VFD's causing chain stretch? - Page 2
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  1. #21
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    Quote Originally Posted by SBAER View Post
    I dont think its the VFDs, I think the chain has started to wear out and now its wearing out faster.
    I definitely agree that the chain stretch is largely due to it's age and is part of the natural process. I'm pretty certain there is a syncing issue between the two motors as well. The part I'm unsure about is how much the sync issue is contributing to the chain stretch (if at all).

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    Quote Originally Posted by Tor View Post
    After 5 mins of googlefu. The powerflex 70 drives comes with torque regulation options, and the 750 etc comes with positioning options too.

    As per the specs for ac 40s communication capabilities.:

    • Integral RS485 (Modbus RTU, Metasys N2, P1-FLN)
    • Optional: *EtherNet/IP, *ControlNet,*DeviceNet,
    BACnet, *Bluetooth®, *LonWorks®, *PROFIBUS DP.

    You say you have plc logic available as id imagine with the system you describe.
    If you want the to run in synch and have control of the vfd loads, etc. Running them as slaves on a profibus dp network, fx lets you do that.


    But those drives do not have the options to run with torque control, positioning feedback etc. So the powerflex 40 isn't really suited for that, it seems to be a very very basic vfd in terms of software.

    Did the motors use to run with a equal load ca before these issues?

    So you will want to upgrade drives, get the correct comm cards etc for your needs. And get it programmed into your plc logic, if thats what you find out is causing the issue in the first place.
    Thanks,

    We've discussed upgrading to a powerflex 70 before but nothing came of it. I will explore this further. Thanks for the specs for the PF 40's. I will investigate this as well. I can't speak for the very being as I wasn't here when the system went live but the unequal load issue has been around for many years now. We're simply taking a deeper dive into this issue because chain stretch is more of an issue for us now.

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    Perhaps you could try running the motors at slightly different frequencies, then swap the frequencies on reversal. Make the motors 'feel' each other's presence. If they are not encoder feedback, they won't really fight all that hard anyways.

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    Quote Originally Posted by HuFlungDung View Post
    Perhaps you could try running the motors at slightly different frequencies, then swap the frequencies on reversal. Make the motors 'feel' each other's presence. If they are not encoder feedback, they won't really fight all that hard anyways.
    That might work if not that the motors are not doing any work . 2.7 Amps on a 7.5HP motor with gearbox thats almost if not idle And 0.2Amp on the other motor means it is beeing pushed
    Motors are way too big IMHO

    Peter

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    I would consider this method:

    First VFD (master) has setpoint 40 Hz as you said.

    Second VFD (slave) has PID regulator activated. Reference for PID setpoint is current from first VFD. Second VFD will follow the first VFD amps.

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    So if the chain reverses, set the motors with electro clutches that release at reverse. That way one motor will always pull the chain in the required direction, no issue with pushing the chain.

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    Two or more independent drives / motors on a chain for a long conveyance is not uncommon. The idea of a PID with a master - slave arrangement is headed in the right direction, but I would deploy it a little differently than described especially given the limited performance of the PF40.

    Give both drives the same reference as described, however, monitor current in both drives and make a small alteration to the slave drive reference point dynamically to balance current. The PF40 is really a tinker toy drive and I don’t know if the drive has this capability built in. You could just monitor current from the PLC and make a slight alteration to the frequency reference to the slave drive for a specific amount of time (say 0.25 Hz for 1 second) and then restore the frequency reference. If you find this is too active make the reference change smaller, or visa versa. If current on the slave is low, give the slave drive a short bump in the positive direction to pick up the load. This is similar to a “Slack take up” feature on a paper machine drive system where you can have dozens of drives and motors all driving a series of rolls and felts all contacting a continuous web at the same time.

    This approach should provide ample load sharing performance for this setup. Make your reference alterations easy to change and monitor / adjust until you get a smooth load balancing routine

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    Quote Originally Posted by Vladdrac View Post
    Thanks for that. The good news is I've measured the chain stretch recently and it was something like 0.02% so that part seems ok.
    On long chain runs its not uncommon to need to frequently replace the sprockets multiple times over the life of the chain. Never had to deal with a twin driven chain, we would always just size up so it had enough tensile capability to be driven off one end only. that said if your only pulling 2. something amps at one end and 0.2 the other then you could probaly just remove one motor and sit it there as a life spare and let it idle. because your not even at 3hp.

    Oh things like only even numbers of sprocket teeth matter greatly too, do to how most roller chains are made only the outer links generally lengthen the inner links stay much the same, odd number sprocket teeth will wear significantly faster even number sprocket teeth tend to rebed and sort of become more hexagonal with sides of differing lengths all be it just a nats, but that nats matters greatly if you want long term life span.

    Correct lube and if low tooth count sprockets getting them cut in high strength wear resistant steel can gain you massive life increase, hardox sprockets work ace for this kinda job compared to plane std free cutting steels. Horizontal chain support details matter too on longer runs, side loads are always bad for chains!

  9. #29
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    Default 7.5 HP is too big

    Quote Originally Posted by Peter from Holland View Post
    I would say remove one motor and see how things go
    Also see how much amps does the motor/gearbox draws freerunning
    If that is more as 0.2 Amps one motor is driving the other
    0.2 Amps freerunning with a gearbox sounds very low for a 7.5 HP motor
    But wear on a chain speeds up over time
    Look at the guiding of the chain If thats worn you even experience more wear
    Replacing the sprockets is no good idea eighter
    Because of wear the pitch of the chain gets bigger so the nominal diam of your prockets needs to be bigger in fact
    Therefore the chain climbs on the sprocket and wears to a bigger nominal daimeter
    With a new sprocket that gets more difficult for the chain to run to the bigger diameter resulting in even more wear

    Letting run the 2 motors at the same amps will improve things IMHO
    I would see the sollution in getting the Amps the same
    More as in syncronising the speed
    But I am no expert in VFD s


    Peter
    7.5 HP is too big for 0.02A


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