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Sizing of DC motors

  1. Jun 26, 2007 #1
    Hi, I am working on an EV and the problem is in sizing or selection of dc motors. I have to select few small PM DC motors instead of a big one, the purpose is to pull a wheelchair, I have the mechanical quantities with me required to turn a wheel:
    Torque required is 43 N-m
    Power required = 135 W
    w(angular velocity)= 3.14 rad/sec

    I want to use 24 v supply for the parallel combination of the dc permanent magnet motors, now first thing how to select the motors and while selecting the motors what should I do?
    1) Is it correct to consider the amount of current flowing through the parallel combination of motors as equal in each motor?
    2) and is sum of all the torques produced by each motor is equal to my mechanical torque required?


    please help me out, thks!!!
     
  2. jcsd
  3. Jun 27, 2007 #2

    xez

    User Avatar

    I'm not an uber-expert on using motors in applications like
    this, so review my comments with a somewhat careful
    perspective.

    a) Using several motors electrically in parallel
    or mechanically in parallel can be complicated, and I
    suggest you consider your engineering of the drive and
    control system carefully. I imagine that maybe you're
    trying to have individual motors for all four wheels, or
    maybe two motors each on each of two primary large
    wheels. Anyway you mentioned having them electrically
    in parallel, and it seems like that could be a bit difficult
    since in the case of different wheel paths or sizes or
    tractions you could have some motors working much
    harder than others, or some might lose traction or stall
    while others continue to work.
    If you're turning, for instance the outer circumference
    wheels will travel a larger distance than the inner
    circumference ones. If you're making an ultimately tight
    turn the inner wheel may need to rotate very little or none
    at all, or may be best driven *backwards* while the outer
    wheel goes forward etc. In cases where one wheel has
    a bad traction or stall it can be useful to independently
    detect that situation and adjust the motor drive for that
    section independently since the stall would be very bad
    for the motor and battery (and maybe other mechanics),
    as well as having a possibly free-spinning "peeling out"
    wheel accelerating to high speed might be bad too in
    the case of lost traction.

    b) The angular velocity specification you mention may
    be relevant to some aspect of your design, though it's
    not clear what aspect you refer to -- presumably motor
    shaft rotation rate, and that would have whatever effect
    on the driven wheels that the transmission / gear / drive
    ratio would imply. Anyway the issue for the motor is
    what torque / power load it must be under when going
    at your specified angular rotation rate. If it has to have
    the specified 43Nm torque @ 3.14rad/s at 135W electrical
    input then that's certainly a set of parameters you could
    look up in a motor specification graph / table to ensure
    your candidate motor is suited for that operation.
    Also account for duty cycle/factor, maintenance, lifetime,
    heat dissipation, weight, voltage, current,
    et. al. in the consideration. It wouldn't be uncommon for
    a motor that delivered 3.14rad/sec @ 43Nm load to
    be able to spin much faster, though, under lesser torque
    loads given the same input voltage drive, though, so
    you'll need some kind of speed regulation drive if you want
    it to maintain that angular rotation rate depending on
    all loads up to the rated maximum, and including the
    case where traction may be lost and the motor is unloaded.

    c) Starting torque is usually a distinct consideration
    since it's easier for some motors to deliver a maximum
    torque under a certain load condition whereas when they're
    running at high speed or stopped they'll have a different
    available torque. If your torque specification is the
    minimum acceptable over all needed
    ranges of motor speed that's fine, but understand that
    the parameter isn't a constant over all possible speeds,
    so sometimes you'll get more than you need depending
    on the motor and its load / design.

    d) Well if you were to literally wire the motors in
    electrical parallel they'd have the same voltage across
    them by definition of parallel, but the current each
    takes would be depending on that motor's load and speed
    and temperature and subtle variances in its construction /
    winding resistances etc. relative to even nominally
    identical parallel units. In general motors of the same
    specification will deliver SIMILAR power/torque at
    SIMILAR voltage/current levels, but don't expect identical
    results, and certainly mechanical issues like friction / wear
    or varying loads will cause sometimes very different
    performances. Usually it's ideal to have a distinct
    motor controller channel for each motor, i.e. something like
    an H bridge driver, and also to have individual monitoring
    of motor terminal EMF and motor current for each
    motor, and perhaps also individual shaft rotation speed
    sensors for each motor unless you can get the speed
    data reliably enough from the EMF/current.

    e) Well IF you have several motors acting TRULY in
    concert then, yes, the sum of their mechanical powers
    or torques will contribute to driving the load. That's
    somewhat of a big IF, though, and it depends on your
    drive-train 'transmission' design as well as the motor
    controller electronics that are driving each of the group
    to help ensure they're operating in most efficient concert.
    If you're reaching the power output limit of a given motor
    then it's reasonable to see how the further output of
    that motor would diminish and another motor that is
    ganged together with the same load would be able to
    do additional work up to its limit of available torque at
    the current rotation rate, etc. etc. so several motors
    wired and geared truly in parallel could all contribute
    to driving a load. However you'd want to be careful
    in the design so that you don't end up with too much
    of disproportionate power outputs among the set of
    motors such that the ones that were a bit stronger wouldn't
    end up doing much more of the work and always
    performing near maximum capacity where others would
    be under-loaded. If the design is operating conservatively
    within the motors power / thermal / speed / torque
    limits and the curves on them are relatively flat over
    the operating envelope, and they're all quite similar
    in mechanical / electrical variances, you'll probably be
    able to have success with a more simple load sharing
    parallel type arrangement. Though with
    simple paralleling you will give up opportunities to
    monitor more closely individual motor
    health / performance, to reduce the usage of some motors
    entirely in the case of light load conditions, to individually
    control motor direction and/or speed, etc.
    The individual speed/direction control can be handy
    for steering, and handling turns and traction differences.

    As to how to select them, well, it's not too difficult
    once you know the drivetrain design, thermal limits,
    duty rating, maintenance or lifetime requirements, and
    the power/torque/speed/efficiency,
    size, weight, cost, availability you'll need, and you
    have figures for some of the key mechanical parameters.

    You're looking into something around a 1/5th HP
    motor, as you said, ideally DC permanent
    rare earth magnet, capable of achieving the power output
    and speed you specify at 24VDC input.

    Whether they should include things like controllers,
    encoders, be relatively more or less environmentally sealed,
    et. al. is all a matter for your design to specify.








     
  4. Jun 27, 2007 #3

    Danger

    User Avatar
    Gold Member

    Welcome to PF, Dave.
    I'm a bit confused about what it is that you're trying to accomplish. Unless you have some independent steering mechanism, you can't run your motors strictly in parallel. Electric wheelchairs steer by differential speed and/or direction of the drive wheels, and are usually set up to be controlled by a joystick.
    I have 3 wheelchair motors (2 with worm-drive gear heads). They're 12VDC, 1/2 hp. Each one has 6 wires coming from it, and 2 sets of field coils inside. Depending upon how the wires are connected, you can have the coils in parallel for maximum torque, or in series for maximum speed with half of the torque. In its natural habitat, that connection option is governed by the joystick. It's almost like having a 4-speed transmission (2 forward & 2 reverse speeds).
    Excellent post, Xez.
     
    Last edited: Jun 27, 2007
  5. Jun 28, 2007 #4
    thks a lot Xez and Danger.......I 'preciate ur kind helps....
     
  6. Jun 28, 2007 #5

    Danger

    User Avatar
    Gold Member

    You're quite welcome.
     
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