On Tue, 30 Mar 2004 17:41:20 -0800, the renowned Tim Wescott
wrote:

Michael A. Covington wrote:

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!


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



Permanent magnet motors can have their magnets demagnetized by over
current events, heat or both. The cheap ferrite motors that you see in
toys (or from Radio Shack) have magnets that are prone to
demagnetization from over current. Samarium-Cobalt and Neodymium
magnets are much better in this regard, but way expensive (and heat can
ruin them). If you have old AlNiCo motors from the '50s then they're
supposed to be _really_ prone to demagnetization from over current.

If you have the time you can take a good one and a bad one and spin each
one at the same rate (with an electric drill, perhaps). Now measure the
motor's terminal voltage. If the magnets are equally good you should
see the same voltage from each motor -- a low voltage would indicate
weak magnets in that motor.

A shorted turn would set up an opposing magnetic field so you would
also see less voltage. But such a motor would coast to a stop much
faster than one with bad magnets.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com