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From: Wild_Bill on 9 Oct 2009 10:16
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.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"dave" <tool_box(a)weirdstuffcox.net> wrote in message
news:rZvzm.94198$u76.12432(a)newsfe10.iad...
> 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 :-)

From: Denis G. on 9 Oct 2009 22:41
On Oct 8, 10:51 pm, RoyJ <spaml...(a)microsoft.net> wrote:
> I'd probably go with the next size larger (still within the specified
> range), should give a bit more starting 'kick'http://www.mcmaster.com/#7245k54/=3z859g
>
>
>
> Denis G. wrote:
> > On Oct 8, 9:03 pm, dave <tool_...(a)weirdstuffcox.net> wrote:
> >> 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 :-)
>
> > 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:  http://www.mcmaster.com/#7245k114/=3z7b3b- Hide quoted text -
>
> - Show quoted text -

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.
From: William R. Walsh on 10 Oct 2009 10:24
Hi!

> 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.

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.

> 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.

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.

William
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