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
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