"eljainc" wrote in message
ps.com...
Hello,

Does anybody know what size power supply I might need for a typical
cordless drill motor?
For example, I wish to power a 19.2VDC motor from black and decker/
Ryobi drill. Could I get by with a 18VDC supply? What amperage would
I be looking at? I think most of those battery packs are 2.4Amps. If
I put a 4 amp supply would that be sufficient or would that overpower
the motor?
I called DeWalt and the technical support person wouldn't tell me (I
think for liability reasons). He said that people have tried to use a
car battery to power a dewalt drill and something exploded or went
haywire on them.

Mike

Do a Google search on the term "back emf"

A rotating motor also acts as a generator and generates a voltage that is
opposite the driving voltage. This limits the current. That limited current
is used to calculate the motor windings and allows for smaller wire sizes.
At reduced driving voltage there may not be a fast enough rotation to
generate a sufficient back emf and the current will rise. As an extreme case
consider a DC motor in a stalled condition. It may rapidly overheat and
fail.Many a little device like the popular Dremel tools have met a sad
ending because of this. Ohm's law works as advertised, but the effects of
back emf must be considered.


Charlie