My apologies - shows how out of date I am.
Regards Peter
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----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@...>
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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