can bad cap = hot motor?

I have an old Baldor grinder that's at least 80 years old. It is a cap start and run motor. So the cap is always in the circuit of the phase shifted winding. There is no centrifugal start switch. It has always taken a long time to come up to speed but these types of motors do that. However, it seems to take much longer to spin up than my newer Baldor grinder motors that are wired the same. The capacitor is a rectangular shape about 5/16ths of an inch thick and maybe 1.5 x 2.0 inches. It has a brass case with cloth covered wires coming out of the tar used to seal the case. I mention the cap construction just to give an idea of how old it is. I went through the motor a few years ago because the bearings were bad. They had rawhide seals that had dried out, shrunk, and let grit in. I replaced them with modern sealed bearings. The motor runs very smooth. Just like a grinder motor should. Today I used the grinder for about 1/2 an hour and it got pretty hot. I was not loading the grinder very much because I was just grinding points on tungsten TIG electrodes with a diamond wheel I have mounted. Still, the motor got pretty damn hot. Could a bad cap do this? I suppose there could be some shorted windings. In any case, if a bad cap could cause hot running I'll take the thing apart again and replace the cap. Thanks, Eric

Reply to
etpm
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If the cap is original to the motor, replace it. 80 years is a very long ti me for such a device, keeping in mind that AC-rated capacitors (80-year-old style) will deteriorate just by sitting on the shelf. A modern polyprop ca p of the correct rating and voltage will outlive most of us here. As to the motor itself, do you have an AC ammeter, with specific reference to the clamp-on type? If so, check each leg of the feed - they should be su bstantially the same when running, and substantially below nameplate when u nloaded. If either of those statements is untrue of your motor, it is on it s way out, however slowly.

But, to answer your question directly, yes, hot-running is a symptom of a f ailed cap - as is slow starting and poor load handling.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

Yeah, I'm betting that the cap is as old as the motor. I'll take your advice and replace it. I don't understand how the current can be different if there are only two wires feeding the motor. What am I missing? Thanks, Eric

Reply to
etpm

If the capacitor's value is wrong, it doesn't provide the proper phase shift. That causes the motor to require more power to do the same work.

Reply to
Michael Terrell

Not likely, but if one of the windings is shorted to ground or the capacitor is leaking to ground the current could be different.

Reply to
Ralph Mowery

I suspect your motor has a single capacitor and is a split-phase motor. Cap start, cap run requires two capacitors and the mechanism to switch between the two.

A grinder has no reason the have a heavy load when it starts, which is the reason for motors with two caps.

Paul

Reply to
Paul Drahn

Well, actually it does. It has the inertia of both grinding wheel to get up to speed.

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Reply to
Fox's Mercantile

The motor could certainly get it up to speed fast with a high torque, but the user of a grinder can wait a few seconds before applying his workpiece... so "low startup torque" :== "low startup load" is acceptable in this application.

Reply to
whit3rd

I guess the cap could indeed be leaking to ground. It has a brass case after all and the case may be touching metal. I will be replacing the cap and that will hopefully solve the heating issue. I'm certain it will speed up the startup. Eric

Reply to
etpm

I doubt the grinder really has that much inertia to prevent starting. It may tke some time to get up to full speed. Most hard starting is like air compompressors and refrigeration compressors unless they have some sort of unloader.

Reply to
Ralph Mowery

Maybe you can restuff it rather than dispose.

NT

Reply to
tabbypurr

While I can see the draw of keeping all the old stuff intact or appearing so this cap is weird. It's a rectangle about 5/16 inch thick. Modern motor caps are a lot different in shape. Eric

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Reply to
etpm

Ignore Tabby, they tend to not read an entire thread before making some, typically, useless suggestion.

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Reply to
Fox's Mercantile

Agreed. However, the official name is a "Permanent-Split Capacitor Motor" which has one permanently connected run capacitor. The "Split Phase" motor has no capacitors. See:

Two-winding Machines

Permanent-Split Capacitor Motor If run efficiency and vibration are important, but start torque can be compromised, the capacitor can be left in the auxiliary circuit at all speeds. Sizing the capacitor to provide balance at a particular load point, the backwards field can be eliminated, improving efficiency and eliminating torque pulsations. Eliminating the centrifugal switch can reduce the manufacturing cost significantly. The trade-off is lower starting torque, since the capacitor is not sized to provide balance at starting, but for run conditions

The design has the disadvantage of having low starting torque, which is the cause of the slow start. At 80+ years old, I would guess(tm) that the capacitor is as dead as the bearings that were replaced. As it gets old, the ESR starts to climb which I presume was the cause of the observed heating.

Drivel: My experience with motors was mostly with industrial sewing machines at my father's lingerie factory. At the time some of the older motors would exhibit the symptoms of a shorted winding. What was happening was that some motors were not designed to handle sustained high temperatures. The copper motor wire would expand slightly, cracking the old enamel insulation. New enamel was quite flexible, but old enamel became brittle. My guess(tm) is it took about 40 years to cause problems. Insulation failures were mostly around sharp wire bends. I think varnish insulation did the same thing, but at the time, I couldn't tell the difference between varnish or enamel insulated motor wire. Therefore, when working with really old motors, I always look for loose insulation flakes, which might be an indication of impending shorts.

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Reply to
Jeff Liebermann

You gave its dimensions earlier. It is presumably a flat paper cap. I meant restuff with higher v non-motor caps.

NT

Reply to
tabbypurr

Modern motor run caps are all large. They use a lot of material. It can't be they do this just because of tradition. Eric

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Reply to
etpm

Start caps are AC electrolytics. Run capacitors are AC rated film, or in case of that 80 year old cap, paper insulation. They are large, to dissipate the heat from continuous operation.

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Reply to
Michael_A_Terrell

You could always look up what properties motor run caps have. But did you also notice that even a 1930s paper cap lasted 80+ years? Modern film caps are massively better & smaller.

NT

Reply to
tabbypurr

I know modern caps are much better than old caps. However, why do you suppose it is that motor run caps are so big for their voltage and capacitance ratings compared to other non-polarized caps? I'm serious. Eric

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Reply to
etpm

That is not a bad question at all: a) Because they are AC-rated caps. b) Because the nature of a motor starting puts a tremendous load on the cap for a very short time, so they must be robust enough to absorb that shock, repeatedly and reliably, in addition to the actual voltage involved. So, w hereas the actual operating voltage may be say.... 240 VAC, the start-cap m ust be rated (at a minimum) of 150% of the operating voltage. c) Typical NP caps are used a great deal in audio applications, where the v oltages are small, and the frequencies high. d) Motor Run caps start around 370V, or so, are typically of much lower cap acitance than a start-cap (and whereas replacement voltage may be higher, r eplacement capacitance should remain very close or identical to OEM).

Hope that helps.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

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