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  1. #11
    I see what you mean...

    This doesn't look half bad for £72, UK stock, it has the closed loop but not the 3 phase.

    Click image for larger version. 

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    Slightly naughty testing at 48V when they suggest 36V typical but good enough.

    0.8Nm falling to 0.5Nm at 25 rps, I could get anywhere in the machine envelope in just 12 seconds. Maybe faster, they say it can do twice that before it clacks out.

    Next question, three 36V 5A PSU's or one 15A?

  2. #12
    I would guess they suggest 36V to allow for back EMF from the motor and variations in mains voltage when using an unregulated supply, which could momentarily bump it up by 10 volts or so. If you were running at 48V nominal with a 50V drive then that would make 58V which would quite possibly fry the drive, though you can add a clamp circuit to prevent this.

  3. #13
    The best solution for these low voltage drives would be a regulated (switched mode) power supply combined with a clamp circuit. That way you could safely operate a 48V drive at around 45V or so, to exploit the higher speed capability to the full. I haven't seen a clamp circuit available or in use (haven't been looking, mind) - is there something readily available on the market?

    It tends to be periods of rapid deceleration that result in overvoltage due to regeneration of the kinetic energy back into the DC bus. One simpler option might be to limit the deceleration setting (or acceleration if not controlled independently). That way you could run fast rapids as long as your e-stop didn't try to force a quicker stop.

  4. #14
    Good point. I just ordered 3 of the next size up and a 36V15A power supply
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    The extra Amp to compensate for the lower voltage
    The hasty purchase because we may be entering interesting times...

  5. #15
    Muzzer you jumped in while I was in mid-reply...
    I shall start at 36V and if that's not enough then 48V and clamps it is.
    When you say clamp is this the infamous crowbar circuit with a fat DIAC hard across the PSU to blow the fuse?

  6. #16
    No, I'm thinking of the "chopping / braking resistor" concept. Acts like a giant zener diode. Usually they have a switching transistor and a low value resistor. Once the voltage exceeds a threshold, the PWM duty cycles increases over a 5-10V band. That's the mechanism used within VFDs and servo drives but they are also available as standalone "braking units".

    CNCdrive do one, although I had to repair the one I bought after it broke down across one of the insulators. A lot of the compts we use on our systems are little more than hobby grade - along with the low price you have to expect questionable quality or design and manufacture.

    A big f*** off zener diode would do the trick but it's not very sophisticated and choice is rather limited.

  7. #17
    I couldn't find one so made my own - like you say a big fat switching transistor and a big (100W IIRC) wirewound resistor - I'll PM you the circuit if you want.

  8. #18
    You can even use a hysteretic controller where the switch turns on above a high threshold and stays on until it falls to a low threshold. Can be implemented with just one comparator. As long as the switch is turned on and off cleanly, it doesn't care - and as long as the resistor value is low enough it will control the transients when they occur.

    I suspect for most of our machines, the resistor would barely get warm. An LED would be a useful indicator to give some idea if it actually does anything. I might try that on mine some day - it needn't be much more than a resistor and LED across the braking resistor.

  9. #19
    Genuine interest... why not a TVS/MOV device? (I can guess at tolerance/variance at the switching voltage - but that is just a guess).

  10. #20
    Yes, usually the combination of the limited number of nominal voltages and the tolerance. They aren't too bad actually but you may struggle to find a voltage that is much above 36V but safely below the max allowable voltage for the drive. I'd be more confident in MOV than a TVS but that probably won't make much difference here. They are both deadly fast, so speed isn't an issue.

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