On Jan 2, 11:21*am, mm wrote:
On Fri, 01 Jan 2010 17:29:15 -0500, "Michael A. Terrell"



wrote:

Andy wrote:

On Dec 31 2009, 7:50 pm, "William R. Walsh"
m wrote:
Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required. *Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

This can't be that difficult that we are all dependent on the keepers
of the secret capacitor knowledge. *I prefer to understand rather than
to ask someone and then believe them. *I want to know how to solve
this riddle myself. *What is the basis for capacitor sizing? *There
must be some relationship between the amount of current in the main
winding and the current in the secondary winding. *If someone on this
site doesn't beat me to it, I will figure out how to size a cap and
then post the solution.

* This isn't a 'site' *It is 'Usenet', even if you are using the lame
'Google Groups' interface.

* There are formulas to calculate the required capacitor. You need some
basic information to use them, so stick your smug attitude where the sun
doesn't shine.

I'm just a visitor here, but I agree with Michael's reply.

And Andy, why are you trying to figure out what size the cap should be
when you said that the motor nameplate says 7.5 uF?

I thought your question was, is it safe to use 10 uF?

I think JW and Wild Bill answered those questions before your last
post, and I would add that, though my info is 45 years old and
probably for a different type of cap, I was told the real value for
caps ranged from -10% of the rated value to +50 or +100%.

I'm trying to figure it out because I want to know. I didn't think of
that as smug, but I guess it is.