This was a post by Kenneth Mayer
on Hobbicast giving me some advice on selecting a motor for my muller :

The horsepower ratings for
homeowner-type air compressors, power tools,

household vacuum cleaners, etc
are really "peak horsepower *consumed*".

They are not "continuous
*output*" ratings. Basically, the manufacturers

load the motor while monitoring
the voltage and current. At the instant

before the motor stalls and
releases its smoke, the product of voltage and

current is maximum; this is the
basis for "peak horsepower". This number

has no relationship with
reality. As everyone knows, all electrical devices

contain smoke which is necessary
for them to operate. Once the smoke

escapes, you are left with a
stinky paperweight.

True motor power output is
measured with a pony brake or similar load

device, and tachometer. The
load is increased while monitoring the shaft

speed. The motor's shaft
speed begins to fall off rapidly at the maximum

continuous load point. The
motor is then derated by the service factor (an

arbitrary number, typically 1.15)
to determine the continuous horsepower

rating. The current
measured at that load is the "Full Load Amps".

Take a look at the air
compressors and power tools at Sears, Home Depot and

other stores. The motor
data plate will list the horsepower as "special".

To determine the true continuous
*output* rating, multiply the voltage by

the full load current to obtain
VA, then multiply that by 0.65 (the average

efficiency and power factor for
small motors) to obtain Watts, then divide

by 746 to convert to
horsepower. That "6 horsepower" air compressor is

really ~2 hp
continuous! Even some of the low-end 5hp compressors are

using phony numbers.
Generally, the industrial type compressors and other

machines are rated using
"real" horsepower. 3-phase motors are rated in

"real" horsepower.

Motor weight is also an
indication of power output. A "real" 5hp motor

weighs +/- 100 pounds! A
motor works by induced magnetic fields

attracting/repelling. In order
to generate more shaft power, the rotor must

generate more torque. More
torque is generated by either increasing the

diameter of the rotor (expensive
and inefficient) or by increasing the

strength of the magnetic
field. Magnetic field strength is determined by

current draw and number of turns
in the winding, and depends on the iron

laminations to concentrate the
field. In order to handle the higher

magnetic field strength without
saturating (magnetically speaking), there

must be more iron in the laminations,
hence more weight. If the laminations

are allowed to saturate, the
excess power beyond the saturation point is

simply dissipated as heat in the
windings. As the First Law of

Thermodynamics states: "You
can't get something for nothing".

As a general rule, 1 hp on 120
volts will draw 11-12 amps, and half that on

240 volts. The largest 120
volt "consumer" motor you'll see is ~1.5 hp, and

that will *barely* operate on a
20 amp circuit, provided the motor starts

without an applied load.
Starting current for a 5 hp motor runs well over

100 amps! Remember to
account for starting current when sizing circuit

breakers and wiring.

The Harbor Freight "5 hp
compressor duty" motor draws 15 amps on 230 volts.

That's ~3 hp true shaft
power. Notice also that it weighs only 32 lb.

There's not much iron, so I'd
expect it to run very hot under continuous

load. The 3 hp Marathon
brand motors in the same catalog weigh 78-92 lb.

The 5 hp Marathon on my air
compressor can run continuously under full load

and barely feel warm. I
just a friend's new "Rigid" brand shop vacuum last

night; its rated 120 volts, 12
amps, *4.5 hp*. In reality its ~1 hp.

>>The bottom line is I just
need a motor that will work for a small amount of

>>money with reasonable
quality. If you gear a motor down enough you could

>>mull the world (well
extremely exaggerated but you get the point). The

>>trade off is time of
course. I would like to mull sand in a decent amount

>>of time and speed.

Harbor Freight has good prices (I
have a Marathon 5 hp on my Quincy air

compressor). Be careful
though- mine had the rotation direction opposite of

the wiring label. Grizzly
also has reasonable prices.

>>Therefore I guess a
logical question to ask is what horsepower rating is

>>really needed? What
size of motors are you guys using for what type of

>>muller and how does it
perform? Keep in mind that I am shooting for 100

>>lbs of sand and cost is a
big issue. Good quality powerful motors are expensive

>>and I do not have one. I
am a cheap person but I also want it to perform

>>well. A reversible
motor does seem to me that it would be nice to have.

Try ~2 hp, 1800 RPM, and adjust
the pulley sizes to obtain best performance.

You'll need a clamp-on ammeter to
determine how much load the motor is

actually seeing. If the
current is less than the rated "full load amps"

with a full load of sand, you can
adjust the pulleys to run the muller

faster. Its possible to
roughly calculate the needed pulley size change

from the current reading.
e.g. if the power (use the formula above) is 80%

of the motor's rated power, you
should be able to safely increase the muller

speed by ~10-15%. If the
motor draws more than the rated full load current,

adjust the pulleys to reduce the
muller speed. Remember the "service

factor" of the motor gives
you an additional margin of safety. Run on 240

volts if possible, and be sure
the machine is properly grounded! Try to

start the machine with the hopper
empty to minimize the starting current

draw.

Ken

:-)