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characteristics of DC motors?

Discussion in 'Boat Electronics' started by Shaun Van Poecke, May 26, 2007.

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  1. just some general questions here to try to get my head around DC motors....

    is there a direct relationship between max RPM and torque for motors of the
    same wattage? ie. if you have 2 ungeared motors that both draw 500watts and
    one has a max 300RPM the other has a max 1000RPM, will the 300RPM motor have
    more torque?

    is maximum pull really available from zero RPM? many people say this, but
    my experience with small DC motors (cordless drills etc) doesnt point at
    this being true. Does the nature of the controller have an effect?

    can all DC motors be wired up in reverse to go backwards?

    Are they really a 'free voltage' device, can i use a 12VDC motor with 24VDC,
    or a 36VDC motor with 48VDC? am i shortening the life of the motor by doing
    so? some motors that i have seen specify operating voltage from say,

    For two motors of the same given rating say, 500W, if i have a 24VDC and a
    48V motor, will they actually have the same operating characteristics? Will
    the 48V motor have more power than the 24V one, even though they consume the
    same ammount of power? Does current draw remain the same as the voltage
    goes up, or does it drop? Some manufacturers say that as the votage goes
    up, so does the HP... ie. [email protected], [email protected] etc etc....

    is one sepcific type of design more efficient than any other?

  2. Larry

    Larry Guest

    Yes, but I forget the formula at the moment.
    Maximum torque occurs in DC motors that are stalled because that is the
    speed at which counter EMF created by the spinning windings in the
    magnetic field is ZERO...generating maximum current. There's so much
    torque available that a starter motor trying to turn over a hydrolocked
    diesel may pull its own armature apart, destroying it, or the massive
    torque will break injector holders because the water in the cylinders
    will not compress on the compression stroke. Something's gotta give!

    The controller on cordless appliances has a current limiter to prevent
    you destroying the motor and battery with overcurrent on stalls.
    Yes. In permanent magnet motors just reverse the connections to the
    commutator. In field coil motors just reverse the connections to the
    When a motor is spinning at a certain RPM, a counter EMF is generated in
    reverse to the source power polarity that is exactly equal to the source
    voltage. Counter EMF generated is directly related to motor speed and is
    not linear. As applied voltage increases, motor speed increases until
    counter EMF increases due to speed up to this new balance point. As
    applied voltage increases further, there's a point at which the armature
    can no longer oppose the centrifugal force trying to pull it apart and
    the motor explodes, destructively, as it armature crams into the stator.
    If you are nearby a large industrial motor when this happens, you will
    never forget the sound or experience....sort of like remembering the 16"
    guns on the USS New Jersey going off when you were nearby. I'll never
    forget either! Life in overpowered motors may be measured more
    accurately in milliseconds than hours. Being a former slot car addict
    and racer of highly modified slot car motors known to melt track
    conductors, I been there, done several T-shirts!
    1hp = 746 electrical to mechanical conversion factor. It's
    not quite that high because the damned windings heat up and the bearings
    drag on it, reducing the actual output power, but it's close...ideal.

    A 500W 24V motor draws half as much current as a 48V 500W motor putting
    out the same torque and speed to power a load. They'll both put out
    500/746 or about 2/3 of a HP at whatever design speed they were built
    for. P = I x E. 500 = I x 24. I = 20.83A 500 = I x 48. I = 10.48A
    Ohm's law applies... If there's no output "power"...torque spinning a
    load...I = the sum total of all the losses in the motor...friction,
    leaking magnetism from the core, resistive heat heating up the wires and
    core, etc. Unloaded motors still draw current because they are not 100%
    efficient machines, but it's very little as they are very efficient.
    That depends on what you are driving. For instance, a series wound motor
    is better for vacuum cleaners, egg mixers, etc., where we want the motor
    to try harder when the load changes. But, to power a washer at a
    relatively constant speed, a parallel-wound motor is superior because it
    has better speed regulation. Listen to your vacuum cleaner when you put
    your hand over the end of the hose. It goes berserk with RPM as the fan
    unloads. You wouldn't want the washer to spin out of control just
    because it's not full, would you?? "Efficient" has a touch of reality in
    it, you see.

  3. Matt Colie

    Matt Colie Guest

    You have asked way many more questions than you realize. As I don't
    expect you would understand compete and accurate answers, I will post
    short answers in line (I hate doing this). The only other solution
    would be to lend you to you the text books from the electric machinery
    courses that I took.
    No, not maximum, but there is for rated load.

    ie. if you have 2 ungeared motors that both draw 500watts and
    If the 300 & 1000 are rated load speed then the one will produce three
    times the torque at its rated speed. (speed*torque/5252=hp)
    That is motor design dependent. A series motor can provide a torque
    curve that is this way. Traction motors used on locomotives are this
    built that way.
    No, the battery stack falls on its face and lets the motor power off.
    Don't try this with a good cordless drill, you will hurt your hand.
    By and large - yes, but because miniature motors are missing lots of
    windings, most are designed to run in one direction
    DC motor design gives little consideration to operating voltage. The
    internal design is all based on ampere-turns. If you run a 12v motor on
    24v it may run faster and have a higher power output, but it may not be
    able to accommodate the thermal load and over heat. If you have seen a
    motor with a rating you mention, it is a variable speed motor and
    actually designed for the maximum source voltage listed.
    That is motor design dependent.
    That is completely dependent on the load that the motor is driving
    Remember the ampere-turns thing??
    Permanent magnet field motors are generally more efficient as they do
    not use supplied power to create the stationary field.
    I don't know what you are planning to try to do, but I suggest that you
    learn a lot more about electric machines before you spend much money on
    anything. If you get a motor that has a different rating than you
    believe you need, do not count on being able to push the curve and make
    it work.
    Good Luck Guy
    Matt Colie
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