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AC stepper motors?

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  • AC stepper motors?

    I have some experience with DC stepper motors (STP-MRXXXX), but browsing, I came across the AC (STP-MTRACXXX) kind. At rough glance, they seem to be superior in almost every way. Is there somewhere I can find more info on them, comparing AC/DC steppers, and notes on when it's best to select either?
    Thanks


  • #2
    The motor themselves are not AC driven, generally it's the drives that are AC powered. This solves a couple of issues: 1. You don't need to supply a separate DC supply and 2. The internal DC voltage bus is at a higher voltage.

    The higher the voltage you use with a stepper, the better its performance. The speed of a stepper is going to be limited to how fast you can commutate it. That's determined by how fast you can charge the windings (inductance limited, higher voltage = better). You're also going to be limited to the current the windings can handle so that's why almost all modern drives will chop the output. It will use a higher voltage than the steppers listed voltage but only momentarily up to the current limit.

    There are some other motor types that are high pole count 'steppers' that are AC driven. Also, microstepping is using AC to drive a stepper but that's more for increasing resolution (to a point) and smoothness, not usually for performance.

    On the other side of things they have closed loop steppers which work a few different ways are are somewhat similar to steppers but have various ways of closing the loop allowing them to do interesting things like running in four quadrant mode.

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    • #3
      "they seem to be superior in almost every way" perhaps your "rough glance" missed the price? or was that the "almost"?
      Sure, one has to buy the power supply separately with the lower voltage models, but it still could be less expensive and in a multi motor application sharing the supply it might be quite a bit less.

      It is a bit annoying that the torque curves in the specifications use different units for the high voltage models making comparison more difficult. It does look like the high voltage ones have significantly faster maximum useful speeds (not surprising, but for high speed a closed loop servo is going to be better yet), but at lower speeds it looks like, in some cases, the torque falls off faster than the lower voltage models, but the different charts make it hard to tell with a brief glance.

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      • #4
        That's a pet peeve of mine too Tinker. Some of the charts I run into are just awful. One line( I forget the name) used oz-in on one chart and next to it, Ncm :-P

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        • #5
          Tinker,
          I've noticed that they are somewhat more expensive, but that price difference becomes less significant when considering the whole machine size, if you are only building 1-2 off them. The biggest issue I run into is that if you're using a gearbox, it's very easy to overcome the max torque if the gearbox with the smallest motor available in that size, if I'm doing the math right.
          For example a 25:1 Nema 34 has output torque of 42N-m, corresponding to input torque of 1.7N-m (ignoring efficiencies). So even the STP-MTRH-34066 risks damaging the gearbox before it stalls.
          In the original post, I was hoping that there is some kind of simple guideline (i.e below 2krpm continuous use a stepper, above 2krpm use a servo). But I suppose that not every situation has one of those.

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