Using DC relay with no known AC rating to control AC circuit? - Page 1
Forum section: electronics / beginners

author: ethereal
author_rank: Contributor
timestamp: June 19, 2017, 01:27:23 am
content: Hi all,I'm looking to control an AC inductive load (3A motor at 240VAC) with a relay, and was wondering if there's any way to tell if this random DC relay I found would be able to handle said load. The rating on the side of the relay is "12V 70A" but I can't find a datasheet so I'm not sure if it has an AC rating at all --- it's a part for an older VW vehicle (part #251919505) and so that may not have even been tested! The control circuit will be 12VDC so no worries about driving the coil.I'm guessing the only reasonable response would be "try it and see if there's fireworks", but I was hoping someone could chime in with some personal experience.Thanks!- ethereal

author: floobydust
author_rank: Super Contributor
timestamp: June 19, 2017, 02:15:40 am
content: No, you can't safely use a low voltage (automotive) relay to switch mains.Mains-rated relays have high isolation voltage between coil and contacts, and between contacts.So there is a danger of arcing across the contacts as they are gapped for 12V, and arcing from contacts to coil- which is disastrous.

author: Unknown
author_rank: Unknown
timestamp: June 19, 2017, 03:24:57 am
content: I wouldn't recommend it, especially with an inductive load.Why not just buy a properly rated relay? I am a fan of solid state relays for motors (overrated, for starting current).See:http://www.crydom.com/en/tech/whitepapers/ac_mc_whitepaper.pdf

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2017, 08:28:35 am
content: Yes, definitely buy the correct relay. The 12V automotive relay certainly will not have adequate separation between the contacts and coil.

author: ethereal
author_rank: Contributor
timestamp: June 19, 2017, 01:47:54 pm
content: Makes sense, that's what I'd guessed. I'd love to have one of those solid-state relays, but unfortunately, time pressure makes it not feasible for me to get one right now. Definitely something I want to add to my parts box for later... I'll just go with a manual control instead of the automatic one I'd hoped for.Thanks for the help!- ethereal

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 19, 2017, 01:52:48 pm
content: When (or if) you decide to get a proper relay for starting a motor, don't go by the ampere ratings alone!Because of the very specific inrush-current characteristics of an induction motor, relays are rated by the motor's HP rating.

author: lypse
author_rank: Contributor
timestamp: June 19, 2017, 05:03:46 pm
content: I believe the type of relay OP is looking for is a "Contactor"https://en.wikipedia.org/wiki/Contactor

author: ethereal
author_rank: Contributor
timestamp: June 19, 2017, 05:09:55 pm
content: Quote from: schmitt trigger on June 19, 2017, 01:52:48 pmGood to know --- I was planning on just over-spec'ing the relay to a large degree (10x or so) but I'll definitely keep my eye out for relays with an HP rating as well.Quote from: lypse on June 19, 2017, 05:03:46 pmAnd that should make my search much quicker. :-) Thanks!- ethereal

author: floobydust
author_rank: Super Contributor
timestamp: June 20, 2017, 07:29:46 pm
content: Motors draw high currents due to the mechancial load inertia and if they jam even higher current.You don't want the relay burning up on startup or if things get stuck.Contactors can take this extra abuse and have safety approvals for motor loads.General purpose relays likeOmron G5LEmight be rated 16A 250VAC but these numbers are for resistive loads only- no mention of motor load.I recommend a high power relay such asTE Potter & Brumfield T92which are basically a small contactor for $14. Rated 40A 277VAC resistive and 3HP 240VAC motor/compressor use.OP's motor is 3/4HP and this is a bit of overkill but reliable.

author: leblanc
author_rank: Regular Contributor
timestamp: June 22, 2017, 01:16:52 am
content: I like GIGAVAC contactors.

author: Mukrakiish
author_rank: Contributor
timestamp: June 22, 2017, 02:00:28 pm
content: Quote from: ethereal on June 19, 2017, 05:09:55 pmCouple things to note, you can get a relay that is built to "connect" an inductive load up to x Amps but it can only break a much lower amperage. This is what accounts for the inductive spike that happens with motors. What you are looking for if it's a relay, would be a "Power Relay" such as this:http://www.futurlec.com/Relays/PR12VDCSPST.shtmlIf it matches your HP rating, you're good to go. If you're concerned about having an overload so it doesn't overheat on a locked rotor, a contactor + overload would be a better approach but cost more as well. Depends on your needs. More often than not, a contactor will cost you more than a Power Relay since they are built specifically motors and can scale up in the 100's of HP.