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Topic: more Re: +Grinder motor start capacitors (9 msgs / 374 lines)
1) From: Edward Spiegel
Hi,
I just got back from a wild goose chase and have some follow-up questions. I went to a Grainger's that is nearby and they had a couple of 15 mfd oval run capacitors but they wouldn't sell them to me because they don't sell to the public and when I showed the guy at the counter the original, he told me that he didn't think that I should use it. He sent me to an appliance parts warehouse, and when I went there and showed them the motor and capacitor, the guy that worked there took them into the back and said 'you have a start capacitor; not a run capacitor; i have a 15 mfd run capacitor but i don't think that i should sell it to you because it won't work'
When I asked how he could tell the difference, he handed me the run capacitor and said 'shake it. Do you hear the fluid inside? Shake yours. You don't hear anything because its a start capacitor and so it doesn't have fluid inside and the two are not interchangeable.' At that point I had to come home to get work done.
For those of you that know about such things, are these guys right? The guy at Grainger's didn't necessarily seem more than a clerk but the guy at the appliance parts place sounded like he might know something.
Any opinions?
Thanks -- and sorry for this endless tiresome question-asking,
Edward
At 9:01 AM -0800 3/11/05, Michael Dhabolt wrote:
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2) From: Philip Keleshian
Hi Edward,
These guys at the appliance stores aren't engineers.  Some of them =
aren't even technicians.
As I pointed out a start capacitor is switched out or the circuit when =
the motor starts.  If you don't have the capacitor connected to a =
centrifugal switch or current relay is in the circuit all the time and =
is therefore a run capacitor.
A run capacitor should be of good quality (low loss so it doesn't get =
hot).  Suitable capacitors use dielectrics of paper and can be oil =
filled (remember PCBs),or wax impregnated. Polymer films are also =
suitable. (no sloshing, this is probably what you machine has)
A start capacitor (remember I said capacitor start motors are used where =
high starting torque is required - think HIGH starting current) is large =
because of the high starting current in the motors which use them. To =
get the large capacitance in a manageable size and cost non-polarized =
electrolytic capacitors are used. Electrolytic capacitors are used in =
applications where the circuit impedance is high relative to the =
reactance of the capacitor or the duty is very short. (eg. motor =
starting)
In short you can always use a motor run capacitor for a start capacitor =
but it will be large and costly.  If you use a start capacitor as a run =
capacitor the fact that it is lossey may cause it to heat and possibly =
fail (sometimes catastrophically).
Phil

3) From: Philip Keleshian
Hi Edward, 
I just proof read this and made some changes.
Also this morning I responded to a post where someone suggested that you =
use a lower voltage capacitor.  He also mentioned polarized capacitors.
My response was to caution against using either low voltage capacitors =
or polarized capacitors.
I have not seen my reply so I am briefly stating it here. My original =
(apparently lost) replies were in the watch building category.
Phil

4) From: Edward Spiegel
Thanks again for all the info. I guess I need to find another source for capacitors other than Grainger's since the 440 V capacitor that they stock is way too big for the grinders case. I wish I could track down a capacitor of a similar size to the one that is in there. The one that Grainger's carries seems almost twice the sie.
Thanks again,
Edward
At 3:28 PM -0800 3/11/05, Philip Keleshian wrote:
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5) From: Michael Dhabolt
This is a multi-part message in MIME format.
Yes - - er, ah,-- What Phil said
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6) From: Michael Dhabolt
This is a multi-part message in MIME format.
Phil
Thank you for the review, edit and corrections.  It always amazes me, the
level of knowledge instantly available at the click of a send button in this
community.
Mike (just plain)

7) From: Deward Hastings
Edward:
*Those* counter guys don't know . . .
Sometimes counter guys do . . . there's a world of useful (and correct)
information behind *some* parts counters.  But they *all* will *claim* =
to
know . . .
Here's one source . . . never dealt with them, but they have what you =
want.
You can trim the URL to get to the home page from the datasheet.  The =
370
volt parts are adequate for a 120V motor.http://www.cpcares.com/NTE/capacitors/pdf/motor_run.pdfGonna cost you at least twenty bucks there, though.
And then there's our old friends at Surplus Center . . . they seem to =
have a
few thousand pieces in stock, at a reasonable price:http://www.surpluscenter.com/sort.asp?numrec=20&lastrec=20&sort=1&k=eyword=EM
RC&catname=electric&UID=2005031119121608
It's getting harder and harder to find the parts to "fix" things any =
more .
. . we live in a disposable society (and yes, you can read that several =
ways
. . .).
Deward

8) From: Ray Owen
This is a multi-part message in MIME format.
Edward, Phil has a good description of the motor capacitor situation.
I about fell out of my chair at this:
"These guys at the appliance stores aren't engineers. Some of them
aren't even technicians."
*On alternate days, maybe it's:
"Would you like fries with that?"
A grinder motor with a start cap? I'd be looking to stick a triac /diac trigger in the motor to relieve the starting switch of switching the cap/ start winding current out when the motor revs up. 
I assume you're talking about a three to five horsepower motor. Must be multiple V-belts driving a grinder with a big flywheel on it? 3450 rpm, I bet. 2-pole motors are cheaper to fabricate and have miserable starting torque.
Egad- Boyer's Coffee here in Denver has some equipment of that size and larger- on three phase power. Do you have three phase available? Starting a motor like that on residential service would put a Hellacious spike on the line- NG for computers...
W.W. Grainger's has quite a parts depot in Chicago, and they can supply parts for all kinds of equipment that's not even in their catalog. Their advantage is that, with a catalog like a mid-size phone book, the stuff is all available off the shelf or within a couple of days, max. And you pay for that.
Do a search online for Motor Start capacitors- I found These Guys  in Nebraska, or here . Thousands on Google.
Cheers and 73 -RayO
*
W. Edwards Deming:
Plants don't close from poor workmanship, but from poor management.

9) From: R.N.Kyle
found this that may helphttp://snipurl.com/deah


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