|Forum topic by srzsrz||posted 07-06-2014 02:22 AM||1145 views||0 times favorited||1 reply|
07-06-2014 02:22 AM
I’m restoring a rusty old Sears 113.29580 10” saw. I guess I was very excited about taking the thing apart and cleaning out all the rust and caked-on sawdust (what? don’t you love sanding old power tool parts?). So excited, indeed, that I never bothered to actually turn the saw on to see if it worked.
Fast forward to the point where I finally get to cleaning the motor. I figured that the easiest way to brush the dirt off the arbor would be to turn the motor on and hold a wire brush to the spinning arbor. So I proceeded to try to switch the motor on.
After a minute or so, the motor started to smoke a little. Then, a loud pop and a flash. I quickly switch off the power. At this point, I gave up and left to see the 4th of July fireworks with friends up in San Francisco. Today, I turned the it on again, and now smoke is coming from the switch box, and the start capacitor is feeling awful hot. No good. Better turn it off immediately.
After reading up on it a bit more, it became clear that the way this particular saw cuts out the start capacitor after it’s done its job is with a little relay operated by the inrush current — quite a clever trick. If the relay doesn’t operate, the start capacitor stays on, which doesn’t keep the motor from running just fine, but it overloads the start capacitor which isn’t made for continuous operation, so after a while it goes kerploof.
But why would that little relay have worked fine for years for the previous owner only to give out when I need it to work? The capacitor itself, sure, it could have dried out over the years, but this was clearly the relay, and the relay looked just fine.
In the end, I found it on a forum. Already lost the link, but it was buried deep inside a thread. These cutout relays don’t work upside down.
That was the difference between what I did and what the previous owner had done. He used the switchbox mounted in the saw in the normal way. I had taken the saw apart, and the switchbox was sitting on a bench. Upside down.
I clamped the switchbox in a vise in the right direction, wired everything up again, and sure enough, it ran fine. (Yes, even with the blown capacitor. I imagine it will dry out much faster now. I can see some crud coming out the vent hole. But it does still work.)