I'm not positive so don't take my word as gospel but, two things I can
think of right off are:
The Black & Decker torque and rpms are probably at the chuck, after the
gear reduction, not the actual motor
Your applying AC motor formulas to a DC motor.
> 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