Re: [CAD_CAM_EDM_DRO] Ordinary AC motors & position control

Ac motors can be run by a vector drive as long as there is encoder
feedback. The only problem is that they generate heat when trying to hold
position with a load on them.

Harko

On 6/19/07, Jon Elson <elson@...> wrote:
>
> maxnc15 wrote:
> > I'm asking for some basic knowledge. I've seen some motors that
> > normally spin free lock up pretty solid on a position by use of
> > a "dither" frequency to equalize both directions of rotation. Slight
> > adjustments can move the motor one way or the other. Now can this
> > principle be applied to ordinary AC motors?
> Yes. A flux-vector drive works this way. Of course, it needs
> an encoder to sense motor position.
> >
> > If not, is there a way to easily convert an AC motor to DC?
> Yes, but I don't know about "easy". First, cut the rotor
> shorting bars off the rotor, insulate and insert coils in the
> now empty slots. Build a commutator out of intricately shaped
> pieces of copper, and attach the wires from the coils to them.
> Build a structure to hold brushes against the commutator. You
> may have to rewire the stator coils, too, to get the appropriate
> field flux. Easy? No, hardly. Possible? Only if you want to
> prove a point, certainly not practical.
>
> > The reason I ask is new AC motors can be inexpensive relative to servo
> > motors of similar power. In my application I'm more tolerant of
> > backlash than expense.
> Price out a flux vector drive, and you will "easily" see that
> the motor cost is only part of the equation. The drive needs a
> very fancy digital signal processor chip to do the flux vector
> calculations, plus ADCs and a lot of additional hardware not
> needed for a simple VFD. The cheapest thing is a DC brush motor
> and a Gecko or other brush servo drive. Brushless motors are
> coming down in price, and I have seen a number of
> moderately-priced ones on eBay recently. (Missed all of them,
> darn those snipers that get them for just a buck above my bid!)
>
> Jon
>
>


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