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DC motors seem to have shorted rotors

  1. Jul 25, 2013 #1
    A practical DC motor armature is nothing but several "textbook DC motor" single armature windings bundled togheter and isolated together right? Like on this picture:

    What really beats me though is that I've made resistance measurements of the commutator on around 4 different DC machines now and they all seem short circuited!

    While there only should be electrical contact with two adjacent bars on the contactors as I see it, there are electrical contact between all of them.

    Yes I removed the rotor from the motor first and made sure that there were no way of short circuiting via the workbench or similarily. They were all insulated properly between the mechanical shaft and the windings and none of them looked like they had had a very rough time.

    What beats me even more is that they all seem to work as they should.

    Am I holding four semi-defect motors or am I missing something here?

    EDIT: Forgot to give the motor lineup.
    One PM motor from a toy scooter
    One Lucas series wound start motor
    Two washermachine series connected motors
  2. jcsd
  3. Jul 25, 2013 #2
    Here is a page handling the subject:


    But then they to a "bar to bar" test like this:

    Here they expect electrical continuity between to bars that are next to each other. How on earth are these armatures constructed?
    Last edited: Jul 25, 2013
  4. Jul 25, 2013 #3

    jim hardy

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    I had the exact same question upon my first encounter with a DC machine.

    Short answer is - armature coils are connected from one commutator bar to the another all the way around , so on DC they read just the resistance of the (large) armature wire. Coils are not isolated but connected together in series or parallel depending on whether it's a high or low voltage machine.,

    The first twenty five pages of this pdf file will give you a good introduction.

    http://www.kuet.ac.bd/webportal/ppmv2/uploads/1364120248DC Machines2.pdf

    take a look at figs 3.12 and 3.13 for "lap" and "wave" connections.

    old jim
  5. Jul 26, 2013 #4
    Thank you, I've been looking around a long time for this information.
    I guess uni-guys like me only know the textbook version of the DC motor, users only know that it's spinning with a battery applied.
    Than there is the PPM's of the population that does motor design who knows it maybe. :smile:
    My book "Electric Motors and Drives" (a great book otherwise btw) omits the whole subject of commutator design saying it's a "nitty-gritty" not worth bothering about.
    But it sure was in the end of the day. :smile:

    So not only the coil that is in contact with a brush then is current conducting, but perhaps several other coils as well. Interesting.
  6. Jul 26, 2013 #5
    Well they are indeed short circuited normally speaking but if you would put current through such a wire when taken out of the motor it would blow the fuses or heat up and burn but you have to look what it does inside the motor and inside the motor it carries current which then puts up a magnetic field which pushes against the field of the stator coils and the rotor starts moving , while moving everything is fine , hold the rotor intact and youll see that this small resistance soon enough makes sure the wires start to burn and short circuit the whole thing and you have a motor that needs to be rewound.
  7. Jul 26, 2013 #6

    jim hardy

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    That you are taking things apart and asking questions says a lot about you.

    I'll repeat this anecdote (old guys do that)

    I was a college freshman working on getting an old $50 car going, a 1951 Chrysler New Yorker. It wasn't charging the battery. I too was puzzled by the seemingly short circuited rotor. I'd asked some upperclassmen about it to no avail. So I cleaned the armature up as best I could and carried it into my EE professor's office. He was an older gray haired gentleman who always wore a white shirt and bow tie and with round Harry Truman spectacles . I looked like a grease monkey so was fully expecting to be thrown out on my ear.

    Instead his face lit up and he swept a pile of papers off his desk and pulled a multimeter from a drawer. He grabbed a an ancient 1920-ish textbook off his shelf and opened it to a fold-out drawing perhaps three feet long. The drawing showed lap and wave wound armatures , had explanatory notes about pitch, and I got a quick introduction to DC machines and commutation.
    It became clear my trouble was not in the generator, so Prof (Grimm was his name) left me with two practical pieces of advice:
    1. It's difficult to load test a car generator on the workbench. If it'll motor when connected across a battery it's probably okay. If at night the headlights get brighter when you come off idle, the whole charging system is probably working okay.
    2. The university library has a book that he knew was a good one on automotive electrical systems.

    So I thanked him and went to library. Sure enough it was a 1930-ish book that described exactly my Chrysler charging system - six volt positive ground generator with three coil electromechanical voltage regulator. From it I learned how to set the air gaps and spring tension on the regulator coils and how to burnish their 'dancing contacts'. Now THAT was educational - you can literally feel with your fingertips that type regulator doing its job so it quickly becomes intuitive how they work. Lacking a 30 amp meter I set the current limit coil tension so as to be on verge of limiting with all electrics running and high beam headlights.

    Point of all that is , One learns a lot by taking things apart to figure out why they work. It ties your theory to your practical reinforcing both.
    Prof Grimm was one DOGGONE PRACTICAL UNI-type guy.

    So - Don't sell yourself short !
  8. Jul 26, 2013 #7


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    I had noticed the same thing on my battery-powered lawnmower's motor, after naively assuming the coils would all be isolated from each other.
    (BTW Jim is referring to figs 3.12 and 3.13 on p. 21 of the pdf, not the figs of the same designation :confused: on pages 31 & 32.)

    Thanks Jim. Even with those figures I found it a little hard to picture what was going on, but you gave me a term that I could google ("lap winding") and I found these images helpful:

    (From this page.)

    (From here.)

    Still a little mystified by the wave winding scheme. It seems to me that adjacent loops produce fields that more or less oppose or cancel each others field. But it works, so obviously I am missing something.

    Anyway, thanks again.
    Last edited: Jul 26, 2013
  9. Jul 27, 2013 #8

    jim hardy

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    There's a bit more here, near bottom of the article.

    Relative merits of lap vs wave is something i'd rather know where to look up than try to remember. I doubt i'll ever desolder and re-wind an armature.
    But I do have a "Growler" for checking them.
    Last edited by a moderator: Sep 25, 2014
  10. Aug 18, 2013 #9
    I like this forum. :smile:

    Jim Hardy: Sounds like you had the best kind of professor. :smile: Indeed all theory should be combined with some level of practice. Some institutions and lecturers unfortunately neglect that though.
  11. Aug 18, 2013 #10

    jim hardy

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    Thanks - Prof Grimm was a good one who really loved to teach.

    Here's a link to a training module that discusses windings, probably you already have some good ones though.
    http://www.nptel.iitm.ac.in/courses...l Technology/pdf/L-35(TB)(ET) ((EE)NPTEL).pdf

    I still have my 1901 electric machinery book which predates even Prof Grimm !


    Same author wrote a book titled
    "Calculus Made Easy: Being a very-simplest introduction to those beautiful methods which are generally called by the terrifying names of the Differential Calculus and the Integral Calculus "
    and a few revisions later it's still selling well.
    Check out the first chapter of the pdf version....... it's just great.

    I suspect he and prof Grimm were cut from the same cloth.

    Thanks for the kind words.
    Last edited: Aug 18, 2013
  12. Aug 18, 2013 #11
    that's good that you have enjoyed the forums , have a nice time:)
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