Will 1 VFD driving 2 motors act like differential? - Page 2
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  1. #21
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    The 7.5 hp drive is likely vector driving the motor, but with the fla setting set for a 7.5 hp motor, at 15hz it will be delivering half perhaps, of the volts the motor needs. The reason why is because a 1.25hp motor might be 70% efficient, a 7.5hp motor 95%. At 15hz and rated torque the 7.5hp motor will be on the order of 60 to 75% efficient, the 1.25hp motor on the order of 40% efficient.

    This is why when the motor is stalled it doesent burn out.

    I have a feeling this is by design.

    Reducing the fla setting in the drive is increasing the current because the drive is already programmed for a set of assumptions for the motor's charachteristics according to the programmed fla.

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    Quote Originally Posted by johansen View Post
    The 7.5 hp drive is likely vector driving the motor, but with the fla setting set for a 7.5 hp motor, at 15hz it will be delivering half perhaps, of the volts the motor needs. The reason why is because a 1.25hp motor might be 70% efficient, a 7.5hp motor 95%. At 15hz and rated torque the 7.5hp motor will be on the order of 60 to 75% efficient, the 1.25hp motor on the order of 40% efficient.

    This is why when the motor is stalled it doesent burn out.

    I have a feeling this is by design.

    Reducing the fla setting in the drive is increasing the current because the drive is already programmed for a set of assumptions for the motor's charachteristics according to the programmed fla.
    Yah but... most of the classical VFD tuning tricks are still helpless when ONE VFD with ONE sensor set has TWO motors on its output.

    One can protect them well-enough. They have done - microswitch to reduce risk of failing the bridge as well.

    But there's No Fine Way to optimize nor predictably control them independently.

    That's harks back to the opener - whether or not the MOTORS can act as even an "open" differential mechanical analog..

    What would be the closer mechanical analog to the actual need is a "torsyn" differential.

    As said.. NONE of the tools really needed to resolve the muddy-wheels progress sync challenge to any level of confidence much above wishful thinking are there to begin with.

    "Can't get THERE from HERE"

    This gear was built to expect human intervention, and "right away, please", same as if it was a houseful of chikn's at risk of the odd visiting fox.

  3. #23
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    Donít recall exact numbers but when I changed the motor FLA parameter from 7.5hp to 2.5hp (12 to 4.6 seems familiar) supplied voltage under normal conditions went from 150 or so to 300 or so. And amperage decreased similarly. This a 480 volt system.
    Iím 92% sure the 7.5hp default parameter is a factory oversight because the last conventional tower uses an identical VFD to drive one motor and it had same parameters set.

    Same deal on three different systems installed over five years.
    Resetting parameters fixes MOST of the trouble.

    To answer some of Billís thoughts-last conventional tower sets pace. Intermediate towers use one alignment microswitch to fire a contactor plus a safety microswitch to kill system.
    Tail uses a potentiometer to sense displacement of the span structure thus controlling the VFD.

    Steering is by gps though older systems used a buried wire.

    So three ways tail can safety out. Displacement, twist, or off track.

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    Thermite,
    I can’t tell if you know what you are talking about and I don’t follow, or you have no idea what your talking about and your just rambling on for the fun of it. Let’s be clear.. are you a seasoned irrigation fix it guy?

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    Quote Originally Posted by Aejgx6 View Post
    Thermite,
    I can’t tell if you know what you are talking about and I don’t follow, or you have no idea what your talking about and your just rambling on for the fun of it. Let’s be clear.. are you a seasoned irrigation fix it guy?
    It's motion control. 'bout as relevant to ask how expert you are with Tainter gates, slow earth movers, fast cars, 'puter tape, document feeders, or missile tracking.

    Your limitations are your own, not mine.

    If you are "stuck", I/we can't help you understand the underlying physics, geometry, and power control engineering "fast enough" to even make a start much before the snow flies.

    "Find the experts". You have done, BTW.

    None of us happen to have had a hand in the design of this equipment early-on, or there'd already BE one VFD per one load-motor.... at the very least.

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    Thermite, your avoidance of the question provides the answer I seek. Svs, I’m sorry for the derail. I’m just trying to cut through the bs. Will you update us when you find the real fix? I mostly work on Valley, but I would like to store this case study for future use.

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    Quote Originally Posted by Aejgx6 View Post
    Thermite, your avoidance of the question provides the answer I seek. Svs, I’m sorry for the derail. I’m just trying to cut through the bs. Will you update us when you find the real fix? I mostly work on Valley, but I would like to store this case study for future use.
    Guess I would be better off trying to teach a pig how to whistle, wouldn't I?

    Pig may not have the proper physical equipment any more than that rig has.

    But the pig at least understands the concept of making sounds, and has the smarts to control them.



    The OP, BTW, is by now well aware that his rig is lacking the ability to make two motors act differently in a controlled manner whilst on the same VFD.

    Up to him what he does about that.

  8. #28
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    Aejgx6
    I think the reasonable cures in descending importance are wheel track maintenance including a dry circle following tillage/before irrigating, running both tires same direction, and spray nozzle on tires. Running a little faster would help too, but a lot depends on soil type.

    Only had to make four circles so far this year and Iíve had one spin/stall fault on one of the two with opposite facing tires. Third system used to fault twice per turn before flipping a tire.

    Havenít been around a modern Valley corner-are they set up about the same?

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    The Valleys work the same way. We don't have very many corner units down here, but they seem to be very reliable. Our systems run at least twice a week and normally have a pretty good trench to run in by now.. I quit working on systems 5 years ago after my first heat stoke. I have forgot a ton of the details, but I still get the occasional call to hunt down a ghostly problem. Have you identified exactly which safety is causing the shut down? I would guess if its stuck the compression/tension safety would be the cause..

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    Yep-thatís the one.

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    Quote Originally Posted by svs View Post
    Don’t recall exact numbers but when I changed the motor FLA parameter from 7.5hp to 2.5hp (12 to 4.6 seems familiar) supplied voltage under normal conditions went from 150 or so to 300 or so. And amperage decreased similarly. This a 480 volt system.
    I’m 92% sure the 7.5hp default parameter is a factory oversight because the last conventional tower uses an identical VFD to drive one motor and it had same parameters set.
    When you say the volts increase from 150 VAC increase to 300 vac as a result of programming the vfd for 2.5 hp instead of 7.5 hp, that is completely believable. The larger motor needs probably only needs 150vac at 15 hz, the smaller one needs a lot more.. though 300 vac that sounds too high, but the amps decreased so that is proof the motor is being driven more properly. is this 300 vac being reported by the drive as what its delivering to the motor or are you measuring that somehow?

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    Got those numbers off the VFD screen.

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    Hashing this over with you folks has stimulated a few new thoughts. So I thank you all.

    Re:motor burning up from stalling-Stall condition canít last more than say 10 seconds before the displacement safety kills the system. Motors are a good inverter rated brand with a thermal safety.

    Finally occurred to me that the alignment potentiometer would be signaling the VFD to speed up motors, and actual problem might be a slow acceleration curve in the VFD parameters.

    Need to wander through a corn field and take a look.

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    Quote Originally Posted by thermite View Post
    Some few faked it really well:

    Telechron Clock Motors, Rotor Sales and Information

    Not recommended for driving an irrigation rig.

    Wudda done right well at time-of-day control of it though!

    Synchronous motors are not "induction" motors. So his original statement stands. How big the synchronous motor is does not make a difference.

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    Quote Originally Posted by Jraef View Post
    Synchronous motors are not "induction" motors. So his original statement stands. How big the synchronous motor is does not make a difference.
    The telechron type actually is (follow the link and see construction), but it does not really matter.

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    Quote Originally Posted by Jraef View Post
    Synchronous motors are not "induction" motors. So his original statement stands. How big the synchronous motor is does not make a difference.
    Straining at a gnat, given the proper term is "synchronous induction motor", eg a special case AMONG the general tribe of induction motors. They are not powered by capacitance, heat, compressed air, nor magic.

    Asea Brown-Boveri seems to understand it. Not exactly their first rodeo.

    A J said, it doesn't matter.

    The telechron example was only because they had been so numerous and so pervasive that between public, institutional, transport, office, and home appliances, it had for a long period of US history, anywhere the power grid had reached, been nearly impossible for a person at home, in school, other prison, or workforce to go a full day and never at least once in that day have been in the presence of a telechron powered timekeeper of some kind, aware of it, or not. They were that pervasive.


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