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Making a AC Motor Variable Speed

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  • Making a AC Motor Variable Speed

    Are there any low cost ways to make a AC Caps motor variable speed?
    I have a ac motor direct driving a homemade table top mill. Making a variable speed, off axis drive is a last option. Any low cost electronic speed controls available?

  • #2
    I'm looking for a plan myself. I've been looking at H-bridge IGBT drivers, but the module alone is over $50. Add rectifiers, caps, PWM controller, and high side drivers, and you're already looking at $150 just for parts. At that price it doesn't make any sense to build one. Not to mention, without a plan, I would undoubtly blow a couple IGBTs before I get the snubber designed properly.

    Someone must have designed something really cheap and simple using inexpensive bipolar transistor with all analog design.



    • #3
      Would it not be easier with a series-shunt DC motor? I am not 100% sure but I believe a lot of low cost drives are done this way. (Such as the Sherline)



      • #4
        IGBTs are a bit more fragile than MosFets, but since motor drive has been done with bipolar even, they will all work OK. The snubbers may depend on load anyway, but for the slow switch rates they may not be a big issue.

        Having designed 300+ kHz h-bridge class-D amplifiers, I think of the 5-40 kHz as slow!

        With a split-phase motor, I bet you could do some real good by bagging the start switch, and using the start winding as a second phase. I said a split phase because the start windings have to carry more current on them, and might suit continuous duty better.


        • #5
          BTW, look at the IR2110 (I think) its a low and high side driver with 500V isolation to the high side. Solves the drive offset problem.
          Then you can roll your own controller with a DSP or standard micro, using parts of your choice.

          Line isolation of the sped control is your problem, but opto parts do that, so the PWM could be all on an isolated +-15 and 5V.

          Fun project, at least for EE types.


          • #6
            Oso, thanks for the reference. Fairchild has a IGBT H-bridge module with all the drivers and isolation built-in. The only problem is that it's $$$. The more I think about it the more I'm drawn to building one myself. However it faces the same danger as so many of my other proejects. It suffers a sudden death once all the parts have been purchased.