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Old December 5th 18, 12:30 PM posted to rec.crafts.metalworking
Terry Coombs[_2_] Terry Coombs[_2_] is offline
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First recorded activity by DIYBanter: Oct 2012
Posts: 3,115
Default Compressors - kinda on topic !

On 12/4/2018 2:16 PM, Gunner Asch wrote:
On Tue, 4 Dec 2018 07:07:25 -0600, Terry Coombs

On 12/4/2018 2:03 AM, James Waldby wrote:
On Tue, 04 Dec 2018 00:44:44 -0500, Clare Snyder wrote:
On Mon, 3 Dec 2018 13:53:29 -0600, Terry Coombs ... wrote:
* Well , Mr. Compressor has a case of the humms . That is , when I try
to start it , it just humms (until the O/L cuts it off if I let it go
that long). This is the original motor on an old Speedaire 60 gallon
tank , 220 volts and 3450 RPM's . Specs call for 15 amps full load ,
rated output is 2.98Kw , about 6 hp . This is an Emerson/Doerr
compressor duty motor .* Tested both caps , one is good and the other
isn't according to the tests with my DMM . Problem is that the bad cap
has rubbed the capacitance and voltage ratings off the label . Got a
couple of calls in , looked around on the internet , can't find a
definite answer . I'm pretty sure it needs to be around 250uf and at
least 250 volts - though maybe higher voltage to account for spikes . I
have the motor type/classification numbers , just can't find a reference
to check ... anybody got a lead on that type of info ?
That will be the start cap - 275 volt and 80-100 uf

Don't take my word for it - take a look where I got the info:
The tables at that link appear to only go up to about 1 HP (.746 kW)
and it looks like 80-110 uf is the .373 kW entry in the 220V table,
instead of corresponding to the 6 HP (2.98 kW) Terry mentioned.

A formula at
suggests about 164 uF for 3 kW. [2652*amps/volts, ie, 2652*13.6/220].
A rule of thumb in suggests 30-50 uF per kW
(for a 220V 50Hz motor) ie 90 to 150 uF for a 3 kW motor. A formula at
sizing/ calls for a power factor number; I didn't calculate that one.

* I visited the same site as Clare and I think his number is low too .
I don't recall which website I visited that had this formula : current x
10-6th divided by 2 x pi x frequency x voltage , the result of that
formula was 180 mfd ... I'm going over to my blacksmith neighbor's this
morning , he has a compressor very similar to mine . I'll check his
start cap to be sure but I believe the value needed is going to be a
little higher than that due to the high start torque needed for this
application . This is an Emerson/Doerr motor , but I can't find any info
on this specific motor ... it's at least 20 years old and the Emerson
site just burps and says "not found" when I enter the info from the tag
. I believe it's going to be somewhere in the 270-330 mfd range . I may
borrow the cap from the neighbor's compressor to see if it works on mine
since they're very similar . If I can't get it going I'm probably going
to order a new motor , they can be had for around 200 bucks and this one
is pretty old . At this point I've got less than that tied up in this
unit including the original (very used) purchase and parts to rebuild
the pump . Nowhere I know of to get a unit of this size and qualityfor
anywhere near that cheap , including a new motor .

Ive got a 6hp Harbor Freight motor on my old Dvillbiss compressor. Has
worked well for..humm...10+ yrs so far. Its mounted outdoors and it
gets rained on (when we get rain) and its held up quite well. I did
have to replace the start cap last year.

I think I paid somewhere around $100 or less for it then...rack up 10%
for inflation....


* And your point is ? This is/was a Speedaire , had a cast iron pump on
it but - it was used in a large cabinet shop , way beyond it's duty
rating and the pump fried . I paid a c-note for the tank and motor , put
an Ingersoll-Rand pump I had on it . Aside from having to replace the
con rods because I got stupid , this is the first repair it's ever
needed . It is sheltered , I have more respect for my tools than to
leave them out in the rain .

Yes , I'm old
and crochety - and armed .
Get outta my woods !