Re: [CAD_CAM_EDM_DRO] Re: AC drives

My apologies - shows how out of date I am.

Regards Peter

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, November 05, 2002 6:01 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: AC drives


>
>
> 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
>
>
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