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

"Michael A. Covington" writes:

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.

Have you checked resistance and you rotate the rotor to check all
windings?

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!

That's a possibility. If you put much more current through the motor
than it expects, this could indeed happen. In any case, it's a simple
matter to compare the magnet strength of a good and bad motor. There
will be an obvious difference. However, I'd expect the rpms for a
given voltage to be different as well.

Another simpler possibility is that there is conducting "crud" on the
commutator. This is a common problem with the motors in CD players
and other consumer electronics. However, your test of the resistance
would seem to rule this out.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: The email address in this message header may no longer work. To
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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"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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

"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!


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



Also look for comuntator or brush wear or damage. Mostly caused by over
current or by lots of use or both.

Common problem in electric R/C cars. Easy to repair.

Hugh

Thanks to all who are responding! (And - Spehro, Sam, etc. - it's good to
hear from you again. I've been away from the electronics newsgroups for a
while.) I'll point my students to this thread.

"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!


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

Another thing to note but which probably doesn't apply here is that if
some PM motors are disassembled without a "keeper" for the PM magnets,
it will become partially demagnetized. This isn't a problem with the
cheap PM motors you are probably using but may be with high quality
steppers or servo motors.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Michael A. Covington wrote:
Thanks to all who are responding! (And - Spehro, Sam, etc. - it's good to
hear from you again. I've been away from the electronics newsgroups for a
while.) I'll point my students to this thread.



If I may interject one more thought before you go?

None of the answers you have so far (assuming I
saw them all) speaks to the greater than normal
stall current.

IMHO bad magnets, or in fact no magnets at all would
influence this.

The 1.5 ohm resistance is interesting, though.
Is it possible that there's a diode in there
that no one's noticed? (arc suppression, reverse
polarity protection, or something)

If so, betcha a dime to a donut that it's shorted

Take care.

Ken

"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

More on the mechanical side:
Damaged mechanical bearing surfaces can also attribute similar effects. Not
just perpendicular to the diameter of the shaft but also the 'play'(in and
out) of the rotor. Caused by: normal wear, cheap gears pulling/pushing the
shaft or out of round, worm spur/spline spur without(or worn) bushing
plates, excessive loading, over revving(exceeding intended rpm). One can
usually pick this out under a no-load comparison of motor rpm to that of a
known good motor to the suspected under the same supply. Quicky fix that
doesn't always work: a few drops of oil on bearing surfaces.

Brushes may also be of a composite that forms an oxide on the commutator if
the intended volt/amp rating is exceeded. Usually noted by a definate black
coloring and excessive arcing when in use. Unfortunately this is similar to
an out-of-round commutator.
Apply a bit of ultra fine sandpaper or gentle scrape commutator for oxide
contamination. Doubt it's worth trying to sure the commutator on a little
motor.

Have fun damnit.
"Michael A. Covington" wrote
in message ...
Thanks to all who are responding! (And - Spehro, Sam, etc. - it's good to
hear from you again. I've been away from the electronics newsgroups for a
while.) I'll point my students to this thread.

"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!


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

Sam Goldwasser wrote:

Another thing to note but which probably doesn't apply here is that if
some PM motors are disassembled without a "keeper" for the PM magnets,
it will become partially demagnetized. This isn't a problem with the
cheap PM motors you are probably using but may be with high quality
steppers or servo motors.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.




I know that's an issue with AlNiCo magnets, and I know it's _not_ an
issue with Neodymium-Boron-Iron (temperature will kill those suckers,
but not demagnetization). Is it a problem with Samarium Cobalt or any
other modern high-performance magnet material?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott writes:

Sam Goldwasser wrote:

Another thing to note but which probably doesn't apply here is that if
some PM motors are disassembled without a "keeper" for the PM magnets,
it will become partially demagnetized. This isn't a problem with the
cheap PM motors you are probably using but may be with high quality
steppers or servo motors.
--- sam | Sci.Electronics.Repair FAQ Home Page:

I know that's an issue with AlNiCo magnets, and I know it's _not_ an
issue with Neodymium-Boron-Iron (temperature will kill those suckers,
but not demagnetization). Is it a problem with Samarium Cobalt or any
other modern high-performance magnet material?

The motors I'm referring to almost certainly used magnets more high
tech than AlNiCo but I don't know what it was. There must be some
info on PM motor maintenance from motor manufacturers on the Web that
would shed some light on this.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Sam Goldwasser wrote:

Tim Wescott writes:


Sam Goldwasser wrote:


Another thing to note but which probably doesn't apply here is that if
some PM motors are disassembled without a "keeper" for the PM magnets,
it will become partially demagnetized. This isn't a problem with the
cheap PM motors you are probably using but may be with high quality
steppers or servo motors.
--- sam | Sci.Electronics.Repair FAQ Home Page:


I know that's an issue with AlNiCo magnets, and I know it's _not_ an
issue with Neodymium-Boron-Iron (temperature will kill those suckers,
but not demagnetization). Is it a problem with Samarium Cobalt or any
other modern high-performance magnet material?


The motors I'm referring to almost certainly used magnets more high
tech than AlNiCo but I don't know what it was. There must be some
info on PM motor maintenance from motor manufacturers on the Web that
would shed some light on this.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.



I was just curious if you knew.

Certainly a motor manufacturer could tell you this if they encouraged
maintenance on their motors. Many really small motors aren't designed
to be disassembled, however. Even the $150 precision motors from the
Swiss -- Escap and Maxon -- are pressed together and don't come apart
without fatally changing their characteristics.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott writes:

Sam Goldwasser wrote:

Tim Wescott writes:

Sam Goldwasser wrote:


Another thing to note but which probably doesn't apply here is that if
some PM motors are disassembled without a "keeper" for the PM magnets,
it will become partially demagnetized. This isn't a problem with the
cheap PM motors you are probably using but may be with high quality
steppers or servo motors.
--- sam | Sci.Electronics.Repair FAQ Home Page:

I know that's an issue with AlNiCo magnets, and I know it's _not_ an
issue with Neodymium-Boron-Iron (temperature will kill those suckers,
but not demagnetization). Is it a problem with Samarium Cobalt or any
other modern high-performance magnet material?
The motors I'm referring to almost certainly used magnets more high
tech than AlNiCo but I don't know what it was. There must be some
info on PM motor maintenance from motor manufacturers on the Web that
would shed some light on this.

I was just curious if you knew.

Certainly a motor manufacturer could tell you this if they encouraged
maintenance on their motors. Many really small motors aren't designed
to be disassembled, however. Even the $150 precision motors from the
Swiss -- Escap and Maxon -- are pressed together and don't come apart
without fatally changing their characteristics.

I'm guessing the motors the original poster was referring to are the
$1.50 variety which probably use ceramic ferrite magnets and really
don't suffer from this problem.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Sam Goldwasser wrote:
Tim Wescott writes:

Sam Goldwasser wrote:

Tim Wescott writes:

Sam Goldwasser wrote:


Another thing to note but which probably doesn't apply here is
that if some PM motors are disassembled without a "keeper" for
the PM magnets, it will become partially demagnetized. This
isn't a problem with the cheap PM motors you are probably using
but may be with high quality steppers or servo motors.
--- sam | Sci.Electronics.Repair FAQ Home Page:

I know that's an issue with AlNiCo magnets, and I know it's _not_
an issue with Neodymium-Boron-Iron (temperature will kill those
suckers, but not demagnetization). Is it a problem with Samarium
Cobalt or any other modern high-performance magnet material?
The motors I'm referring to almost certainly used magnets more high
tech than AlNiCo but I don't know what it was. There must be some
info on PM motor maintenance from motor manufacturers on the Web
that would shed some light on this.

I was just curious if you knew.

I'm guessing the motors the original poster was referring to are the
$1.50 variety which probably use ceramic ferrite magnets and really
don't suffer from this problem.

I would go along with that, too. When they first came out, we were
astonished how good the retentivity is and for all practical purposes,
taking apart, putting back together etc, the field strength was retained.


--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.