a heat sink on it though since the combination of summer sun and current through the device got it too hot. I also had trouble with a contactor on the water heater. It was a 110V device and made noise. I found out that that is a really common problem with contactors, they eventually start to buzz. The SSR I put in there solved the problem nicely and didn't need a heat sink since it was indoors.
I only have a contactor on one of my AC units, so far. The contactor started to have trouble and needed to be swatted with a wrench every once in a while to knock crud out of it. I tried to keep the various animals that were pooping on the AC condensor and causing the problem away, but you can't watch it all the time and a cactus wren just never gives up.
I've had the $$$ ants in the contactor issue. The contacts in the contactors have a thin coating of silver that can result in more issues if some of the silver is removed during cleaning.
OK, you need input from other people as well on this, but:
That one will do the job, but in the US, we have split phase 220 which means that each leg has 110V to ground and 220 between them, so I always use 2 SSRs to control this. I shut off both legs so that I don't have to worry about one leg to ground grabbing me when I stick a screwdriver in there to tighten something.
Also, I am really careful about shutting off the power if I need to get in there with a screwdriver. These little devices have a tiny amount of leakage, and although I've never had a problem, I'm chicken and shut the power off before touching anything with anything besides a meter lead.
There's usually some suppression circuitry built into the SSR that causes the leakage, and that can confuse you, so read up on it a bit so you understand.
You can also use a double SSR or even a tripple and leave one leg unused if the price works for you.
Thanks a lot for the suggestion!! Will surely check up on the points you mentioned especially about the leakage current which i guess is the biggest disadvantage of SSR's compared to electromechanical ones.