General motor power concern...

General motor power concern...

Hello all, I am a bit puzzled about some calculations I have done
regarding motor power. First, I found some formulas relating motor
power, rpm, and torque at http://www.patchn.com/motorformula.htm. Now,
as can be seen from this site,

Power [HP] = Torque [ft-lb] x rpm / 5250,

and simple electronics tells us that

Power [Watts] = I [amps] x V [volts].

My motor (Black & Decker 18V cordless drill motor) is rated at 10
ft-lb of torque and 425 rpm's. The first formula above, then, gives
P = .9 HP. This of course assumes that the given torque and rpm occur
simultaneously. It may be, however, that these values are maximums
which then the max power would occur when these values are halved thus,
P = .2 HP.

Now, based on actual experiments, with no load at 18 volts my motor
draws about 4 amps. The second of the above formulas gives P = 72
Watts = .09 HP.

My problem is the considerable difference in the two calculations. I
understand that the specs on the motor given on the Black & Decker web
page may not be precise, but I still feel my results should be a little
closer than they are. The reason for my concern, I am currently
looking for a larger DC motor, and I hear one good source is old
treadmills. Most of these are rated at 1 to 2 HP. My intuition tells
me that a treadmill motor should blow the socks off of a cordless drill
motor, but how can I compare them if I get power calculations anywhere
from about 1 HP to less than 1 tenth?!?! Any
help/advise/expertise/recommendation would be appreciated.

Thanks, Lucas McGill

.

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