Hi Graham,

At 08:45 PM 7/26/99 +1200, you wrote:
>Hi Sean,
>I believe that you have forgotten that frictional loading will be
>largely proportional to speed, therefore reduction in speed gives a
>reduction in frictional loading.   Work = force x distance, less
>distance (rotations) = less work

Are you saying that the work done is proporional to speed,or that the force
(torque) is proportional to speed? In a recent project where I had to
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
large change in RPM. This lead me to believe that the major frictional
torque was caused by surface to surface kinetic friction, and therefore the
torque required to maintain speed was constant,and the work needed was
proportional to speed. This is what I said in my previous email.

>There are of course many other factors,
>-Magneto striction,
>-Air flow drag,
>-Viscosity of lubricant,
>-Temperature of lubricant,
>-etc.

I realize that there are many things involved (hence the 60mA change in
current shown above), however, I am trying to create a first order model,
so I can see the major factors involved.
I do think, though, that the kinetic friction will really dominate in a
small,cheap DC motor,and that's what I feel my experiment bore out.

That being said, it WOULD be interesting,as you say, to do a more detailed
study and see what other factors play a role in the friction. I'm sure that
the ones you mention would top the list.

>
>Sometimes it *really* helps to try the real thing out before developing
>general theories !

Well, I realize that I am no expert on motors, that's why I prefaced by
discussion with "someone please correct me if I'm wrong". BUT, I wasn't
simply talking with NO experience. I had just finished a month long project
in which I controlled a pair of small DC motors with attached gear/crank
systems using a PIC (gotta get the PIC in there <G>), did a computer
simulation of how the PWM affected the motors, thought about it quite a
bit,and used my scope and other tools to test my theories.

>regards,
>Graham Daniel.

Thanks,

Sean


|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
mailto:shb7@cornell.edu ICQ #: 3329174
________________________________________________________
NetZero - We believe in a FREE Internet.  Shouldn't you?
Get your FREE Internet Access and Email at
http://www.netzero.net/download/index.html