Re: [CAD_CAM_EDM_DRO] AC Servo Motors???

jmkasunich wrote:

> The dumpster at work has quite a few AC Servo motors in it this
> week. Are there any cheap (Gecko price range) ways to use these
> things, or do you need the matched $$$ drive unit? The motors
> are Yaskawa (sp?) and other brands. I didn't write down all the
> data, but several were labeled 29 kg-cm continuous, which works
> out to about 400 oz-in. I imagine peak torque is quite a bit
> higher. The motors are significantly bigger than NEMA 34
> steppers. About 4-5" square by 6-8" long. They have attached
> encoders. Some of them are labeled as absolute encoders. I
> have reason to believe they work fine, they're simply longer
> needed. The trash-picker in me hates to see them go out.
> But if I can't use them, or find someone else who can, then
> they really are junk. They are too bulky/heavy to haul home
> on a whim.

Gasp! These are worth REAL money! People are selling them on
eBay. The problem with 'AC' servos is that there are no detailed
standards. So, if one company's amp is expecting, say, 1000, 2000
or 4000 cycle/rev encoders, and your motor has a 1024 cycle/rev
encoder, it just won't work. Now, a true AC servo is different from
a brushless 'DC' servo, in that it doesn't have permanent magnet poles
built at fixed locations on the rotor. So, the brushless DC servo either
has a separate set of 3 commutating sensors (either optical in the
encoder or hall-effect) built to be at the right relationship to the
stator and rotor poles, or it counts from the encoder and index pulse.

But, an AC servo is really just an induction motor, and so the rotor poles
are created by the stator currents. So, it should just have an encoder
with index.

These motors should work fine with a fully-programmable VFD, or
with most flux-vector servo drives. The only really difficult incompatibilies
would be a servo drive that can't accept the encoder count on that motor,
and a drive that can't be programmed for that particular motor's
V/F curve. Now, many AC servo drives just have programming
settings for their own motors. If you find compatible motors, then
all you might have to deal with is making the right connections.
And, if you used a generalized AC servo drive (rather than A-B,
Yaskawa, Mistubishi, etc.) it should be programmable for most of
the reasonable range of values.

I think Copley, Servo Dynamics and such outfits are bringing out lower-priced
AC servo drives. Maybe not quite down to Gecko prices yet, but getting
closer. Also, these drived DO turn up on eBay from time to time.
There are also guys selling specifically Yaskawa drives on eBay, but they
seem to want real money for them.

Jon