sizing of "run capacitor" for table saw motor needed

johnpeterFebruary 17, 2013

I have an old Sears Craftsman table saw, with a 3450 rpm motor. The capacitor just failed, and I could easily replace it with one from Grainger, if only I knew how many microfarads to select (i.e., capacitance). The old one is an oil-filled rectangular can, and the markings are illegible.

I have read things here and there, with one person in this forum saying that it's OK to select an overly-high microfarad value. I am deeply suspicious of that comment, though I cannot speak with authority.

I believe the value is somewhere between 5 uF and 70 uF. The capacitor is used to impart a phase shift to a secondary winding in this AC induction motor, and the amount of that shift is critical towards maintaining efficient operation and longevity in the motor. On the other hand, the motor runs only briefly, so I suppose I could err without burning the motor windings.

I'll probably buy several capacitors, and use a trial-and-error method. Of course, I would appreciate any experienced advice, on this matter.

More about the motor; it's a nice heavy motor from the 1950s and I have been using it for about a week. It was given to me 10 years ago, and I only recently resurrected my old saw, which needed a motor replacement. Suddenly, it stopped working. The motor did not burn.

Motor's original manufacturer: Emerson Electric
"Sears & Robuck" on the motor's plate.. . type CR260K20 Model 157167. 3/4 HP. 3450 RPM. 115 VAC, 9.3Amps, 60 cycle.

Dimensions of the old capacitor, 4,88" long x 3.15" wide x 1" thick.

Thank you,
John Peter

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Emerson Electric is still in business. Why don't you contanct them!

    Bookmark   February 18, 2013 at 11:14AM
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Assuming that Sears cannot provide that information, then there are a few approaches, none of which leads to the-bigger-the-better.

a) Measure the inductance and resistance of the winding the capacitor feeds and calculate the capacitance that will produce some phase shift suitable for the type of motor (PSC induction, I presume). An ideal 90-degree phase shift is probably not realizable with adequate current flow, and there is some other design value that I expect is normally used. This can probably be searched out on the www.

b) Try different sizes using an dual input oscilloscope to observe the phase shift. A test under load is probably warranted. Unless the scope is dual differential, be sure to isolate the scope from ground, connect power neutral to the common, and be careful .

c) Trial and error. You want rapid start, but too large a capacitance and the motor may sound bad. Too small and it won't start. If the cap is a bit too small, you won't be able to supply much power under load. I would start small and work up.

I have a three phase motor that I run on single phase to generate three phase power to a smaller motor. I used the trial and error approach to select the starting capacitor (the capacitor is only used momentarily so it is not so critical).

You must use an ac motor cap and not a dc polarized cap. When you think you have the solution, check for abnormal capacitor temperature and motor temperature.


    Bookmark   February 18, 2013 at 11:19AM
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I don't know what capacitance you need, but I wanted to warn you that in the 1950s oil filled capacitors usually were filled with PCB oil. That has been banned since about 1989 and you should not let it contact your skin because it has been linked to cancer and other diseases. It may not be disposed of in landfills. Modern oils have replaced PCBs and polypropnlene has largely replaced kraft paper insulator films. Look for "metalized" polypropylene in ads I believe that is the latest kink but not really sure it is best.. A lot of stuff is electrolytic now and may not last as well as you old Sears product.

    Bookmark   February 18, 2013 at 3:54PM
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