AC Servo Motor control

[teilhardo]

Hi Everyone,
I recently came across some fairly big AC servo motors (with encoders) for less than $25 made by oriental motor. What is the difference in terms of cost between DC and AC servo systems? Will an AC motor have the same torque as a DC one of comparable size?
Thanks,
Tei

[arvidb]

The good news first, then the bad news:

An AC motor will have significantly more torque (or at least power, which translate to either more speed or more torque or both) than a DC motor of the same size, and no brushes to wear out, either. The big, expensive machines use AC motors and drives.

The bad news:

A driver for an AC servo motor is much more complicated than a DC servo motor drive. There's almost no AC motor drives for the hobbyist. I think rutex makes one (www.rutex.com/us) that might work.

If you are serious on using AC servos and don't want to pay $1000's for a single servo driver, be prepared to do a lot of reading of motor controller techniques, manufacturer data sheets, application notes and the like and then buy from Ebay.

Or maybe you are lucky and the Rutex drive will work for you .

Please post your success/failure stories here, it would be interesting to me, at least.

Arvid

[teilhardo]

Trust me, I don't have the $$ to buy the controller but I have the surplus store that has the AC servo motors pretty cheap. Last time I looked it was $25 for a 5" long by 3" wide servo motor with encoder. Is there any way to make a homemade controller for it simply?
Thanks for the advice,
Tei

[arvidb]

Simply put: no.

Do a google search on "Field Oriented Control" and you'll have a new project that's at least as difficult as the DIY CNC machine itself, involving power devices and DSP programming.

I wanted to do a project like this myself, but after reading up on it I realised I *have* to finish a few of my already started projects first, or I'll get nowhere

Arvid

[MaX-MoD]

Hi everyone!
I'm bringing this old thread back to life cuz AC servos are a cheap and available on surplus stores for us hobbyists, but unfortunately we don't have the power to drive them!
50$ a 1KW AC servo with 2000ppr encoder seems sexy, but what to control it?
300, 600 1K+$ and more, controllers can be VERY expensive for that type of servos.

but AC are not -technically- impossible to drive for a hobbyist.

the major default of induction motors is that torque is not linear to current and current is function of the load, speed of the rotor and 'electrical speed' of the stator's poles voltage.
but it can be controlled -or at least approximated- by a cheap and available dsPIC (or any µC that has */ periph and about 20-80MIPS speed) like 33F family.

Well, I am actually making one.
AC servos is a type of servos I'll include in my servo controller project.

I'll tell you more about this project later, when the dsPIC will be actually doing something, as I am still beiginning with my tiny 33F12jmc202.