"Michael A. Covington"
wrote in message ...
My students are having the following experience with small
permanent-magnet
DC motors:

After some kind of misfortune (overload? nobody knows), the motor
starts
drawing much more current than it should. For example, a damaged
motor will
draw 1 amp at 3 volts, while the undamaged one will draw only 200 mA
at that
voltage. (Normal voltage is much higher, about 10 V.) Both motors
turn
easily; the damaged one seems to run fine except for requiring
excessive
current; and both have a resistance of about 1.5 ohms measured with an
ohmmeter.

This has happened to several motors.

Are the permanent magnets getting demagnetized? How? Or what else
could be
going on? My electronics background is analog and digital but not
much
about motors!

Just my guess, but the brush material might be wearing off and gathering
on the commutators, shorting between them and causing excessive current.
The small motors are probably not made for continuious and extended
duty, and just wear out. I've taken out some of the tiny motors from
defunct CD-ROM drives and played with them. These were made to open the
drive door and eject the CD or similar, and get used for a few seconds
very intermittently. If you have something like these, then running
them continuously for even a relatively short time might be as much wear
as a CD-ROM drive gives them over years of service. Just my $.02 worth.
Stop by more often and gab a bit, will ya? I haven't heard from you in
quite a while. ;-)


Many thanks,

Michael A. Covington - Artificial Intelligence Ctr - University of
Georgia

"In the core C# language it is simply not possible to have an
uninitialized
variable, a 'dangling' pointer, or an expression that indexes an array
beyond its bounds. Whole categories of bugs that routinely plague C
and C++
programs are thus eliminated." - A. Hejlsberg, The C# Programming
Language