"GigDaddy" wrote in message
...
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

RP wrote:

"GigDaddy" wrote in message
...

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.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

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

"GigDaddy" wrote in message
...

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

RP wrote:

"CJT" wrote in message
...

RP wrote:


"GigDaddy" wrote in message
...


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.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

"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

"Bubba " ReMoVe wrote in message
...
On Mon, 1 May 2006 14:32:24 -0500, "RP"
wrote:


"GigDaddy" wrote in message
. ..
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.

RP, you do realize that 10% of a 35 mfd capacitor is only 3.5 mfd for
a total of 38.5 mfd?

Quite aware, and since the next size available is 40 mfd, this is why I
said
that he'd have to go back with a 35 mfd.
Award #5 goes to Bubba

Richard Perry

Rule of thumb is DONT use rules of thumb. Thumbs belong on hands.
Use the right part the first time or dont do it.........PERIOD!
Bubba

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

If it were my unit, I'd replace it with the correct cap. At 45, the
one you have is about 30% too large, which is quite a bit. That cap
is optimized to produce the correct amount of phase shift when the
motor is running. With the oversize cap, it will mean that the motor
will not be operating as efficiently as it should. And since an AC
unit consumes a lot of power and efficiency is a top concern, I would
want mine running at peak efficiency. Also, if it's not operating at
peak efficiency, it will likely be generating more heat, which over
time, could lead to premature failure.