"Motor run" or "Motor start"? (mystery capacitor)

[dai h.]

been looking for a long time for some inexpensive large value (hopefully not too physically big) film caps to use in place of the bipolar alu electro. in my Marshall Power Brake and SE100 (attenuator, and attenuator/speaker emulator/DI rackmount unit, respectively), just for a bit less worry/greater reliability (AFAIK lots of current passes through the cap here for this app.). (The load portion where the cap is located is like this basically but with slightly different values--partly because mine are 8 ohm loads and not 16:

dummyload

)

Found some 40uF 125VAC in surplus so I bought a couple. I didn't know exactly what they were when I bought them, but suspected they were some sort of (film) cap used with motors (air cons, etc.). Got home, googled a bit and couldn't locate a datasheet, but they did indeed appear to be a film cap, probably metallized, known as a "type SH" ("SH" = "self-healing") but them stumbled on R.G.'s "Immortal amp mods" article (some of the ideas of which I've seen mentioned here a few times) :

The Immortal Amp Mods, Pt. 4 - Premier Guitar

where he advises (when subbing alu electros for film) to *not* use "motor start" caps. Can anyone differentiate the ones I bought? They have huge tabs, so I'm thinking (guessing) they are meant to be used in high current situations (and therefore robust), but...? (Also they measure about the same at 50Hz, 1kHz, and 10kHz on my LCR meter, so that also seems to confirm that they are film and not some sort of alu. electrolytics.) Thanks for looking.

[R.G.]

I think the "self healing" is the key. Self healing is mostly confined to film types. It refers to the property of metalized film caps of the metallizing actually burning away around a spot where the insulation has failed. I don't think aluminum types have any similar action. My best guess without more data is that they are film caps. Use them. If they get too hot, or change value with use, they weren't. 8-|

As an unfortunate side note, I just got a flier from Electronic Goldmine offering 3uF 450Vdc film caps for $0.99 each.

[nashvillebill]

Surplus Center Surplus Center - Electric Motors, Gearmotors, Relays, Toggle Switches sells Commonwealth Sprague motor run and motor start caps cheap. Here's the datasheet on the C-S caps http://www.icmaster.com/Datasheet.as...5P106H66C36N4X if the link works.

I have not tried these but have been tempted to.

[dai h.]

thanks guys.

(mental light bulb went off. Googled for a bit...)

R.G. I take it the reason you recommended against "motor start" is that they are alu electrolytic?

re: the metallizing: I figured they must be some kind of metallized film cap (datasheets for other SH caps I could find showed metal. polyester and metal. polypropylene--metal. polyesters had a size advantage IIRC--i.e. could be smaller).

The price on these was 525 yen ea.(not sure if that included 5% consumption tax), which is about USD$7.23 (x4 = around $29 for 160uF to replace the stock alu. electrolytic). (As a side note, from what I've seen Solen from N.American vendors seem to be quite competitively priced (as far as large value films), but here in Japan I think I would usually have to pay more for a comparably sized film. Whether that's due to taxes on hi-fi parts or distributor markup or whatnot I have no idea.)

[R.G.]

I was mostly interested in alternative caps for power supply use. In a power supply, being aluminum electro, yes, they'd have the same wear-out mechanism as polarized aluminum caps; more to the point, they would be subject to continuous charging pulses, and the heat burden that would bring. I don't know that they would not work as well as polarized electros, but since I was trying to get away from anything that would wear out on its own, and motor run caps are continuous-duty rated film caps, I wanted to make the distinction clear. It would be easy to ask for a "motor cap" and get a motor start cap.

Motor start caps are rated for intermittent use, not constant. They are intended to start the motor and drop out after a few seconds, and not to run constantly. I don't know that start caps wouldn't do OK in a speaker sim because of the high ratio of peak to average audio power, but as long as you can get film caps, to me it's not worth taking the risk. Film caps for sure will not have wearout problems.

I've been tempted by the Surplus Center caps as well.

[dai h.]

R.G., thanks for the clarification, that was very helpful!

[sgelectric]

Question regarding motor-run cap voltage rating

Please excuse me if this seems a bit off-topic

What is the DC rating of a 375VAC rated motor-run cap when used as a power supply filter? TIA

[R.G.]

Good question. With 375VAC across it, it certainly has to withstand the peak of that AC waveform, which is 375*1.414 = 530V peak. That at least means that 530V won't instantly punch through it. However, since they're made for AC situations only, I suspect the makers don't rate them for DC at all. I checked a number of film caps looking for AC versus DC ratings, and found that DC ratings were generally higher, sometimes much higher, than AC ratings.

If it were me, I'd get some, charge them up to 500+ volts DC through a 100K or so, then monitor the voltage across the 100K. If it never drops below 10V, that indicates a leakage less than 100uA, and I'd probably call that good for a 400-450Vdc power supply. If I had to have 500-550Vdc, I'd probably go with 440Vac rated caps - and test them too, remembering at all times that these things will hold a charge for days, unlike electros, and that some way to safely discharge them after testing is needed, as well as a way for *me* to survive the testing. Just things to think about.

The makers often spec them for 1.1 times the rated voltage for line variations, and often test at twice the rated voltage for quality purposes, but these are of course raw generalities. If you're going to rely on it, test it.

And please don't try any of this unless you already know how to do it safely.

[Gingertube]

Guys,
Just a flag here - my understanding (I may be wrong - would'nt be the first time) is that the difference between motor start and motor run caps is that the motor starts are not intended for continuous duty. That is, they are in circuit to start an electric motor and then switch out. Motor runs on the other hand remain in circuit continuosly and so for tube amp power supply use, then the motor run is what you want.

Motor Starts MAY be OK but may have a problem since they were never intended for continuous duty.

Let us know if I have this wrong
Cheers,
Ian

[nashvillebill]

That's true and so a motor start is somewhat of an unknown. Tossing in another factor, though, the motor start cap has to deal with the inductive load of the motor, so it's going to see a momentary inrush current that's quite severe, so is the cap made any diferent to handle that high current? I dunno, I'm no expert.

When my AC at the house turns on, apparently there's enough momentary current draw that the street light at my house will go off (it takes a minute or so for it to restart, so I finally correlated the street light going off and my AC kicking on)

[dai h.]

I remember reading something about bipolar alu electros for speaker network use being (having) non-etched alu foil to better withstand higher currents, so maybe there is some construction difference (assuming all motor start are alu electro). (So if they are meant for higher current, motor start might not be an inappropriate choice for reactive load use--but if they have the liquid inside which can evaporate they should still have the more finite lifespan vs. film.)

Also, one thing I thought of re:the caps (for reactive load use) is that since my replacement caps (original : 160uF BP alu electro. new: 4x40uF film) are not in a single package (each has thick-ish-looking plastic case) this makes them worse in volumetric efficiency (space taken up), plus each has two giant lugs (which while looking tough and reassuring, take up even more space vs. smaller leads or lugs). I compared the caps to my Marshall Power Brake (attenuator), and there is no way four will fit (might be able to fit a couple inside my Marshall SE100 since it's got a rackmount chassis and is much more spacious inside). Not really complaining (they weren't very expensive after all and not always getting exactly what you want is the nature of surplus), but just a 'heads up' to anyone else trying to find a film cap for the same sort of use.

[R.G.]

I keep coming back to the reason I was after film caps instead of aluminum electros, no matter how good the aluminum electros may be, film caps don't have a built-in death mode. All aluminum electros degrade with time. Period. They have a shelf life of perhaps five years, and a working life of maybe one to four decades if used regularly. If left sitting in a closet, they're eating their shelf life. This is one reason "closet queen" amps often die when first turned on after long storage.

Film caps probably degrade with time too, but over many human generations, not over a couple of decades.