Diagnosing motor problem

[Tom Deutsch]

I recently picked up an old (1953 or 4, I think) Unisaw. Some time ago, the motor had been replaced with a 1 hp Leeson. The motor was wired for 115v and the capacitor is new. Wiring is correct – matching the diagram on the motor. Mounted in the cabinet, with the belts attached, it just made a “buzzer” sound when switched on. No movement on its own.
I took it out of the cabinet and opened the case to investigate. Many of the thin wires that wrap the heavy windings were brittle and frayed. I heard them making slightly musical sounds when I spun the motor by hand. Not the kind of music you want to hear! I trimmed off the ones I could see – just on the theory that they might be grounding things out. And figuring I can’t hurt a dead motor. The centrifugal clutch looked fine. I cleaned and lubed it up lightly and made sure the contacts were in good shape. Reassembled.
Sitting on the bench with no load, the motor will spin slowly, but not enough guts to get it into the run circuit. I can give the shaft a spin by hand and then it will run. Once it is up to speed, I can unplug it, let it coast down beyond the point where I hear the clutch go back to the “start” position, and it will spool up to speed.
Like I said – I had replaced the cap with a new one of equal value to the one that was on the motor. I do not know if this was the original capacitor, however. It was hanging there when I bought the saw – as if replacing the capacitor was the last thing they tried before putting the saw in storage (I bought it from an old lumber yard/custom shop).
I’m tempted to try two things. First – wire it up to run on 230v, thinking maybe that would boost the legs enough to make the start circuit more effective. Second – try a larger value cap – again for more oomph in the start circuit.
Alternate thoughts and theories? Advice? Declarations or opinions? :)
I don’t want to dump a couple hundred bucks on repairing this 1 hp cap-start motor – so “take it to a repair shop” has already crossed my mind and moved on.

[Don Jarvie]

Leeson has a good website so you might want to see if that's the right cap since you said it may have been switched out before you. 1hp motors are pretty cheap on eBay so don't rule out getting another. They may have switch the cap to a run cap instead of a start cap.

Only issue is the original 1hp was a better motor than what we get today so you may find a 1 hp motor may be too weak and bog down. Ideally a 3 hp motor is what you need.

If your not a member of OWWM it's worth joining and see what others have done. You may be able to get a motor for a Unisaw from that error. There also a ton of info about motors.

[Frank Pratt]

It probably was bad or incorrect capacitor, or centrifugal switch. But now it's ruined for sure if you've clipped any wires.
Go get yourself a new motor.

[Tom Deutsch]

329965 Hi Frank. When I say I trimmed wires - I mean ones that were already frayed/broken and sticking out. There was already no continuity in anything I trimmed off - I just removed any portion that was sticking out with broken ends. And the photo (not my motor - just for illustration) shows which "wires" I'm talking about - they're the ones that look like netting or something. I suppose their function is to hold the winding wires in place? Some motors actually have paper tape or other material wrapped around the windings, instead of the braided wires. I'm not sure how they would have become damaged to begin with, though.

[David Malicky]

Right, those strings are to retain the windings during manufacture.
Yes, clearly it's a bad start circuit. The switch sounds fine.
Start capacitor values are usually not critical. A common formula is
C (uF) = 2688 * I / V
http://www.elkoncapacitor.com/capacitor.htm
(Technically, the ideal cap depends on the windings, inductance.)
So a 120V 1 HP motor start cap would typically be in the ballpark of 200 to 300 uF.
A dual-voltage motor is more complex and depends how the cap is wired with the windings. If the cap is designed for the 220V config, the correct value would be ballpark of 100-150 uF (even wired at 110V).
A larger cap is not necessarily more 'oomph', just a different phase angle.
Final cap sizing can be done by timing how long it takes to come to rated speed -- e.g., 2 seconds.

Don't try 220V on 110V windings. 110V has the windings in parallel -- 220V on them would ~double the current and fry them.

[Tom Deutsch]

Thanks, David. Lol - yes, it's a dual voltage motor. But I will definitely try a different capacitor first. The one on there is in the 500uF range! This gives me hope. Man, it's easy to jump to a "give up" conclusion.

[Frank Pratt]

I understand now. But the motor's still no good. The fine wires are the start winding & if they're cut or otherwise ruined then the only solution is to rewind the motor. That's not something that is practical with a small motor like that.

If it was just the string that was cut & the start winding still has continuity then maybe you could use epoxy to secure the damaged string.

[Tom Deutsch]

I'm still not sure we're talking about the same thing here. Here are a couple diagrams I found. I'm still not sure that the "netting" is the auxiliary/starting winding. All the diagrams seem to show that the auxiliary/start winding runs partway round the radius, like the main winding. Also, the fact that sometimes the "netting" is made of string or tape (or not present) makes me think it is non-electrically involved.

329979329980

[David Malicky]

500uF might still be correct if it was designed for the 110V config, and if the rated amps are very high (an old and very inefficient 1HP motor). But since the cap looks new, probably someone tried the wrong one after the original died. We'll be curious to see if it runs on a lower uF cap.
The start and run windings use similar gauge wire. That netting/string/tape/fine-wires that run perpendicular to the motor windings are not electrical -- they just hold the windings in place. Yes, good idea to epoxy where they were cut.

[Frank Pratt]

Test the winding leads with an ohm meter to see if there is continuity.

[John McClanahan]

It sounds to me like the start winding is not energizing. Make sure no wires have pulled loose in the junction box. Even though you can hear the start clutch clicking, the contacts may not be making contact.


John

[Myk Rian]

I trimmed off the ones I could see – just on the theory that they might be grounding things out. And figuring I can’t hurt a dead motor..
Well. If you clipped wires, it's dead now. Replace it.

[David Malicky]

If the start winding is disconnected, the motor shouldn't rotate at all. Slow rotation and low startup torque imply bad phase angle (bad cap value), low voltage to start circuit, or very high friction. Since the run circuit works normally, the input voltage and friction sound ok. Fouled switch contacts could increase resistance and deliver low voltage to start circuit, but he checked those. All that and the hanging cap when purchased suggest trying a lower uF cap as the first change.

[Tom Deutsch]

I should edit my original post. After looking more closely at the frayed "netting" this is definitely string. Unless my understanding of conductivity is wrong, trimming off the frayed strings is/was not the problem. I cut no wires.
I discovered, when re-assembling, that the clutch armature was not making contact with the switch surface. No wonder it would not start! After fixing that, I reassembled and it ran fine on the bench with no load. Mounted it in the cabinet and put the belts on (fairly loosely) and no go again! Grrrr! Going to try the new capacitor again (the old cap is in there because it seemed to work fine on the bench.) Cross fingers!

[Charles Lent]

On the motor shaft there is a plastic ring that operates the centrifugal switch. It needs to slide on the motor shaft. Weights on arms located on the motor side of this plastic ring fly out as the motor spins, moving this plastic ring toward the end of the motor shaft. The centrifugal switch in the motor end cover is actuated by this sliding ring. Clean the motor shaft where this ring needs to slide and then apply a very thin layer of oil or synthetic grease. Re-assemble the motor and try it again.

Charley

[Tom Deutsch]

Thanks for all the good input.
In fiddling around with the motor, I noticed that the centrifugal switch wasn't quite making contact with the rotating plastic/phenolic disc. So I epoxied a bit of plastic washer on the contact and now it works just about as well as an old, overworked 1 hp Leeson on a table saw can work. :rolleyes: