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
:-)
