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VFD questions

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  • #16
    look in the manual. Generally, a VFD is only UL recognized if it is associated with the correct size of fuse, or breaker, which will be specified in the manual.
    CNC machines only go through the motions


    • #17
      Originally posted by J Tiers View Post

      Why do you say that he "needs" that?

      Are you suggesting that the V/F would not operate the lathe?
      I always set up machine tools for sensor-less vector control
      Instead of a fixed V/f, with sensor-less vector the VFD uses a vector algorithm to find the best output voltage to run the motor.
      This uses current feedback from the motor itself. where the processor uses current readings and rapid calculations for constant torque.
      With this method usually auto-tuning is needed to ensure the VFD has as much motor data as possible.
      Also generally noted for tighter control. much better speed regulation also low-speed operation without sacrificing torque.


      • #18
        Well, I know what it does.......

        The question is, how low do you want to go? Generally, if you "need" that, you are probably going too low in motor speed on a machine tool, and ought to have a mechanical speed change.

        Everyone cites "torque", but totally forget, or sidestep around, the fact that it is "power" that cuts metal. For example: Suppose you slow the motor to the point that the lathe is turning the work at 30 rpm, when the natural speed at full motor rpm is 240 rpm. An 8x reduction in speed.

        With constant torque (generally a feature of both V/F and "vector" except at extremes) , you can remove the exact same chip size at 30 rpm as at 240 rpm. Sounds great, right? Woweee.

        Problem is that with the "old-fashioned and obsolete" mechanical speed reduction of the same ratio, you could remove a considerably larger chip. That might be as much as 5 to 8 x larger, because the motor could apply full power to the cut, and the 8x speed reduction would increase the torque 8x also.

        The "electronic speed control" has let you down, even though it does exactly what it is specified to do. It slowed the motor at constant torque, just as advertised, but it reduced the power applied to the cut in the same proportion as speed (basic mechanical engineering 101) because it could only hold the torque constant, it could not increase torque in proportion, the way mechanical speed reducers do..

        A VFD is a handy thing to have, but it is wise to not try to stretch speed control down to low speeds where the power loss becomes excessive and affects the work adversely. That tends to be the same range of speed reduction where the "vector" algorithm becomes important.

        OK, sure, people often say that in the "hobby shop" such things are "not important" and "hobbyists do not need to take full power cuts". Says who? So many "hobbyists" have low powered machines, that asking them to accept a cut in power to 1/6 of the already low power is a steep request.

        The workaround is to use an oversized motor and VFD, so that the reduction in power with slow speeds still leaves "enough" power. Maybe the OP is planning that. Or maybe you are assuming that.
        Last edited by J Tiers; 04-01-2021, 01:20 AM.
        CNC machines only go through the motions


        • #19
          Max and Jerry's conversation in the last two posts has made me think about what I really want out of this potential motor swap. My original thought was something along the lines of "hey look! a free 3 phase motor. Tho cool kids in machinist-ville have those on their lathes! I might want that some day!" I honestly hadn't thought too much about speed control and other stuff like that. It was more about smooth operation, a bit more power than what I have to start with and easier integration of a safety E-stop switch.

          The workaround is to use an oversized motor and VFD, so that the reduction in power with slow speeds still leaves "enough" power. Maybe the OP is planning that. Or maybe you are assuming that.
          Maybe this is the best idea. Would it hurt anything to use this 1.5hp 3450rpm motor at 1725rpm thus generating about 3/4hp? Then just leave it running at that rpm and change speeds using the belts as the lathe was intended to be used all along? Heck, 3/4hp is much better than the 1/3hp I've got now but no so much higher that I would have to worry about breaking stuff immediately.

          Also, I looked closer at the photos of the VFD at the link in my original post. Image 6 or 7 has the input specs which shows that it will draw up to 15.4A. So Paul was right on the money, a 20A circuit will do nicely.

          Thanks everyone for this conversation, as always I learned more than I knew before and got more out of the follow up than from my original questions!