On Sat, 29 Sep 2007 23:44:21 -0500, Tim Wescott
wrote:

On Sat, 29 Sep 2007 22:22:15 -0500, Louis Ohland wrote:

Leo Lichtman wrote:
snip
A theoretically perfect DC motor has a 1:1 relationship between armature
current and shaft torque, and a 1:1 relationship between armature voltage
and speed. It is, in fact, the same relationship if you use the right
units (Newtons, meters, volts, amps, radians and seconds) to do your
calculation.

Real DC motors, unless they're horribly inefficient, don't stray too far
from the theoretical -- so you can pretty much calculate the torque vs.
current characteristic from the speed vs. voltage characteristic.

snip

This is only true of permanent magnet motors or shunt
field motors operated with a fixed field voltage.

In the case of a perfect lossless SERIES field motor
the shaft speed is directly proportionate to supply voltage AND
inversely proportionate to supply current..

Because increase of supply current increases BOTH the
field and the armature excitation, the shaft torque is
proportional to current squared.

With regard to the original query, because the
manufacturer quotes RPM/supply current, these are clearly series
wound motors possibly intended for wheeled transport.

There is little to choose between them. The higher
speed motor is likely to be slightly smaller and lighter than the
lower speed machine. The lower speed machine is a bit more
efficient and the lower speed and higher torque may be an
advantage.

Neither machine is suitable if the main aim is
constant speed with variable load.

Jim