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    Reversing Field-Wound motors
    by DodgyGeezer πŸ‡¬πŸ‡§ ( Sub-Lieutenant)
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    RNinMunich
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    πŸ“ Reversing Field-Wound motors
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    Country: πŸ‡©πŸ‡ͺ Germany
    Online: 12 hours ago
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    You're right Dodgy!
    I do have some issues, at least with the circuit diagram.
    But 'I see where you're coming from' - I thinkπŸ€“
    More later when I've dug a bit deeper.
    Cheers, Doug.
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
    RNinMunich
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    Natch Dodgy πŸ‘
    As I said 'If I'm happy with it'.
    I.e. if it survives rigorous testing!
    I have various ESCs (old and new) to test it with.
    Goody box just arrived so awaaaay we goπŸ˜‰
    1
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
    DodgyGeezer
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    πŸ“ Reversing Field-Wound motors
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    I would pause a bit with promises. AFAICS, this approach is quite novel, so it's untested beyond my desk. And while I may think it's fine, someone else may find all kinds of issues with it. Different frequencies and/or pulse formation compared to my ESCs, for instance, might create instability, even though the simulations look OK. This is still 'bleeding edge' work, I'm afraid...
    RNinMunich
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    πŸ“ Reversing Field-Wound motors
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    That's what I intend to do Dodgy, with my Target.
    If I'm happy with it I'll fit it to my fish cutter.
    As you say it eliminates the voltage drop across the diodes, and also the heat generated in the bridgeπŸ‘

    @ Colin-H; if you like I'll send you one to replace the rectifier version you have?
    Cheers, Doug
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
    DodgyGeezer
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    That would be very useful! It's not a lot of components - and you don't need to drive a relay. Just an LED would show the operation, though to ensure that you don't get relay chatter you really need a complete circuit and a big, noisy motor to test...

    Here is a pic of one of my test boards - power from the battery on the left, while the (input and output from the ESC) are on the right...

    Correction - for 'input and output from the ESC' read 'connections to the cross-over coils'. πŸ€”πŸ˜Œ!
    RNinMunich
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    πŸ“ Reversing Field-Wound motors
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    Seems reasonable DodgyπŸ‘
    Got loads of Op Amps in the stash and some suitable FETs and 15A relays coming today.
    So I'll be 'avin a bash shortly.
    Doug
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
    DodgyGeezer
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    Field-Wound motors - of which Taycols are the commonest example for model boating - used to be used extensively as hobby electric motors until the Japanese started producing decent permanent magnet motors in the 1960/70s. They are not easy to reverse, however, requiring a polarity switch of the field coils, OR armature coil, but NOT both. This means that vintage modellers cannot easily use a polarity-reversing radio control ESC to control such a motor.

    A common technique to address this problem uses a diode rectifier to maintain the initial polarity on one set of coils while letting the others be driven in reverse. This is a simple to apply solution, but it has a few drawbacks - it lowers the voltage available for forward running, for instance. An ideal method of reversing would be to switch the coil connections as specified by the original designer.

    Such switching could be done physically by an extra servo, but that brings its own ergonomic difficulties. A better method would be to detect the polarity change and switch the coils using a relay. The circuit below is designed to do this, with few components (though more, of course, than the rectifier!)

    The PWM signal is smoothed, then fed into an op-amp acting as a comparator. Hysteresis around the switching point is achieved with a resistance feedback. This seems to cope with the problem of generated back-emf interfering with the polarity detection quite effectively. The output switches a 5-20A relay (depending on the motor being used) via a mosfet. The relay is, of course, set so that it is on for reverse, so that the only circuit drain during forward running is the op-amp, which is a matter of uAs.

    This approach appears to be novel - I can't find anyone else doing it - so I am cautious about recommending it for widespread use. I have tested it on several ESCs and motors, and put it through simulators on a circuit forum that I follow. The circuit seems to work reliably with these component values, but I would appreciate it if someone else would trial it with their system. There are so many different ESCs out there that it would be hard to test them all!

    There are some PIC-driven polarity-detector units on the market place for other purposes - reversing steam engines or water-jets, for instance. I have tried one, and found that motor interference made the logic circuits very unstable. Using a standard op-amp avoids that problem.
    1



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