Re: ATM electronics question

["Bob May" <bobmay@nethere.com>]

Everybody is making a common problem when it comes to DC motors.  This is
that the motor is essentially a DC resistance.  This is not true at all.  DC
motors (I'm talking about straight permanent magnet type motors as opposed
to the motor often used in fans - an electronically controlled motor which
will have completely different characteristics!) have something called Back
EMF which is the fancy name for the motor acting as a generator.  If you
want to know what the DC resistance of a motor is you need to stop it from
turning.  When it starts turning, the rotating of the armature generates a
voltage which opposes the voltage of the power supply.  Thus, the current of
a motor will be something less than what the DC resistance of the motor
indicates.  A lightly loaded or steadily loaded motor thus will tend to have
a pretty constant current draw irrespective (I've built controllers for
motors that either use the current draw or the backEMF of the motor to
maintain the speed of the motor) of it's actual turning speed.  Since the
power is going to push air somewhere, there is the differential pressure
across the fan (the load) and the friction of the fan moving through the air
to determine the load of the fan.  In the end, if the fan is drawing .23A
(current measurement is usually done at max. flow rate) at 12V, it's
probably going to still be over .2A of current draw at the same pressure at
9V.  If you drop the pressure differential then the current will go down a
bit.  In the end, it's generally something that you will have to measure to
find out exactly what the current draw is going to be.
The motors used for fans tend to be current limited (meaning that they have
smaller wire than needed for the job) so that they don't burn up when they
get stopped for some reason.  Those motors also tend to be very low power
motors (no current draw thus no HP).
Bob May
My new web space address is http://webu.wigloo.com/bobmay/ or
http://nav.to/bobmay
and my new email address is bobmay@nethere.com
Bob May