Re: [CAD_CAM_EDM_DRO] Ordinary AC motors & position control

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