On Tue, 30 Mar 2004 20:01:21 -0500, the renowned "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!
Are your students applying more than rated voltage and/or "plugging"
the motors? (Suddenly reversing the voltage while the motor is still
rotating without a current limit). The motor can momentarily draw
close to double the stall current under such conditions, even with
rated voltage, which, of course, is much higher than the normal
running current. Some motors can stand up to this, but others
apparently can not. Another possibility is a shorted turn caused by
overheating the magnet wire insulation, which your simple resistance
check might not show up.
Suggest you disassemble a bad motor and possibly a good one too and
eyeball the differences. If the coils are smoked it should be obvious
(you may be able to tell with your nose without even taking it apart).
Similarly, a large difference in the magnets should be easy to see.
Here's a spec sheet on a small motor:
http://www.cs.rpi.edu/courses/spring...outs/gnm21.pdf
Note that at nominal voltage, the maximum armature current at stall
for the 2130 is only 5.7A. However, if the motor is turning with no
load and a 12V supply and the input is suddenly reversed, the current
will be very close to the maximum (especially at 25°C). Probably not
all motors are as good as these ones, and not everybody sticks to the
nominal supply voltage.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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