My table saw has an relay on it, controled by an on/off switch. Today it wouldn't turn off unless I unplugged it. When I plugged it back in, it ran immediately.
I figured either the on switch had shorted, or the relay had fused on. The relay has 4 poles. When I move the wires to another pole, it works fine; so I guess the relay fused.
But, I know very little about relays, so I don't know how good an idea this is. When a relay fuses, is it just the one pole affected and the others are okay? If so, I should be fine until this one fuses also. Or does it damage the whole relay, in which case I should be replacing it?
I guess the real question is, is using a different pole a reasonable thing to do?
Welded contacts most likely. Relays like this are often a single "activator" with multiple spring-loaded contacts. One pair of contacts has welded closed.
Or could be broken spring, a piece of grit or something.
If you have the spare contact pair, sure. But it's a little odd that you have a spare pair. Perhaps a 120V/240V convertible saw wired for 120V.
That being said, in some relays a stuck contact pair _may_ begin to interfere with relay operation. Stretching springs a bit much, distorting activator movement, increased likelyhood of jamming etc.
If possible, I'd try to free up the bad contacts to prolong the lifetime of the relay (even if you're not going to switch back to the burnt contacts).
If you can't open the relay housing, give it a good sharp rap with screwdriver handle. If you have a VOM, attempt to test whether you've managed to free the contacts. Don't worry if you can't, it may be a broken spring (which means you probably can't get them to stay apart).
If you can open the relay housing, examine the contacts, and pry them apart if stuck. Don't bother putting the wire back on the contact unless you can polish the contacts - ie: with an emery board (and there's enough metal left on the contacts).
Generally speaking there's no hazard in continuing to use the relay even if the unused contacts remain fused other than it may refuse to stop again in future.
No, I think he just used something surplus. It is a 4 pole 120v relay, max
1hp. Since the motor is 2hp he jumpered two of the poles and used both. One more pole is used to maintain current to the relay after you let go of the the start switch. That leaves one extra pole. Since only one of the original two poles are fused (or have a broken spring), I was able to use the extra as a replacement.
I would have to pull the whole thing out to even look at it; just changing the wiring was not easy. I will save pulling it out for when I have to replace it.
Since you know about relays, let me ask you another question. This is a $100 potts and something or other. In the McMasterCarr catalog they have a
240v (if I have to replace it I will rewire from 120v to 240v) 25a relay for $12. Will something that cheap actually work? The motor is 8a at 240v, so it is well within the 25a limit, except at start, where it is probably 28a or so. Will that be okay?
Just make sure your motor is capable of operating at the higher voltage. And that you convert it correctly. Otherwise your next post is going to be "where can I get a replacement motor for my table saw?" :-)
You need to find a HP-rated relay - because the startup surges are many times the plate rating. Indeed, "stop" spikes (inductive kickback when current thru an inductor is interrupted abruptly) can be pretty nasty too. That motor will probably spike well over 30A.
_Big_ amp zaps just when the contacts aren't quite in full contact.
They _can_ be that (or near that) cheap, but be sure to find one that actually _says_ it can handle 2HP motors.
Dredges up some fond memories from the cobwebs. We had nightmare problems with inductive kickback on some computer equipment until some smart soul(not me) came up with the idea of putting a magnetic reed switch in parallel with the motor switch around which was wrapped a coil of #14 wire in series with the motor, now when the motor was on the reed switch merrily toggled on-off at 60 hz, no current flowed through the reed switch, when you turned the motor switch off, current flowed through the reed switch for 1/2 cycle till the zero crossover and since no(or almost) current flowed through the coil the reed opened and no kickback.
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