On Wed, 9 Feb 2011 11:15:34 -0800, "Jon Danniken"
wrote:

wrote:
1HP is not the absolute limit on 115v but if it has a NEMA 5-15 plug
on it (15a breaker max) that is about all the motor you can reliably
start.
There are some 1.5 HP motors that run on 120v but they will require a
20a breaker and a NEMA 5-20 plug if they want to get a U/L listing.

If you want to believe you have a 3HP compressor, go for it but I
would look at the air output myself.

There are about 100 articles on the net about "SPL" rated motors and
they all have the word "lie" or "inflated" in them

I never stated that I had a 3HP compressor, I stated that the motor on my
compressor is a 3HP SPL motor, with a rating of 15A at 115V.

You stated that " If it is 120v it will not be more than 1HP," which is
complete and utter bull****, as is most of the rest of the horse**** you
have tried to pile on to cover your ass.

I should know better than to attempt to have an honest discussion with
people on usenet.

Jon


Jon, Sorry to burst your bubble, but a 3HP SPL motor is NOT a 3HP
motor. It is a special application motor that has the starting
torque(roughly) of a regular 3 HP split phase motor. This is
accomplished by the use of 2 capacitors (usually) which affect power
factor and the phase shift between current and voltage in the motor.
Once the motor comes up to speed it looses that advantage and it is
simply a 2HP motor (maximum, under ideal conditions) with a little
more "slip" built in (if it is a dual cap - or cap-start cap-run
motor), which allows it to produce slightly higher torque at slighly
lower RPM for short periods without stalling and going over-current.

1 HP is defined as either 745 watts or 550 ft lbs/second.

Since an electric motor will ALWAYS be less than 100% efficient, to
get 745 watts (or 550 ft-lbs/second) of power out, slightly more will
be required to be put in. If the motor was 90% efficient (and precious
few are) that would require 828 watts in. At a line voltage of 123
volts (high side of average) that requires 6.75 amps at 100% power
factor (or unity power factor, or Power Factor of 1, depending at how
you want to quote it).At 117 volts (more realistic line voltage) that
takes 7 amps.

So, at the very MOST you have a 2HP motor. And that is being extremely
optimistic on both efficiency and power factor.(real world efficiency
is closer to 80%)
More realistically, a motor that will start and run on a 15 amp
circuit in a 120 volt system is a 1.5 HP motor.

You can slice it and dice it and spin it any way you want - the laws
of physics prevent anything more than 2 HP from being developed on a
120 volt 15 amp circuit, absolute best case.

That said, the 3HP SPL motor will start a bigger load without tripping
the breaker or blowing the fuse than a regular 2HP motor will start.
It might even, very optimistically, start the same load as a 3HP
regular duty AC motor would start (under the right combination of
conditions), and installing a 2HP utility motor on the compressor in
place of the 3HP SPL would LIKELY reqwuire a smaller pulley to start
the compressor reliably.
I'd love to see a compressor with a variable ratio(variable speed)
drive that would start the compressor at a high numerical ratio, then
speed the compressor up under light load, and slow it down under
heavier load, to get the maximum air power out of the least electrical
power input possible - but that would not be your typical low-cost
compressor any more.