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Three phase, controls etc......further to cow shed topic.

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  • Three phase, controls etc......further to cow shed topic.

    Yesterday was a pleasant day in the countryside, driving and flying from farm to farm looking at this problem of travelling gate becoming misaligned.

    One place in particular requires attention. The travelling gantry and gates are moved by two 750watt geared 3 phase motors, they have three modes, forward, backwards, stopped. They are controlled from a control panel in a pedestal in the milking shed about 50 yards away. There is one cable from the control to the gantry.

    Even a casual examination shows that the gantry spends a lot of time misaligned and grinding against the side rails.

    This the remedy under consideration ..Click image for larger version

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    The tracking sensor will detect when the wheel (which is free to move on its axle 50mm or so) is off centre indicating the gantry is skewed, the tracking control will then speed up or slow down motor B so as to re-centre the gantry.

    Motor A is started, stopped and reversed directly from the feed cable. I have some reservations with this scheme, the tracking and recovery process is I think valid but the motor control of Motor B requires further work.

    Consider, at the start of operations when the supply cable is livened Motor A will start immediately but how long will it take for the VFD to wake up and start motor B? Any delay in starting motor B will cause immediately mis-alignment.

    Is it even OK to be chopping the supply to the VFD and will it even start when the supply is restored? How will the VFD know which way to turn motor B when the 'rotation' is reversed back at the control panel?

    Is a VFD even required? I am thinking that the tracking sensor will control both motors, not by VFD but by simple contactors to pause the appropriate motor to make the alignment correction. Such simple 'stop-start' avoids any complications at starting and of reversing etc and might even be possible without the logic in the 'tracking sensor'.

    KISS, but will the contactors give reliable service even with frequent operation?
    Attached Files

  • #2
    Had to repair a 200 ton gantry crane on a long decommissioned power station to aid with the dismantling of the old turbines and generators. This was a asymmetric gantry where one side would run on rails at ground level and the other 20 meters high. 60+ years of wheel wear had it moving unevenly and the power company had modernized it a few years before the closing down.
    To automatically align the crane, 2 long distance laser ranging sensors were mounted on each side to measure the distance to the reflective targets at the end of the rails. They installed a vfd to each of the old 50hp wound rotor motors and a few cm of miss alignment would slow the faster leg.
    I had to replace some rat damaged wiring.
    Helder Ferreira
    Setubal, Portugal

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    • #3
      Originally posted by Noitoen View Post
      Had to repair a 200 ton gantry crane on a long decommissioned power station to aid with the dismantling of the old turbines and generators. This was a asymmetric gantry where one side would run on rails at ground level and the other 20 meters high. 60+ years of wheel wear had it moving unevenly and the power company had modernized it a few years before the closing down.
      To automatically align the crane, 2 long distance laser ranging sensors were mounted on each side to measure the distance to the reflective targets at the end of the rails. They installed a vfd to each of the old 50hp wound rotor motors and a few cm of miss alignment would slow the faster leg.
      I had to replace some rat damaged wiring.
      That sounds like a good control system, but there should be a controller to take signals from sensors and issue a command to VFD.
      There is no way farmers would do something like that. Too technical and too expensive.

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      • #4
        Originally posted by mikey553 View Post

        That sounds like a good control system, but there should be a controller to take signals from sensors and issue a command to VFD.
        There is no way farmers would do something like that. Too technical and too expensive.
        Nowadays there are small vfd's with plc functions integrated.
         
        Helder Ferreira
        Setubal, Portugal

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        • #5
          Originally posted by mikey553 View Post
          There is no way farmers would do something like that. Too technical and too expensive.
          That’s a remarkably uninformed statement.

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          • #6


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            In this scheme both motors have a connection to the supply cable via a contactor for each motor. The remote control panel can thereby start, stop and reverse the two motors.

            The tracking sensor will react to misalignment by stopping the appropriate motor required to bring the gantry back to alignment.

            Some logic is required to flip the correction process when the gantry is moving in reverse.

            I think this is the best idea so far.
            Last edited by The Artful Bodger; 01-16-2023, 10:23 PM.

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            • #7
              I would't use a tracking sensor which would be excellent if there was a rack and pinion. Soon there will be dirt to alter the radius of the sensor wheel and have wrong readings. A better and tested way with simple micro switch or inductive sensors without any electronics, is used on the irrigation pivots and can be used here as well. on the crap"O"cad drawing attached, all you need is the center sections to be mounted on some pivot mounts and 4 sensors.
              In reality, only 2 sensors are needed but since the control logic inverts depending on the direction of movement, one side of the sensors control one direction and the other side, the inverse.

              Normal 3 phase contactors can be used but small vfd's can be used as well without any extra electronics. Vfd's have a a "motorized potentiometer" that uses 2 digital inputs with one as increase speed and the other decrease. This would give a smoother ride.
              You may only view thumbnails in this gallery. This gallery has 1 photos.
              Helder Ferreira
              Setubal, Portugal

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              • #8
                Thanks for your comments, however we will not be using rack and pinions and we will not be reading distance from any sensor wheel. The sensor will be a simple device to detect if rail is moving towards rubbing the sides of the wheel.

                This is what the wheels look like..
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                I expect the sensor will be a Hall effect proximity sensor.

                There is no need for pivots like you show and if a contactor will do the job we will not be installing VFDs, a smooth ride is not a high priority.

                We need a bit more logic than that on the pivot irrigators as the corrective action must be flipped when the gantry changes direction. Incidentally, every one of the sheds we visited yesterday were built near or even around the centre pivot of an irrigator.
                Last edited by The Artful Bodger; 01-17-2023, 05:25 AM.

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                • #9
                  I would put a VFD on each motor, the VFD motor will not start and stop in the same way as the directly wired one. Putting a contactor on the outputs of a VFD is not recommended practice unless the VFD manual says you can. If wheels can slip then you'll need a laser ranger on each side, one side should be master, other is slave, use a PID to keep the slave within the distance of the master. Perhaps I am over complicating things, wouldn't be the first time.

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                  • #10
                    Imagine the two rails are not perfectly parallel to each other. (shocking concept I know) The gate can be moving in perfect alignment but the sensor will detect the wheel moving back and forth as the distance between the rails changes. What does the control algorithm do in this case? Why it misaligns the gate.

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                    • #11
                      Originally posted by RB211 View Post
                      I would put a VFD on each motor, the VFD motor will not start and stop in the same way as the directly wired one. Putting a contactor on the outputs of a VFD is not recommended practice unless the VFD manual says you can. If wheels can slip then you'll need a laser ranger on each side, one side should be master, other is slave, use a PID to keep the slave within the distance of the master. Perhaps I am over complicating things, wouldn't be the first time.
                      I am keeping away from VFDs if I can. I am hoping this type of contactor will coddle the motors for frequent stop and starts https://www.omron-ap.co.nz/products/family/980/ or there may be better alternatives.

                      Wheels can and of course will slip but I do not see the need for ranging as the need is for alignment. Misalignment is the issue as the wheels rub against the side of the rail and in a serious situation the wheels climb over the rail, so the object of this project is to detect the rail being close to the wheel flange and taking corrective action.
                      Last edited by The Artful Bodger; 01-17-2023, 01:48 PM.

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                      • #12
                        Originally posted by Erich View Post
                        Imagine the two rails are not perfectly parallel to each other. (shocking concept I know) The gate can be moving in perfect alignment but the sensor will detect the wheel moving back and forth as the distance between the rails changes. What does the control algorithm do in this case? Why it misaligns the gate.
                        The controller keeps one end on track and the wheels at the other end of the gantry accommodate the varied rail gauge.

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                        • #13
                          This problem, where the gantry and gates derail is fortunately not so common but when it does happen it is a serious event.

                          You can imagine a hundred or so cows already in the pen wanting to move up to be milked but they are blocked by the jammed gantry and gates while a few hundred more cows are making the way up the laneways. Tractors and tele-handlers, staff too, have to be summoned from wherever to come and lift the gantry back on the rails. Meanwhile a thousand cows have had their routine disrupted and none are being milked and of course the tankers have already left the factory to come for the milk pick-up.

                          Even when there is not a serious risk of derailment the misaligned gantry wheels are forever grinding against the sides of the rails leading to accelerated wear, increased running costs and premature failure of motors and drives.
                          Last edited by The Artful Bodger; 01-17-2023, 02:09 PM.

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                          • #14
                            One thing to watch out for is motors can overheat if stop-started often. The high inrush at start eventually builds up as heat in the motors. Softstarters are recommended for these kinds of situations.

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                            • #15
                              You could synch control logic to phase rotation with a tiny three phase gear motor turning a drum switch part of an arc via a viscous coupling. (I’m sure there is a fancy name or solid state version of such a device.)

                              Only input to gantry is three phase off/fwd/rev? What controls the gate winch?

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