Will 1 VFD driving 2 motors act like differential?
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  1. #1
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    Default Will 1 VFD driving 2 motors act like differential?

    Long intro, apologies.

    Farm has a zimmatic irrigation system that uses one 7.5 hp AB VFD to drive two separate 1 1/4 hp motors mounted to wheel drives and 4’ tall tractor tread tires. Standard practice is pointing tread in opposite directions so one tire will self clean going either direction.

    Occasionally I’ve had the system “get stuck” without digging a hole and the backward running tire is slicked over with mud.

    I suspect the slick tire spins, and the one with good traction stalls just like a vehicle with an open differential. Is this possible?

    Motors run @ 15 hertz while irrigating.

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    Are these nominally 60Hz motors? Wonder what their torque rating is at 15 Hz. Is gearing/chain ratio change an option, or is it direct drive?

    Being an irrigation system, could you devote a nozzle to spraying off the tire all the time, so it doesn't get caked with mud? That water will get to the crops eventually...

    Disclaimer: Not a farmer. But I did stay overnight on a farm a time or two.

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    Shouldn't act as a mechanical differential, the other motor is still powered and both should run (the VFD is run in a V/Hz fashion), one could measure the current going to each motors with one spinning and one stalled. The torque should remain flat down at 15Hz, but the Hp would be around 0.3 at 15 Hz so most likely the motor with traction is stalled trying to drive a 4' tire by itself. If this is the case, I would be concerned about the longevity of the stalled motor over time. Ideally one would change the drive ratios so the motors operated at a higher RPM for more Hp. Also not a farmer.
    VFDs Can Control Multiple Motors

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    Correct on nominal 60hz.
    Motor mounts to a 43:1 which drives the 60:1 wheel drive so substantial reduction already and no easy way to get more.

    Typical center pivot tower is driven by one 3/4hp motor at 60hz. compared to these corner system towers with 2.5 hp at 15hz.
    Spray nozzle is something I’ve considered-Good idea I think.

    Was really fighting one of these systems a couple years ago and finally pointed both tires same direction. Won’t back up but goes forward better.

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    Yes, the VFD can't "act as a differential"; it has one output, that output is shared by the two motors so both motors are getting the same output of voltage and frequency from the VFD, it can't tell the difference between them. So if #1 is turning but #2 isn't, then the motor on #2 is stalled.

    But if you have the VFD go into Current Limit at a value less than or equal to the motor nameplate FLA, and it's a TENV motor, it can do that virtually forever. That's what I have seen done in situations like that.

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    Quote Originally Posted by svs View Post
    Correct on nominal 60hz.
    Motor mounts to a 43:1 which drives the 60:1 wheel drive so substantial reduction already and no easy way to get more.

    Typical center pivot tower is driven by one 3/4hp motor at 60hz. compared to these corner system towers with 2.5 hp at 15hz.
    Spray nozzle is something I’ve considered-Good idea I think.

    Was really fighting one of these systems a couple years ago and finally pointed both tires same direction. Won’t back up but goes forward better.
    I'd not much fight the mechanicals.

    I would go to separate, smaller VFD, one per motor. It's easier, for one thing. No significant "re-engineering", just some nameplate picking and choosing.

    Many VFD are built for remote, centralized "bus" control, so you do all that with safe, low-voltages or even storm-resistant fiber optics on the control side as far as syncing their efforts.

    Long conveyor lines can use a LOT of motors and VFD that way. Not a new need. More box-tick than head-scratching.

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    On reflection “differential” is a loose/poor analogy. Motor rpm will be either synchronous or stalled.
    What about torque? Always equal?
    Till the smoke gets out at least..

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    Quote Originally Posted by svs View Post
    On reflection “differential” is a loose/poor analogy. Motor rpm will be either synchronous or stalled.
    What about torque? Always equal?
    Till the smoke gets out at least..
    DC motors can be. They are - or "may be" - built as "torque animals". Their own internal commutator (or Hall-effect sensors and switching for "brushless") controls their behaviour.

    AC motors are "frequency" animals. An external source - mains or VFD - has control.

    Clever use of dirty-beach sand (electronics) can overlap their strengths, minimize the weaknesses of each, but at day's end?

    They still "are what they are".

    Not that is is magical, anyway. Well... "Magical?" Maybe a little bit - just not free nor even cheap.

    Two DC motors, same DC Drive, the more heavily loaded (not spinning) one would not stall - just slow down and grab a greater ration of the shared power. As RPM drops under load, CEMF drops, too, offering lower resistance to current flow. On its spinning twin, CEMF has climbed, increasing resistance to the flow of power into it.

    DC motors are not exactly "constant torque" below base RPM. They have "constant RESERVE OF torque".

    If the power is THERE, a locked-rotor DC motor can even eat itself to death, eg: "move the load or die trying". The commutator bars - trapped, unmoving, in one position under the brushes - cannot hand-off the load. Around 90 seconds or more in, they can overheat by enough to pop up out of position from expansion of the metal. Called commutator "wedging".

    There needs to be a current-limit set in the DC drive, a fuse or circuit breaker.

    Usually, the drive itself is (also) sized so it cannot possibly deliver the two to nine times normal power that the motor might ask for, no matter WHAT.

    Grokking this sort of "edge case" wants reading a Reliance "white paper". This is the sort of lab testing to destruction they have actually done.

    We chikn's don't really WANT to repeat that on our dime, anyway!

    Not with 5 HP Reliance DC Motors having MSRP's of over eleven thousand Mike Foxtrot yew ass dollars EACH we do not!

    Splitting to two VFD look a little less expensive, now, does it?


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    The two motors CAN have different "slip", and therefore different speeds.

    No induction motor runs synchronous. In order to cause the needed rotor current, there must be a small difference between the motor speed and synchronous speed. That difference depends on load.

    So, if the two motors need to turn at different speeds over a few percent, AND if the torque can be larger on the slower one, then they can run at different speeds. Otherwise not.

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    Quote Originally Posted by JST View Post
    No induction motor runs synchronous.
    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!


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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!

    Those are EXTREMELY specialized motors. "Fractional HP" does not even begin to describe them..... I have a reference on them, I'll have to read up on them again at some point.... thanks for the reminder. Had forgotten about them

    But as being motors to supply any sort of actual "power"...... not in the picture. Not sure they even could have been made in a form to supply much power, and IIRC they are not very efficient. Not that it matters for the clock usage.

    I have somewhere (if I did not get rid of it) a clock in the same case as the last one on the main page you linked. But it is not a Telechron motor.... it is a tooth wheel synchronous motor, you have to start with a knob, Will run time backwards (at least on the clock). It is somewhat similar to a switched reluctance motor......

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    Thanks for the input everyone.

    Here is a pic of such a system.


    Scott

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    Quote Originally Posted by JST View Post
    Those are EXTREMELY specialized motors. "Fractional HP" does not even begin to describe them..... I have a reference on them, I'll have to read up on them again at some point.... thanks for the reminder. Had forgotten about them

    But as being motors to supply any sort of actual "power"...... not in the picture. Not sure they even could have been made in a form to supply much power, and IIRC they are not very efficient. Not that it matters for the clock usage.
    ???

    Dunno what you class as "actual power, J. I'm sore puzzled by that.

    Synchronous motors have topped 135,000 HP:

    http://www04.abb.com/global/seitp/seitp202.nsf/0/5248d211abe89d48c12571fe005383d5/$file/NASA+wind+tunnel.pdf

    Mere 43,000 HP and 66,000 HP came online nearly 20 years ago:

    World's Largest AC Motor - Electric motors & generators engineering - Eng-Tips

    Yah need to get that 1952 "Heathkit" put back up into the attic and get out more...

    Google did that in mere seconds, wall-clock time, not CPU time..

    Mind - I already knew they would BE there.

    I "get out" so much I have never really been "in".


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    Quote Originally Posted by thermite View Post
    ???

    Dunno what you class as "actual power, J. I'm sore puzzled by that.

    Synchronous motors have topped 135,000 HP:

    http://www04.abb.com/global/seitp/seitp202.nsf/0/5248d211abe89d48c12571fe005383d5/$file/NASA+wind+tunnel.pdf

    ....
    Not the same thing.... "telechron" motors and huge power synchronous motors are worlds apart in design. Just take a close look at the page you linked to see what a telechron rotor is

    I guarantee you that the motors you reference do not look or work like the "telechron" motors. I do not think it would be very practical to make a telechron motor that would put out even 1 HP, due t efficiency.

    But this is wandering way way off the point.

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    If one is slicked and the other is not I suspect it’s not stuck. It could be that it can’t keep up(depending on the tower) or a contractor/drive is failing to start and causing the “safety” fault. Swing pivots shouldn’t ever be re-engineered by anyone but the manufacturer. They can be a finicky beast

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    Does it safety out with a clacker on it?

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    Quote Originally Posted by Aejgx6 View Post
    Swing pivots shouldn’t ever be re-engineered by anyone but the manufacturer. They can be a finicky beast
    +1 Best can be done may be that the OP take up that "lazy motor" issue up WITH the OEM and ask WTH is going on, and what their "usual fix" is for it.

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    Quote Originally Posted by Aejgx6 View Post
    If one is slicked and the other is not I suspect it’s not stuck. It could be that it can’t keep up(depending on the tower) or a contractor/drive is failing to start and causing the “safety” fault. Swing pivots shouldn’t ever be re-engineered by anyone but the manufacturer. They can be a finicky beast
    It’s not a classic pushing mud and digging deep hole stuck like you see with a conventional tower.

    Instead the slick tire/ stalled tire faults the “stretch safety”micro switch-meaning the tail is not keeping up with outermost conventional tower which sets the pace for the entire system.

    I don’t know if factory has figured it out yet but as of two years ago they were still sending the VFDs out with Allan Bradley s default settings for 7.5hp. Nobody I talked to could see that this was a bad thing. Setting the motor FLA address appropriately made a big improvement. (Raised voltage, reduced amperage supplied, better low hz torque)

    Could be I should turn the FLA down even further.

    Finicky beast is true. Neighbor was standing on catwalk of a tail(probably holding the safety in) and it fell down flat. No injuries at least.

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    Having trouble understanding how a stalled motor didnt burn out.

    If the vfd is programmed to deliver lower than rated v/hz to the motor to intentionally prevent the motor from burning out, then that is a feature not a bug. Need to fully investigate both the motor and drive parameters. Simply turning up the volts per hertz may not be smart.

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    Quote Originally Posted by johansen View Post
    Having trouble understanding how a stalled motor didnt burn out.

    If the vfd is programmed to deliver lower than rated v/hz to the motor to intentionally prevent the motor from burning out, then that is a feature not a bug. Need to fully investigate both the motor and drive parameters. Simply turning up the volts per hertz may not be smart.
    Bad - or non-existent - Engineering at the outset.

    OP is stuck with dealing with the symptoms two feedback levels away from the disease.

    Even IF, each wheel had resolver feedback and controls to keep rim-travel matched, even IF it had slip-detection and a second wheel, each support point to hand-off to?

    It would at least still need logic akin to what ABS brakes have evolved into. The ability to tap-dance on each brake independently, plus accelerometers to keep a motor vehicles progress in obedience to what is asked of it.

    That is still one level away from ACTUAL progress over the ground in this case.

    Keeping progress in sync wants the means - independent wheel control - and the measure.

    A sonic or RF "guide fence" at one edge of the swept area, or at least flexure detection on the span-bridge used as corrective feedback. EG: keep correcting on-the-fly to "straight", not just a binary "too late, we failed" microswitch.

    There's none of that in place to work to.

    It's as if the designer assumed irrigating a flat, level, uniform-surface traction, EMPTY, football stadium carpark. Or WAS a no-brushes carwash designer!

    Some competitor may have the needfuls, but priced accordingly. Of course.

    Best fix it soon.

    Once China or Japan owns that farmland, body could get taken out and shot for humidifying Indiana with 70% plus water waste instead of delivering a lesser amount at ground-level or underground to ... ta da... the ROOTS of the crop!

    "Cowboy economy" and cheap water. Its a real BITCH becoming a third-world country agricultural-practices-wise and not even trying to adapt to that...



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