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## Re: General motor power concern...

 Subject: Re: General motor power concern... "George" 19 Jun 2006 13:55:04 -0700 sci.electronics.basics
 ```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 and Your applying AC motor formulas to a DC motor. George lmcgill2@xxxxxxxx wrote: > 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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