Don Klipstein wrote:
In article , Jeff Wisnia wrote:

You are correct on the wattage, but you didn't include the motor's
efficiency, and a NEMA B motor in the 1-4 HP range will have an
efficiency of about 79%. So, to get the motor to produce a true 2.5 HP
of mechanical output the current would be about 19.9 amps. That's
theoretically possible on a 20 Amp circuit, but really pushing it. :-)

But almost everybody in the "shop world" knows that there was and still
is far too much liberty taken with HP ratings on compressors. It was so
bad that class action suits were started and settled in favor of purchasers:

http://www.aircompressorsettlement.com/faq.php3

Your 2KW bathroom heaters are 100% efficient and thus should draw only
16.7 amps on a 120 volt line, certainly OK for use on a 20 amp circuit.


Also, motors are said to typically have a power factor of .8. So that
19.9 amps becomes almost 25 amps (for a supposedly typical figure).
Heaters have a power factor of 1, hardly less if they also have a fan or
electronic controls.

- Don Klipstein )

Well, I definitely overlooked the power factor issue, and I won't even
try to BS you by saying that it's because I've fitted power factor
correction capacitors to every motor in our home. :-)

But while I'm bloviating on the subject, during the energy crisis in the
70s, some guys at China Lake looked into improving the efficiency of
fractional horsepower induction motors by putting an AC capacitor across
the starter switch contacts, which rurned the motor into a two phase one
as soon as those switch contacts opened. 20% eenergy savings were
reported when using this technique on motors with relatively constant loads.

When that energy crisis cooled off that idea went away too. But I always
tought it was a darned clever approach.

The only reference to it I've seen is a letter published in Electronic
Design magazine in 2001, the last one on this page:

http://www.elecdesign.com/Articles/P...ArticleID=4058

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."