Re: AC drives

Thanks all,

It's a little clearer now, the conversion of a vfd to flex-drive
sounds interresting, hope to try it some day.
Thanks

Hugo

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>
>
> Peter Seddon wrote:
>
> >AC servo and DC brushless motors are I believe one and the same
for all
> >intents and purposes.
> >
> >An induction motor cannot be described as a servo motor as its low
speed
> >performance is generally a bit rough. To produce torque you have
to induce
> >current in the rotor that at very low frequencies and speeds is
not very
> >efficient. With permanent magnets on the rotor (ie your ac servo
motor) the
> >rotor flux is permanently established.
> >
> >
> No, the newest servo drives are, indeed, using AC induction
motors! The
> reason is that an induction
> motor can run much hotter than the permanent magnets could stand.
They
> are also cheaper, and when
> rotor inertia is a factor, the rotor core can be made much lighter
than
> a permanent magnet rotor.
>
> Yes, there are tradeoffs, but a motor designed from the ground up
as a
> servo motor can perform
> much better than a PM brushless motor. I don't know how much worse
a
> commercial 60 Hz
> induction motor is, but clearly, they make little attempt to keep
the
> rotor inertia down.
>
> AC flux-vector drives are needed to control the rotor
magnetization, so
> the motor can always be
> accelerated instantly when needed. DSP processor chips make this
> possible at very reasonable
> cost. Basically, you replace the cheap CPU in a standard VFD with
a $30
> DSP, add an encoder
> counter, and you have an AC flux-vector drive.
>
> Note that ANY servo motor, DC brush, brushless or AC, when
producing
> torque at zero speed,
> has an efficiency of zero!
>
> Jon