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DC machine speed/torque characteristic

  1. Jun 21, 2014 #1
    Hello everyone!

    In the following speed-torque characteristic of a DC shunt motor, we see that if the speed exceeds a certain value, then the machine becomes a generator.

    I don't understand something. How is it possible for the machine to have a negative torque and a positive speed? Shouldn't they always be of the same sign? For example if the machine (motor or generator) rotates clockwise, then speed and torque should be positive.

    So what does torque and speed of different sign mean?

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  2. jcsd
  3. Jun 21, 2014 #2

    Simon Bridge

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    If the speed is positive but the torque is negative then the acceleration is in the opposite direction to the velocity.
    Remember - torque causes acceleration, not speed.
  4. Jun 21, 2014 #3
    Thanks a lot! So the kind of rotation (clockwise or not) is defined by the sign of speed and not by the sign of toque. Is that right?
  5. Jun 21, 2014 #4

    Simon Bridge

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    That is correct - just like the direction of linear motion is not determined by the sign of the force.

    A clockwise applied torque produces a clockwise acceleration.
    If the initial velocity was anti-clockwise, it will take the rotation a while to change direction.

    Usually an applied torque will be opposed by something - a load say and always friction.
    The friction (or other losses) is often speed dependent, so here will be some speed where the friction+load is the same as the applied torque and the acceleration will be zero.
    This is the concept behind torque-speed relations.

    You have to look carefully at the description to see where the "torque" mentioned comes from.
  6. Jun 21, 2014 #5
    Thanks a lot again. However it still bugs me: If speed is positive, that means that the rotation is clockwise and so the Laplace force is positive and the torque is positive. Similarly if speed is negative then the torque is negative. So I come again to my first question.
  7. Jun 21, 2014 #6

    jim hardy

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    Intuitively one gets a feel for this running up and down steep hills in an electric golf cart.

    Arithmetically maybe this'll help ?

    Positive torque X Positive speed = positive power , a motor
    Negative torque X Negative speed = positive power , a motor wired to run the other way

    Negative torque X Positive speed = negative power , a generator

    sorry to butt in - just i sensed a need for some abject simplicity, and i'm as simple as they come..
  8. Jun 21, 2014 #7
    jim hardy thanks. I get that about power, but the exact thing I don't get is this: How is it possible to have negative torque and positive speed? Because it seems to me that both speed and torque must have the same sign as the "sign" the rotation.
    Last edited: Jun 21, 2014
  9. Jun 21, 2014 #8
    they are different things. Torque is a force, speed is a change in position. There is nothing that stops them from having different signs.

    When a car is braking, its applied torque to the wheels is negative but it is still moving forward.
  10. Jun 21, 2014 #9

    jim hardy

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    As SimonB said,
    What is providing the "twist"(torque) ?

    Have you ever started a lawnmower or outboard motor?
    You must supply significant torque to spin it,
    but once the engine is started it produces plenty of torque.

    Viewed from the top my lawnmower rotates clockwise
    and i must supply clockwise torque to get it started.

    Once it has started i could apply counterclockwise torque to stop it, or to extract power from its shaft.
    Note reversal of torque..

    Perhaps confusion stems from this fine point about action-reaction pairs. They act on different objects.
    So when you swap from motor to generator keep in mind who's doing the twisting and who's feeling that twist.
    To start the mower i apply clockwise torque from the top, and the motor's inertia produces equal counterclockwise torque.
    When mowing with it the motor produces clockwise torque and grass applies equal counterclockwise torque from the bottom.
    Observe torque felt by mower engine - clockwise to put power in, counterclockwise to take power out.

    These guys express it more succinctly :
  11. Jun 22, 2014 #10
    Thanks a lot!
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