Yeah, well Marathon is said to be among the best--and with the 3600 rpm 1-1/2
HP unit listing for $320, they should be!
"Character is much easier kept than recovered." Thomas Paine
My point was to show that motors are still rated in output HP from reputable vendors.
Delta, for example, list their saws in
traditional HP whale others list input amps or that fictitious peak HP baloney.
Having worked with AC motors and machinery for almost 25 years, I understand how one
arrives at the BHP and torque requirements for
a machine and tow to match a motor to the application. Again, the only thing that is
meaningful is the HP and Torque at the output
shaft. With that you can do a reasonable comparison.
The other interesting thing is that when these ratings are used, there is no easy way
to compare. Just look at how many different
Peak HP figures there are for 115v 15A input devices. Saws at 4.4HP, compressors at
"It ain't what you don't know that gets you into trouble. It's what you know
for sure that just ain't so." --- Mark Twain
A motor _should_ be rated by the HP it can deliver continuously.
Additional ratings are often given for various duty cycle such as a 1
hour rate (normally 10 to 20 % higher than continuous) at which the
motor can be loaded for 1 hour without overheating.
Most AC induction motors can develop two to four times their
continuous rating for a short time. This has nothing to do with stall
condifiton since with a stalled rotor, not power is being developed -
although a lot of power is going in and being turned to heat.
The current rating of a motor should be the current it draws at rated
voltage when developing rated continuous power.
As to efficiency - it depends on many things. It varies with the
motor constuction which can provide in excess of 90% maximum efficency
to less than 60% for a lousy motor at peak effciency. It also depends
on load. Peak efficiency normally occurs under light load and drops
as the motor is loaded to its continuous rating (whick can still be
over 80% efficiency) and continues down as the motor is loaded closer
to its peak rating.
If you will note, most of these inflated power ratings have the words
peak or maximum. This peak can be reached momentarily either by a
fused circuit for a brief time as the fuse becomes overloaded and then
reacts to the overload, or by a circuit (just for testing) that can
supply whatever current the motor asks for.
The best lies are 99% true. I'll bet these inflated ratings can be
backed up by actual data. What isn't said is that these test
conditions can't be approached in ordinary use.
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