What part defines the basic type of dc motors?

  1. a. the commutator
    b. the brushes
    c. the field
    d. the armature



    I know this is a basic question, its off an old exam paper im looking at. I think its the brushes because if it doesnt have brushes, then its a brushless dc motor, which isnt basic right?

    Thanks for your help! :shy:
     
  2. jcsd
  3. jim hardy

    jim hardy 5,001
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    An old test you say? Might date clear back to my school days then.

    I'd have said the field because that defines series, shunt, or compound among wound field motors, or permanent magnet field as in car windows.

    A brushless DC motor is really an AC motor , usually synchronous, with electronics to make a rotating magnetic field that the permanent magnet field follows round and round.. I remember when they were just a pipe dream. I looked into using one ~1973 when they were still exotic, and the ten horsepower size i needed wasn't yet practical .

    http://en.wikipedia.org/wiki/Brushless_DC_electric_motor

    that's my two cents, and it might be overpriced at that.
     
  4. Thanks for the reply :smile:

    Would I be correct in saying that both the field and the armature vary from shunt, series and compound?
    Or would it be the field just changing with respect to the armature?
    :bugeye:
     
  5. jim hardy

    jim hardy 5,001
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    You COULD use the same armature in a series, shunt or compound motor,

    but that's not to say you always would.

    So i'd say those three adjectives describe the field and do not tell you anything about the armature.

    In my DC machinery course we had in the lab a machine with both series and shunt field windings brought out to terminals, so we could wire it up any way using same armature.
     
  6. sophiecentaur

    sophiecentaur 13,771
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    Are you sure there wasn't more to this question?
    I think what they may be getting at is the need for Commutation, for a rotating motor to work from DC. Without a Commutator (or some other mechanism to achieve it), any motor fed with DC will execute half a turn (or less) and then stop.
     
  7. Before I could answer this I would have to attend the class in which I learned "what are the basic types of DC motors". Answer that first. Is it "series, shunt, or compound" as Jim suggested?
     
  8. sophiecentaur

    sophiecentaur 13,771
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    Unless the student had been coached in a particular direction, that original question could have been a poor one. This is often a problem with home-brewed questions from a teacher who has a particular axe to grind and loses sight of the bigger picture (and possibly can't express himself properly). I still say that the thing that enables a motor to work off DC has to be a commutator (of some sort). Other items on that list are optional.
     
  9. sophiecentaur

    sophiecentaur 13,771
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    I have been trying to read the question accurately. The question really does not appear to be asking for a definition of the various types of DC motor.
    If you actually parse the sentence, the first thing you come to is "define the type" and not 'types'. I interpret that as 'give a definition of the class DC motors in terms of the parts'. The part that actually distinguishes DC motors from AC motors is the part that alternates the supply to the armature.
    If you fall into the trap of deciding what the question is before reading it*, then you can "read it any way'. I was trying not to do that and had to assume that the person who wrote the sentence was being as precise as possible and using English correctly.

    *This is a 'do as I say and not as I do' statement, I realise. :wink:
     
  10. jim hardy

    jim hardy 5,001
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    hmmmm well, yes, "part" and "type" are singular while "motors" is plural... so your parsing challenge is valid.

    But a universal motor has rotating armature with commutator, brushes and field yet runs on AC.....

    Were I given a pile of similar sized brushed motors I'd label them according to how their fields were connected. A differential compound generator could be handy for powering gas discharge offroad lamps.
     
  11. sophiecentaur

    sophiecentaur 13,771
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    "hmmmmm" :smile:
    I guess you will find AC/DC motors with a commutator but you won't find a DC motor without some commutating mechanism. If we are discussing Brushless motors and the like, we'd have to define what actually constitutes the motor and what constitutes the supply circuitry. But that would be venturing into classification, to which, as you know, I am allergic.

    I think the original question would have been aimed at 'student level' and probably the part of the course not far beyond the introduction to motors. That, and treating the wording of the question 'literally', makes me pretty certain that it requires a simple answer.

    Jim: your experience of motors is at a much higher level than your average EE student. Some of them wouldn't necessarily recognise an electric motor if it landed on their foot - they tend to deal with diagrams and the dreaded animations. As usual, we really needed to know the level at which the question was set.
     
  12. jim hardy

    jim hardy 5,001
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    Yep, throwing in universal motor was a "slime" in debate jargon . ....

    meant in good clean fun though.

    :devil:'s advocate
     
  13. sophiecentaur

    sophiecentaur 13,771
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    The real clue is that it's a multiple choice question. That makes it a no brainer. We've been treating it as a 'discuss' question.
    Thinks . . . We should try more multiple choice posts. :devil:
    Eg.
    Is light
    A) a wave
    B) a particle
    C) a banana
    D) other ?
     
  14. I vote for Jim's answer in the 1st response.
     
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