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Inverter In Torque Control Mode

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  • Inverter In Torque Control Mode

    Sorry for my English.
    I wonder how the inverter in torque control mode works.

    I attach the picture.

    The load's weight is equal to moment of force of 100 Nm (After the pulley, friction etc)
    The maximum motor torque is 200 Nm.

    The joystick is stepless, and program to inverter according to scale:
    Start position 0, to 100 Nm (in the middle) and ended position is 200 Nm.
    The torque direction is clockwise.

    Then I think the process will be like this:
    - Start in 0 position, no movement.
    - When we move joystick a bit, and the torque is about 10 Nm, then the (coil) rope will moving upward slowly.
    - When we move joystick further away to 100 Nm, the upward speed is faster.
    - At about to lift the load, we decelerate the motor by moving back joystick to about 10 Nm.
    - The rope will be in tension but the load will not lift.
    - Up to 100 Nm, the load still not lift
    - After the joystick position pass the middle part to 200 Nm, the load will be lifted.

    The question is:
    - After the load lifted and the joystick position is back to 100 Nm, is the load will standstill in the air?
    - If the load hold in the air by 100 Nm, is decreasing the torque (to about 75 Nm) will make the load lowering slowly?


    Warm Regards.
    Herbrata
    Attached Files


  • #2
    I have used ABB and other drives in torque mode quite a bit. It is possible to lift a load in this manner, but I would not recommend it.

    Looking at your picture, the motor is directly connected to a cable take up spool.
    This means that there is no gear reduction and no self locking condition with no motor torque.

    The load will remain motionless only when the motor torque is equal to the load. At this condition, the motor will be under load, but not rotating. This will lead to excessive motor heat if not cooled and kept in this condition.

    The load could be kept motionless in this condition, but will most likely require a continuous feedback on the joystick. It may not be repeatable in the exact torque conditions, as the cable being wound around the spool will wind up being different diameters even during each revolution.

    Being in torque mode, the drive will continue to accel to its max setpoint when the torque is greater than the load and will require additional torque as the cable is wound up.
    The same when the load is being reduced, with the exception that the torque required will drop off as the cable is unwound.
    Both of these conditions will make it difficult to control the height of the load safely and consistently.

    Comment



    • #3
      If the motion is transmitted via a reduction box instead of directly from motor shaft, then the static resistance of the worm gear against counter-drive could prevent the load from coming down, however will not stop the motion. Still need a small brake on motor shaft to completely stop the load at a certain height. Reduction gear should be chosen with care, not all support static load.

      Trying to control the drive directly with a joystick without proper implementation of safety could lead to accidents. The automation needs to be solid and human factor needs removed, otherwise is just people in traffic. Human should not be involved in appreciating how much is too much, only the automation should decide motion is safe.
      Last edited by Alexandru; 09-12-2018, 09:45 AM.

      Comment



      • #4
        If one is ignoring real world problems like friction ( https://xkcd.com/669/ ) then I think the answers to both questions are "yes". Although at 75Nm the load will be accelerating down, so while it will start "lowering slowly", it will soon become fast.
        Since this is just someone's homework problem we don't need to be too concerned about safety issues.

        Comment



        • #5
          Originally posted by RogerR View Post
          I have used ABB and other drives in torque mode quite a bit. It is possible to lift a load in this manner, but I would not recommend it.

          Looking at your picture, the motor is directly connected to a cable take up spool.
          This means that there is no gear reduction and no self locking condition with no motor torque.

          The load will remain motionless only when the motor torque is equal to the load. At this condition, the motor will be under load, but not rotating. This will lead to excessive motor heat if not cooled and kept in this condition.

          The load could be kept motionless in this condition, but will most likely require a continuous feedback on the joystick. It may not be repeatable in the exact torque conditions, as the cable being wound around the spool will wind up being different diameters even during each revolution.

          Being in torque mode, the drive will continue to accel to its max setpoint when the torque is greater than the load and will require additional torque as the cable is wound up.
          The same when the load is being reduced, with the exception that the torque required will drop off as the cable is unwound.
          Both of these conditions will make it difficult to control the height of the load safely and consistently.
          It just an example pic. The actual drive has gear reducer and mechanical safety brake.

          The real problem is, my subkon supplied a General Purpose Inverter for crane application.
          And it is not working in speed control mode.

          It is a jerking movement when starting and stoping.

          So I try to modify it to torque control mode.

          Comment



          • #6
            The example in the beginning of the thread was a hypothetical problem.
            Did this crane operate on an inverter to start with?
            I would check to see if the supplied inverter meets all code for a crane use. If not, do not use it.

            Going to torque in this situation will most likely not be better.

            Look at the accel/decel settings.
            Also make sure that the brake is not electrically operated, part of the motor circuit, or requiring an output when the crane is being moved.

            Comment

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