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Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
"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/ +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. |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
"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 |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
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. |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
"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 |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
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. |
Failure mode of a small PM motor
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 |
Failure mode of a small PM motor
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. |
Failure mode of a small PM motor
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. |
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