Restricting my followup to rec.crafts.metalworking only.

On 2009-10-09, dave 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

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

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)

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.

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?

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

or

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

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?

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On 10 Oct 2009 04:43:49 GMT, "DoN. Nichols"
wrote:


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.
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.