Motor control

Here's a solid state relay that might work, better than designing your own.

Bill

Not very exciting.

Better go for something that will commutate reliably with an inductive load such as alternistor or back-to-back SCRs. Or a relay.

Look at the LRA of the motor for an upper bound on the current, pick an SSR or specify output components that will handle the LRA for long enough for the compressor to start, will commutate reliably and then worry about the power dissipation when it's running and how much heatsink you'll need. Solid-state devices tend to fail on, so you may wish to double up on the safety relief valves (maybe one electrical and one mechanical)-- consider the safety aspects in any case, a ruptured air tank is like a bomb.

Personally, I'd go with the relay in most cases. You'll have fewer worries (but you'll have to have EMC under control to keep your micro from going off to meet Michael Jackson in neverland...).

Best regards, Spehro Pefhany

Who told you that? If correctly dimensioned, a triac will not blow apart. It may fire repeatedly (because an inductive load will create an excessive dv/dt), but that does not have to blow it apart. You may need a snubber, or snubberless triacs.

Best,

Compressors use itty bitty pressure switches to control the motor, and amplify the signal from the pressure switch with a big relay. They're not called relays, though -- IIRC the term you want to look for is "motor starter" or "contactor". If they're not available in solid state versions assume it's for a good reason.

I'd look for those terms, or I'd do a web search on "compressor wiring". If you find what I'm talking about you'll still have to switch current to the contactor coils, but it'll be less than an amp. You _won't_ be able to do anything fancier than turning the compressor on and off, but it sounds like that's all you want to do anyway.

You could also ask for the correct term on rec.crafts.metalworking, but do some web searching first.

Hello Spehro,

I'd second that. One has to keep in mind that where a compressor is located there may be other rather noisy gear, such as electric welders. Those can kill "semiconductor relays" in a jiffy. Then there are large angle grinders, shop vacs and whatnot.

BTW, Jackson ain't there no more...

Regards, Joerg

You seriously underestimate TRIACs. However an SCR, is the 'better' device for an inductive load... However the big problem is that you generate nasty interference, unless you are careful, because the devices can switch very fast. Look at the FAQ section of the Crouzet site, at:

and the catalogue for simple SSR's at:

Note that these devices contain a built in 'snubber'.

Best Wishes

As others have pointed out, SSR's have a nasty tendency to fail closed! You would be better off using a Contactor, which is nothing more than a relay that's designed to handle larger currents. Try your local electrical supply house, or even better

Triac based solid state relays routinely handle many dozens of amps and startup transients.

Why is an SCR better for inductive loads? I have to use two back to back, and isn't a TRIAC just 2 SCRs back to back, anyway?

Commutation (and not in the sense of lifting a death sentence or of taking I-96 to and from Novi every day).

A triac (it's not an initialism, so not capitalized) is NOT two SCRs back-to-back despite frequent misleading depictions as such. It's a single structure, and has only a very short time to commutate (turn off) even at 50/60Hz under the best of conditions. A triac with a high dv/dt rating with a snubber network added can be used, but two SCRs (or alternatively, an alternistor) probably would prove a more satisfactory solution.

Best regards, Spehro Pefhany

I did the same thing. I had a double pole contactor in there for a while, but the clang when it pulled it would make me jump more than the compressor itself. I put one SSR in there, but the starting current draw killed it. The replacement is a 50 A, 400 V unit, and it has been rock solid for several years. This is on a 2 Hp 240 V motor.

Jon

I always thought it was called a triac because it has 3 terminals and controls AC, hence TRIAC. I've never bothered to verify this, however.

Well I hope I-96 is at least better than Mound Rd :-)

Hey what brand/part number did you use? I usually design small delicate electronics, so I'm a little low on experience with this stuff. It's for a friend and I want to make sure it's reliable.