"CJT" wrote in message
...
RP wrote:

"CJT" wrote in message
...

RP wrote:


"GigDaddy" wrote in message
m...


My 9 year old goodman ac quit working over the weekend. Had a
friend
that knows a little about ac look at it. He said that the run
capacitor for the compressor was bad. The one that was in there was
rated at 35 micro farads. He put in a 45 micro farad one and said
that
there would not be a problem. The system runs now but the different
size capacitor has me concerned. Will this cause a problem or not?

Thanks,


45 is too big. Rule of thumb is "within 10%", thus in this case
nothing but a 35 will do.
If a capacitor is oversized, then too much voltage may develop
across it, exceeding its voltage rating and causing premature
failure. Also the start winding is not designed for the extra
current that bigger capacitor will send through it.

Richard Perry



Given that most of them have a tolerance of 20% or more, your rule of
thumb surprises me.


Run capacitors typically have a tolerences that range from +/- 5% to
(-6%+10%). I don't know where you got your information. All of the
run capacitors in my stock are either +/- 5% or +/- 6%. OTOH, this
has what to do with the correct size of the replacement capacitor?

Richard Perry





Perhaps +/- 10% is more common than +/-20% as I earlier asserted:

http://www.newark.com/product-detail...alog/7248.html
http://www.newark.com/product-detail...alog/7154.html

What it has to do with sizing is that it shows motors must tolerate
a range of values.

If you oversize the capacitor by 10% of the *rated* capacitance (suppose
you are replacing a 50 mfd with a 55 mfd) then at most, because of the
10% tolerance, you are going to have 60.5 *actual* mfd installed where
a 50 mfd is called for. That's roughly 20% oversized. Thus a 20%
*actual* increase in capacitance is the most that you can oversize the
capacitor above what is recommended for the application. IOW, because of
the 10% tolerance you can only oversize by 10% of the rated capacitance,
which introduces a possible 20% oversize. If you exceed this, then
warranty is voided, and for good reason, the motor windings will
overheat. The voltage developed across the capacitor will also be
excessive (in most cases), causing the capacitor to short prematurely,
perhaps causing main winding failure. If OTOH the tolerance of the
replacement capactor is 5%, then obviously you can get away with a 15%
increase in rated capactance, since you would then still be at most 20%
over *actually*.

It's best to go back with the correct capacitor, however. If you *must*
go up in size, then check the running voltage across the capacator
before leaving it in the system. If the actual voltage reading is below
the capacitor's rated voltage, then you're good to go. Typically a 440
volt replacement capacitor will cover the increase in voltage that will
be developed with higher mfd.

Upsizing a run capactor, though allowable when the above guidelines are
met, is no substitute for maintaining the correct truck stock. IOW,
there is generally no good excuse for doing this. It's intended as more
of an emergency measure.

Richard Perry