Pancake Motors - permanent magnet magnetisation

Discussion in 'General Electronics Chat' started by Skeebopstop, Nov 1, 2012.

  1. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Dear All,

    I was reading up on pancake motors at the following URL:

    http://www.phys.unsw.edu.au/hsc/hsc/electric_motors5.html

    They have some interesting properties. The only thing I couldn't understand from this description of them is the fact they leave a black wire in the motor so they can drive a few thousand amps (one off) thus fully energising the permanent magnets once the 'magnetic circuit' is completed after construction.

    I don't quite understand this for the following reasons:

    1. Is it true that permanent magnets retain their magnetisation better when placed within a region with a flux return path (i.e. magnetic circuit)?
    2. The rotor is composed entirely of copper, so there is little permeance for the magnetic circuit to satisfy the previous claim?

    Many thanks for anyone who can help me understand.

    Cheers,

    James
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    I have some very small DC motors that are "core-less". Their rotating armature is just core-less coils. The housing has fixed magnets. Without power the motor turns very freely, unlike the jerky rotation where the magnets attract the iron cores in a normal motor.
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Answers to your questions -

    1. Yes, a return path is very beneficial to the life of a magnet. When you buy a "horse shoe" magnet they will most times come with a "keeper" (a piece of steel that goes between the poles) that acts as a return path. The newer rare earth magnets are so powerful and able to retain their magnetic power that keepers aren't used much any more.

    The iron/steel discs that the magnets mount to and the outside casing of the motor makes up the return path.

    2. The copper printed circuit lines on the rotating disc replace the wires in a normal motor armature.

    The magnets in the motor are not magnetic until the amperage makes them into a magnet. No man made magnet is "magnetic" until it is charged or made magnetic.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    The rotor is not magnetic but it is sandwiched between two magnets or magnetic circuits that are very close (as close a manufacturing tolerances allow) to the rotor. Thus the magnetic circuit is through the magnetic material and across the small gap where the rotor is located. The permanence is provided in the magnetic circuit, not the rotor.
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    We had some motors that had printed circuit rotors (low inertia) and were physically held together by the field magnets. Alignment pins maintained proper alignment. The heavy gauge black wire was used to magnetize the permanent magnets after assembly, AND, to demagnetize them so the unit could be disassembled for any repair. These were used on a 1960-1970 vintage Jones and Lamson automated lathe.
     
  6. davebee

    Well-Known Member

    Oct 22, 2008
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    Many years ago my father was a stepping motor salesman, and he told me that same thing about the magnetic rotor in a stepping motor - they had their final magnetization after they were assembled, and just disassembling then reassembling the motor permanently damages the motor from its design parameters because of loss of some magnetism.

    The reassembled motor would still run, so some guy in his garage could take one apart, put it together again and not notice any difference, but the motor would not have the same strength that it originally did.
     
  7. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Thanks for great replies.

    I think we are now near where my concern lies. In a printed copper rotor motor, given copper is not of high permeability and no currents are flowing, what completes the magnetic circuit? Seems like the answer will simply be copper is better than air.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    An air coil is also an electromagnet but not as strong as one with an iron core.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Copper has about the same magnetic permeability as air (Air is 1.00000037 and copper is 0.999994 compared to vacuum according to Wikipedia) so the magnetic field flows through the copper, the same as it flows across the air gap between the rotor and the field magnets.
     
  10. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    So what is completing the magnetic circuit in a pancake motor with copper rotor (with relevance to pre/post assembly and magnetization of permanent magnets)? :)
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    The same thing that completes the magnetic circuit in a permanent magnet speaker. The field force created around the wire when current flows through it. Remember the experiment where a vertical wire carrying current is fed through the center of a piece of cardboard and iron filings are sprinkled on the cardboard? What completes the magnetic circuit there?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    I believe you confused about what constitutes a completed magnetic circuit. The magnetic field is generated by the magnet, crosses the air gap containing the rotor, continues through the magnetic material in the stator to the other side of the magnet and then through the magnet to cross the gap again. It's a roughly circular closed magnetic loop with an air gap. It is similar to any magnet with an air gap.

    Make sense?
     
  13. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Revisiting the amperian loop model i believe i may have found my error. Correct me if i am wrong but a static magnetic field enclosing the copper rotor WILL induce a current which rei.forces the field. Only time varying will create opposing voltages. This then makes sense to be closing the magnetic circuit.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    I don't understand what "rei.forces the field" means or what that has to do with closing the magnetic circuit. :confused: The magnetic circuit is there whether the pancake rotor is there or not.
     
  15. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Keep in mind the original query is why magnetize the permanent magnet stator ONLY after rotor assembly. My original confusion was based on the confusion of copper having no better a permeability than air and thus why does it make a difference if rotor is in or not.
     
  16. BillB3857

    Senior Member

    Feb 28, 2009
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    I think I answered that in post #5 The motor is loosely assembled with pin alignment, then the magnets are magnetized. The high field strength holds the unit together as well as providing a field for the armature to react against.
     
  17. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Thanks Bill. I guess I was still focused on the necessity of the rotor after post #5 but I can now see that perhaps it is just the practicality of 're-magnetizing' and 'disassembly' as you said that also warrants the black wires being left in. My fundamental confusion was "why not just pump up the magnets and remove the black wire" given the copper rotor doesn't seem to reduce the reluctance of the magnetic circuit present from the stator magnets alone.
     
  18. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I think the one thing everyone is missing about these motors is that the opposing magnets are of like poles, N-N and S-S. This is the reason that the magnets are magnetized after assembly. They would be too hard to assemble with the magnets trying to push each other apart.

    The black "magnetizing" wire is left in because there is no reason to take it out. It would add an extra step in the manufacturing process and cost to a motor that is a basic throw away product. These motors aren't normally repaired due to the low cost throw away nature of them.

    The magnetic "path" is between adjacent magnets not across the rotor. The end disc's and shell of the motor is what creates the path. A pancake motor is an axial flux motor and the magnetic path is different from the normal radial flux motor.
     
  19. crutschow

    Expert

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    Not true. If the magnets were opposing then there would be no net magnetic field flowing across the gap. They are of opposite poles to maximize the field across the gap. The magnetic path is directly across the gap and axially through the windings of the pancake rotor. When the magnetizing pulse is applied it magnetizes everything with a N-S-N-S orientation to generate the complete magnetic circuit.
     
  20. BillB3857

    Senior Member

    Feb 28, 2009
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