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Why can one not make an inside-out dc machine?

  1. Jan 6, 2017 #1
    In synchronous ac machines, the field is usually on the rotor and the power winding on the stator, where it's easy to cool. The field draws a small current and needs only small brushes.
    In dc machines, the field is on the stator and the power winding on the rotor, where it's harder to cool and needs a big commutator and brushes. Why can't we turn a dc machine inside out like an ac machine?
    Looking at the schematic of a dc shunt motor, for example, we see 2 windings in parallel, one connected directly to the line and one commutated. I see nothing that dictates which is field and which is power winding. The commutator ensures that the active winding's poles are always approximately perpendicular to the stator's poles, to provide maximum torque and maximum back emf. The torque is proportional to the product of the field ampere-turns and the armature ampere-turns. What am I missing?
     
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  3. Jan 6, 2017 #2

    NascentOxygen

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    You are proposing to put the motor's stator current through a commutator and feed the rotor through slip-rings? It sounds like this would require a doubling in the number of brushes, or else in addition to the commutator you'd need a second set of heavy duty slip-rings in order to get the DC onto the shaft and into the commutator. We normally want to control the field current separately.

    There may well be other considerations that I have not thought of.
     
  4. Jan 6, 2017 #3
    Nascent Oxygen:

    Sorry I did not make it completely clear. I propose a structure virtually identical to a conventional shunt dc machine. The only differences would be that the rotor winding is of thin wire and is called the field, and the stator winding is of heavy wire and called the power winding. (I avoid use of the term "armature" because not everyone realizes that an armature can be a stator.) Whether as motor or generator, why wouldn't this work? As a generator, all that should matter is the RELATIVE motion of the two parts. And, as I have already said, as a motor, the torque is determined by the product of the two fluxes.
    If you like, do this thought experiment: Put a completely conventional dc motor on a platform rotating at the motor's speed, but in reverse. Now the armature is stationary and the field rotates. Would it stop working?
     
  5. Jan 7, 2017 #4

    Svein

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    The simplest motors are the "rotational field" motors. They do not need rotor winding and therefore do not need brushes. Creating a rotational field from a DC voltage used to be big, bulky and expensive, but advances in microelectronics has made very small motor controllers possible. This wikipedia entry https://en.wikipedia.org/wiki/Brushless_DC_electric_motor explains it better.
     
  6. Jan 7, 2017 #5

    NascentOxygen

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    Yes, that's how I understood it. You'll still need a commutator of some description to operate the motor off DC.
     
  7. Jan 7, 2017 #6

    jim hardy

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    two practical considerations

    1. It'd have to be mounted by the center shaft not the outer frame. Where do the feet go ?

    2. Outer cylinder would experience flux reversals hence need to be laminated .

    Probably it was done around 1880 when electrical machinery was the cutting edge of technology, have you tried searching on term concentric dynamo ?

    My search engine turned up this patent from 1883, which i think is what you describe.
    https://www.google.com/patents/US278516

    concentricdynamo.jpg
     
  8. Jan 7, 2017 #7
    Svein:

    Your response, while it may be correct, is a non-sequitur.

    Nascent Oxygen:

    You have not read my posts carefully.
     
  9. Jan 7, 2017 #8
    Jim Hardy:

    I think that describes a homopolar generator. Nice, but well-known, and irrelevant to my question.
     
  10. Jan 7, 2017 #9

    jim hardy

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    of course not. Ever have your drill bit grab and almost break your wrist ? That's why powerful hand drills always have provision for a handle.


    But in your thought experiment the armature is still in the center.


    Webster agrees with the definition I was taught for "Armature", ie the winding where QV cross B takes place.
    https://www.merriam-webster.com/dictionary/armature
    Usually rotating....
     
    Last edited: Jan 7, 2017
  11. Jan 7, 2017 #10

    tech99

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    LIke you, I cannot understand the objections that have been raised. We are going to reverse the rotating field using a commutator. I must be missing something also.
     
  12. Jan 7, 2017 #11

    NascentOxygen

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    A wheel-hub motor? They work just fine.
     
  13. Jan 7, 2017 #12

    tech99

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    I think the problem must be that the field winding has very high inductance, which will prevent us from reversing the current rapidly.
     
  14. Jan 7, 2017 #13
    Tech 99:

    The armature of a conventional motor would have about the same number of turns, therefore the same inductance. Objection overruled.
     
  15. Jan 7, 2017 #14

    Baluncore

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    So the middle bit is held steady while the outside rotates. The winding on the static field is no longer rotating so can be fed through the mounting shaft and bearings. The commutator will need to be on the rotating shell with the brushes mounted on the static shaft, or externally.

    But the radius of the motor shell is now greater which makes it a massive flywheel. It will need to be very carefully balanced and have windings firmly held in place by something like epoxy, which will reduce cooling and increase the rotating mass.

    The bearings will need to carry a greater load on a thicker shaft so will need to run slower, or be more expensive.

    How will you enclose it to make it safe?

    If the rotating motor is supported by both ends of the static axial shaft, how might you quickly change a drive belt?
    If the rotating motor is cantilevered on only one end of the static shaft it will be a huge shaft and a lower speed bearing, or you will have all sorts of shaft or bearing failures.
     
  16. Jan 7, 2017 #15
    Baluncore:

    You seem to have lost the point. Remember, the rotating platform was merely a thought experiment to elucidate the original post, not a proposed motor design. The rotating platform should be thought of as having a battery on board, or having dc fed in through slip rings.
     
  17. Jan 7, 2017 #16

    NascentOxygen

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    It is clear that the first post in this thread should have included a neat diagram showing exactly what is envisaged, this would have saved our volunteers a lot of fruitless guessing.

    This thread is closed temporarily to give Alfred Cann the opportunity to sketch the construction he has in mind, showing the intended positioning of windings, pole faces, slip-rings and commutator and brushes.

    Alfred, please PM me and include your diagram once you have it ready so I can reopen this thread.

    Also, you'll see there are a number of questions you have been asked which require answers, so please attend to this immediately the thread is reopened.



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