>figure all this out, I found that the current drawn by a small DC motor
>attached to a gear/crank system was almost constant (varying from only
>0.240 A to 0.3 A) over a wide range of voltage (from 2v to 6v), causing a

Having played around with "small DC motors" at large input currents (up to
60A sustained - model boat racing, of course), let me give you a little of
the rules of thumb:

A no-load motor will pull a relatively constant current across the RPM
range for a small motor.  When you get into the performance category,
things change.

As RPM increases, it isn't uncommon to see the current rise significantly
as "other things" come into play.  Other things include (but not limited to):

        Brush drag - related to square of linear speed (this is why racers usual
ly
cut the commutator down - it reduces diameter --> linear speed).

        Air drag - at very high speed (60K RPM is often attained in these sucker
s
no-load, 30K loaded), the aerodynamics of the armature start to come into
play.  FWIW, I've found that epoxy-balanced armatures tend to do better, I
believe because they smooth out the rotating airflow.  The outer edge of a
loaded armature may be moving 25 MPH linear speed.

        Bearing drag - that's a big one!  Improperly lubricated bearings (wrong
oil, too much oil, too little oil) affect performance drastically.  This is
related to RPM^2 also.

        Heating - as the motor warms up (they DO get warm!) the heating of the
windings reduces their efficiency as conductors.  My motors are often
hotter than 100 C after a 90-second run.

Knowing this, the typical way to help rate a motor is to find the no-load
current (it's easier to measure) at a particular RPM and use that as the
basis for efficiency.  That way we can work out the approximate efficiency
under load, knowing the loaded RPM and the loaded current.  I usually set
my motors up for 9A no-load current.  9A vs. 240mA!

Anybody wanting to get deep into this should check out "The Motor Handbook"
by Bob Boucher.  It's available from www.astroflight.com online.  He goes
into _practical_ motor stuff in an easy-to-understand way.  Check out his
bio while you're there.

Andy

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Andy Kunz               Life is what we do to prepare for Eternity
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andy@rc-hydros.com      http://www.rc-hydros.com     - Race Boats
andy@montanadesign.com  http://www.montanadesign.com - Electronics
==================================================================