Reversible motor runs fine one way, blow circuit breaker in reverse.

S

silence dogood

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I have a Lathemaster 8x14 about 10 years old. The magnetic on/off switch started chattering, then only start but would not run. Traced the problem to a loose clamped on lug. Replace and still had some chattering when turned on. And just for grins tried to run it in reverse. Blew house breaker. I don't use the reverse much but it did work before. Traced problem to a couple of contacts in the forward/ reverse switch. Both contacts measured about 20 ohms when it should be zero. Not good, Before ordering anything traced the wiring and made a schematic. The guy that wired this thing must be totally color blind and get his kicks out of staring at bowls of cooked ramen. Example. the hot wire on the power cord is black, good. Then it is connected to a GREEN wire, then red, then white. The neutral on the power cord is white, good, then brown (huh). I won't even describe the mag switch and all the circuit loops. By the way the mag switch has no nomenclature of any sort. So started from scratch, Replaced both the mag switch and the forward/reverse switch with common standard mag switch and drum switch and made the wiring simple. Works great, except that it still blows breakers on reverse. The motor has a start and run caps. Can that be the problem and if so why? I know that motor caps are really two caps in one. Could one be shorted? They don't measure that way. Any help would be greatly appreciated. Mark
 
One thought would be to check and make sure the switch itself, or the wire from the switch to the motor is not doing something weird, like shorting to ground. You could test that with an ohmmeter, with the lathe unplugged from power. At the motor, with the switch connected, both forward and reverse wires should read the same resistance to neutral (they may take a few seconds for the capacitors to charge to whatever tiny voltage your ohmeter applies before the reading settles). Both should read infinite resistance to ground.

Note that at least one convention I've seen is that on a reversible motor, the BLACK wire is neutral, white and red are for forward and reverse. In that case if you hooked up neutral to white, and are switching between black and red for F/R, that could explain the problem. I know this contradicts the home wiring standards but in DC, red is positive and black is zero/ground (as in a car battery/jumper cable). So standards aren't universal ...
 
Can you power the motor directly to the line bypassing the lathe switching to verify the motor itself is ok? If it runs in one direction I think the motor itself is probably not the issue- see below however
I suspect the switching is the culprit. Does it trip the breaker even with the motor disconnected? Rabler's advice is sound.
Post some pics if you can
-Mark
We had a recent member who had the exact same problem. The culprit? One of the cap terminals was shorting to the cover such that in one direction the terminal was cold and the motor ran and in the other direction it was hot and tripped the breaker.
A variation on this would be if one of the caps had an internal short from one terminal to the metal case.
 
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Thanks guys. I'll check again. Here is more info that should have been there. The motor is 3/4 hp. Start cap 40uf 250v. Run cap 20uf 450v. Motor draws 8.8 amps which is about right. 5 wire3 come out of the motor. U1 and U2 should be the run coil. W2Z2 and V2 should be the start/run caps and start coil. And of course ground. And they seem to measure that way. This problem happened before I replaced the mag switch and drum switch. This is why I suspect the motor. However, being human(I think) I'll recheck the wiring. I do have and extra 20uf cap, so I'll try that. Besides the caps, could the centrifugal switch work one way but not the other way? I've worked on electronics for decades and know a little about motors, but this one problem does not show up on the net and I never have come a cross anything like it. First chance that I get, I'll try to get some pictures and schematics to show you. Mark
 
Spent some time checking the wiring. It's good. Then did a resistance check, everything fine until tested the V1 lead to the drum switch. Turns out the V1 lead coming out of the motor case is shorted to the motor ground. May have to pull the motor and take it apart. This is not good.:bawling: Went back and read the past comments. Mark, I hope that you are right. Really hate to spring for a new motor. Anyway, thanks for the help guys. I'm done for the night and deal with it tomorrow.
 
OK. Hopefully it's not a shorted motor winding, but it does happen sometimes. Make sure to test just the windings, all caps disconnected
If the V1 line is part of the start circuit there's still a slight chance the short may be at the centrifugal switch and could be fixed
-M
There's also a last resort which is to change the connections so that the shorted end stays cold. We can explore that if necessary, later.
 
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Found the problem. The wire, I know t's green. It is NOT a ground wire. It is part of V1 and connected to the one side of the caps. It looks like that it was cut by the centrifugal switch. You can even see the extra piece of insulation at the bottom of the picture. So when the motor was running one way, the wire was neutral, not good. But not enough current, if any to cause problems. Reverse it and now the wire is hot. Bambo, blown breaker. Now, I got to figger the best way to repair this since the wire itself has been cut half way through. Thanks for all the comments, they have really helped. DSC01814.JPG
 
A butt splice should do the trick.

Another way is to cut the wire and slip heat shrink tubing on. Then strip about 1/2 -3/4" of insulation. Flare the strands iut slightly and slide one wire into contact with the other . Compress the strands to a small diameter and solder the joint. Slide the heat shrink over the joint and shrink the tubing. I will sometimes double up on the tubing or wrap the joint with electrical tape before sliding the tubing over the joint for added insulation. You will lose about a 1/2" of length in the wire.
 
Yay, another one bites the dust. Very similar situation to the other member's problem I mentioned earlier.
That doesn't really look green to me, more like a bluish-aqua
-Mark
 
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