oh turds! compressor motor starting cap's 'blown out' but it's a "non-standard"

my ancient 4hp sears craftsman air compressor needs a starting capacitor, I've determined (through a process of elimination). I still have the original owners manual, and the capacitor is "part of the motor", not shown in the parts blowup, or parts list, as a seperate orderable part. further, the friggin' thing seems to be a 'non-standard' mfd rating, too (it doesn't 'fall into' any capacitor size-groups of any maker's starting cap's I can find online. matter of fact, it's not even close):

the one in it now is

110 volt, 485-580 mfd mallory #139852-49 seems a slightly non-standard SIZE as well: diameter: 1 13/16ths inch, and length, overall, excluding 'spades': hair over 4 5/16ths (but clearly under 4 3/8ths)

atttempts to locate same by the mallory part number on it entirely unsuccessful. so (physical size and 'look' issues aside) can I "daisy chain" two cap's and achieve the same net effect, somehow? appearance "not an issue", I just need a working compressor. I know I can replace the cap with a higher voltage cap, and that'd be fine (maybe even desireble) but what about the mfd rating? if I must, is it best to go 'bigger mfd numbers' or lesser?

thanks for tips on this, guys :-)

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It looks like the value of the cap is not that critical. They give you a range 485 to 580 microfarads. The average value is approximately 540 microfarads. If you order from McMaster Carr, this should work:

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Reply to
Denis G.

The capacitor values are a range because the caps are made with a large tolerance. Even if you got one marked exactly 580 mfd, it could actually be much smaller or much larger. I don't think Mallory has made capacitors for many,many years.

One trick we used to do in a emergency is to get two DC electrolytic capacitors of twice the needed AC capacity and connect them in series, either both + leads or both - leads together and wire the remaining leads to the motor to replace a single AC capacitor. Gets you going while you look for the correct replacement.


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That is a common value, either of these two will work at about $8 each

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Reply to
Bill Noble

CDE bought the Mallory capacitor business. Take a look at this catalog page:

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Thus looks to be the closest size:

460-552 MFD PSU46015A 110/125V 1-13/16" 3-3/8"

$6.15 each, 42 in stock.

Reply to
Michael A. Terrell

On Thu, 08 Oct 2009 21:03:20 -0400, dave put finger to keyboard and composed:

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- Franc Zabkar

Reply to
Franc Zabkar

On 10/8/2009 7:03 PM KD7HB spake thus:

That can't be safe, can it--running AC through two polarized capacitors?

(Hey: two explosions for the price of one!)

Reply to
David Nebenzahl

Take the old one to the Grainger's nearest you and walk out with a suitable replacement Easy.

If no Grangers nearby, look in the yellow pages under motor rebuilding and they can set you up also.

Reply to
Roger Shoaf

You shouldn't have too much trouble finding a suitable replacement from an electrical jobber supplier.. Johnstone, or local independent electrical supplies dealer.

If you have a local motor repair shop, they will know what you need by referring to the 120VAC motor voltage and 4 HP motor rating.

Start capacitors need to be for AC usage, and are typically sized at about

500uF per HP. The numbers on the original cap indicate that the actual value of that series can be as low as the minimum value, or as high as the maximum value shown. The actual measured values (microfarad, or uF, or mfd) of start capacitors can vary widely, as much as about 30%, especially on parts that are decades old.

The voltage rating for start caps is generally higher than the AC line voltage, and different manufacturers choose to use different rating numbers. Another common marking is 377VAC.

The other source of problems in split-phase motors would be the centrifugal switch for witching between the start and run windings.

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Why? You have no idea how it actually works. Look up 'Electrolytic Rectifier'. What happens when you have two series connected rectifiers, with both cathodes connected? Nothing, if you don't exceed the PIV of either diode.

Sorry, but you're the biggest loser. :(

Reply to
Michael A. Terrell

Roy, you're probably right, however, I wonder that if you skew the value to the higher end if you pay a price by decreasing the life of the contacts on the centrifugal switch.

Reply to
Denis G.

You have a range of possible values to work with here.

The physical "sizes" given for most caps are a bit larger than the actual size -- think of it as a "worst case" size. I don't think that 1/16" makes any difference at all -- except perhaps in diameters (which you don't specify) so a particular clamping strap will work with it.

What do you mean by "daisy chain"? To me, that sounds like connecting them in series, and if you do that you will get a lower value, not a larger one.

Series is: O---|(---O---|(---0

In series, add the reciprocal of the values and take the reciprocal of the answer to get your value. For example, taking a 40 uF cap and an 80 uF cap and hooking them in series, you get:

1/40 + 1/80 = 1/26.67


0.0250 + 0.0125 = 0.0375 1/0.0375 = 26.27

so the value is 26.67 uF. Not likely the way you want to go.

Instead, you want to connect them in parallel to directly add the values.

Parallel is: (View with a fixed pitch font like Courier to avoid distortion of the drawing and confusion of what it means. :-)

0---+---|(---+---0 | | +---|(---+

The numbers you got from the capacitor markings show the range of values which it could take on depending on luck in the manufacturing process.

I would say that probably 450 uF would work fairly well, and maybe 600 uF as well, but look for something marked about 540 uF (near the average value of the range marked on the (ex) capacitor which you have) and it will probably do quite well.

Larger values would be preferred if you were somewhere where the line frequency is 50 Hz (UK and maybe Canada), while here, go for something in the range marked.

Good Luck, DoN.

Reply to
DoN. Nichols


Funny you'd mention that. I bought an old furnace fan assembly ($10) and found that not only was the motor rusty inside, but the start cap was bad. The motor cleaned up well and that got it starting a lot better, but it was still stubborn at times.

So I popped the cap out and looked at it. Turned out it was a Mallory capacitor. I took another look at it and said "that's been a long time". What I took as a date code suggested 1983 or 84 (don't remember which). I'm sure that little motor started up lots of times between then and now.

I'm going to have to remember that! (Or at least remember to try it and see the next time a capacitor drops out.)

I'd have thought that finding a replacement capacitor for my fan would have been easy. (Find one at the hardware store, right?) Nope. Struck out at various hardware stores, Farm and Fleet (now, come on!) and only had any luck at a little hole-in-the-wall motor repair shop.

$7 later I had a slightly larger "Rotom" ("motor" spelled backwards-- clever, huh?) capacitor and everything was fine once again. I was fortunate in that it did fit the capacitor compartment perfectly.

If the OP can't find an exact replacement, mounting it elsewhere on the motor should be acceptable.


Reply to
William R. Walsh

Actually Canada has NEVER been 50hz. We were 25 for a few years WAYYYY back. (Niagara 1 was 25 hz) but switche to 60 across the board somewhere in the early 50s.

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