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scottj963 06-19-2011 03:35 AM

Wiring a capacitor in a motor
Is it possible to wire a capacitor backwards? In other words, is there an IN and an OUT?

AllanJ 06-19-2011 06:25 AM

In an AC circuit it does not matter if a capacitor (intended for that circuit) is wired backwards.

In a DC circuit, some capacitors can be wired backwards, others cannot. Color coding for the negative terminal is quite varied from one brand of capacitor to another, although some are labeled + for positive and - for negative.

Be careful. Sometimes what seems to be an AC subcircuit is really DC because of hidden diodes.

micromind 06-19-2011 12:04 PM

A standard basic two terminal motor start or run capacitor is not polarity sensitive. It doesn't matter which wire goes to which terminal.

If it has 3 terminals, then it certainly does matter which wire goes where.

scottj963 06-19-2011 10:46 PM

Thanks for the help
Thanks to you both for the information. Unfortunately it was not what I wanted to hear. Let me start at the beginning.
I have a Craftsman 3/4 Horsepower, Capacitor motor 3450 RPM, 115 Volts, 60 cycle, 9.3 Amps, with an Overload Protector. It is from an old (early 50's) Craftsman (King Seeley) Bench Saw. When I turn the power switch on, there is a drain on the household power, but the motor will not start and will shut off if left on for too long. If I wrap a cord around the shaft and pull on the cord at the same time that I flip the switch to on, it will start running and then run fine. My neighbor said it was probably the start switch inside the motor (not the power switch). I took the motor apart and the neighbor came over and looked at the start switch. The spring loaded part would not move freely. We got it cleaned up and now springs out.
While I had it opened, I wrapped electrical tape around some of the wires where the brittle insulation had broken though I could not see any bare wire. I got the thing back together and turned the shaft with my fingers. I heard a ticking sound. It was probably a wire rubbing against a moving part. Getting the motor opened and then back together took the better part of two days; I was not about to open it again to move a wire. I plugged the motor in and turned it on. Power drain, but would not start. I spun the shaft again with my fingers to make sure it moved freely and then tried turning it on again. Same thing. I did the old trick of wrapping some cord around the shaft and pulling it while turning on the motor. Voila! It ran, but made a much faster ticking noise. After running about 10 seconds, there were three loud pops, a flash of light, and as I was reaching to unplug the thing I could smell smoke.
I got the motor apart again and found a wire that was almost burned in half. I then noticed two wires not connected to anything. One was white and the other was white with black stripes. It was obvious they were not one wire that burned in half, but I didnít know what they were for.
A couple weeks later when I was ready to tackle it again, the neighbor came down and I showed him what had happened. As I showed him I noticed that the capacitor was not connected to anything. That explained the two white wires.
Yesterday I mended the burned wire, soldered the two loose wires to the capacitor, got the back together again and turned it on. It didnít run, but made an awful noise like an old fashioned door buzzer. I spun the shaft with my fingers and it turned freely so I wrapped some cord around the shaft and gave it a pull as I turned the motor on. Same noise, but I lifted the motor off the work bench because the shaft would not spin. I turned it off and was able to freely spin the shaft with my fingers. Something was not right and I hoped it was that I had wired the capacitor backwards.
Any ideas?
The motor was not grounded so I put a ground wire around the bolt that attaches the mount to the motor. The bolt holes are actually tapped into the metal laminate, non-moving part of the motor. The mount also houses the capacitor and the power switch. Is this an acceptable ground?

micromind 06-19-2011 11:52 PM

Generally speaking, if the shaft will turn freely when the motor is de-energized, but binds up when power is applied, it's because the rotor is not centered in the stator.

The rotor is the part that turns, the stator is the part with the windings.

When you turn the shaft, is there any play in it? Can it be moved up and down at all? Even 0.010" will cause a problem.

It's also possible that the spark has caused two or more of the winding wires to become shorted to each other. If there is any evidence of this (black stuff on them, or arc marks), then the motor is likely beyond repair. It'll need to be rewound by a motor shop, it's usually less expensive to get a new one.

If you do decide to replace it, the new one will not fit. The mounting bolt holes will be different, and the shaft will likely be a different diameter. The new one will have a 5/8" shaft. The old one is likely either 1/2" or 3/4". It's probably belt drive, so you can get a new pulley, and you can very likely drill new holes in the mounting plate to fit the new motor.


scottj963 06-20-2011 01:10 AM

Will check for these
Thanks Rob. There is a very good possibility that the rotor is not centered in the stator. I didnít have the bearings seated completely in the casting. I will get is seated properly and check. I did see black stuff on the stator, but not sure about the windings. I will look more closely. I did see an arc mark, but I canít remember where it was positioned.I am hoping I donít need a new motor. The two ends where the shaft comes through are cast iron as well as the mount. I would guess it weighs over 50 pounds and the weight of the motor tensions the belt.Again, thanks for the help Rob. Iíll let you know what turns out, but it will probably be a few days until I can talk myself into tearing that thing apart again.Scott

scottj963 07-08-2011 01:14 AM

4 Attachment(s)
I wish I was superstitious since they say the third time is a charm. Iím having a final try at this motor, but have run into a bit of a snag.
I labeled all of the wires as A, B, C, and D. A and B were no problem, they were the source.
The Cs and Ds go to a nonconductive plate with two studs on it that serves the same purpose as a wire nut.. The wires labeled C go to the C stud. The D wires go to the D stud. I put one of each wire on their respective studs and then took off my labels. Now I canít remember which stud is C and which is D. I have one more D wire and two more C wires to go onto the studs.
Since a picture is worth a thousand words, here are three thousand words and my attempt at a schematic. Any idea as to which stud (I numbered them 1 and 2 in the drawing) is C or D? I did the drawing in SketchUp and saved it as a JPEG then resized the JPEG so that it would fit on this site, but lost some of the clarity when I resized.
I really appreciate any help I can get.

Jackofall1 07-08-2011 01:28 AM

Interesting story, but I am afraid from your explanation that you have probably smoked a field winding and have turned your motor into a large inefficient elctro magnet.

You may be able to take it to a motor shop to have it rewound, but my guess it McMaster Carr or Grainger for the closest matching replacement would be more cost effective.


scottj963 07-08-2011 02:00 AM

Thanks Mark. No offense, but I am really hoping that you are wrong. When I had put the motor back together, I knew the rotor was not seated properly in the bearings so I am hoping that is why I got a heavy magnet. I did look pretty closely for for signs of burning, melting, and arcing and didn't find anything except for a spot on the part that activates the start up switch. The wires appeared okay.

Jackofall1 07-08-2011 02:35 AM

Non taken, if you have an ohm meter you can check the resistance through the feild windings, they should all be similar in resistance.


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