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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