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How to calculate the minimum torque required for a motor?

  1. Jun 25, 2015 #1
    Hi i want to choose a motor for a project i am working on and i have a structure weighing approx 2kg which is to be run at 1000 RPM. So my question is what should be specifications of the motor i am to buy ,more specificly the torque required to run the structure and the power rating?
    i was looking to use an input supply of 12v or even 24 v for the same.

    thank you
  2. jcsd
  3. Jun 25, 2015 #2


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    Welcome to PF.
    What is the "2kg structure" doing? That will determine the power requirement and therefore the torque.
    What speed range is acceptable? Will it be direct shaft coupled or will it be belt driven?
  4. Jun 25, 2015 #3
    You could get an approximation of the torque you are going to require by winding fishing line around the axle several times then adding hanging weights off the end until the point your structure starts turning comfortably through a full revolution.

    The weight required (in kilograms) times the radius (in metres) of the axle gives you the torque in kg.m and 1 kg.m converts to 9.80665 N·m (which is the standard unit of torque).

    From there you should be able to select a motor based on it's data sheet or specifications web page.
  5. Jun 25, 2015 #4
    Mechanical Structure .jpg

    This is the entire structure . i havnt yet decided on the size of the structure but the 2kg part i meant was the 'U' shaped part in the above pic.
    it wudnt be 2kg but as i said i havnt yet decided on the size so i thot i would put a safe mass of 2kg.
    It is connected to a motor at the centre .
    I plan for direct shaft coupling.
    the speed restriction....well i ve put 1000 rpm however a min of 500 rpm is also acceptable.
  6. Jun 25, 2015 #5


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    Unfortunately it's not easy to work out what torque is required. There are two possible answers ..

    1) The torque required to maintain the required speed. That depends on friction and air drag (unknown).
    2) The torque required to accelerate the object from stationary to the required rpm in the required time (not specified). That depends on the moment of inertia of the object and friction/air drag (both unknown).

    Usually one or the other will be larger and that's the figure you need.

    It's probably easier to measure it. See post #3
  7. Jun 25, 2015 #6


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    Well, that certainly is an interesting 2kg of structure. Once it is running it will continue with very little little torque needed. Speed of rotation can be set approximately by adjusting the voltage to the vertical axis DC motor.

    But there is an unknown. The gimbals will be subjected to forces due to the rotation on the axis of the cross shaft, and that by the rotation of the inner 3 point star. We cannot calculate that without mass and RPM details for all rotating components.

    Without the vertical drive motor, bearing friction and wind resistance you would find that you had built a gyrocompass. It would tend to align it's horizontal axis to the N-S meridian of the Earth.
  8. Jun 25, 2015 #7

    jim hardy

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    Uhhhh , rhtsree

    you are aware that 1000 rpm is about the speed of your central airconditioner's outdoor fan ?

    Surround that thing with a stout wire mesh cage to catch the pieces.
    Last edited: Jun 25, 2015
  9. Jun 25, 2015 #8


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    Staff: Mentor

    At 1000 rpm, for a structure that "interesting", drag is going to be an issue, but will be difficult to quantify. I'd find a fan that is of similar size and rpm and use that motor size as a guestimate.
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