Re: [CAD_CAM_EDM_DRO] Re: AC drives

gittt2000 wrote:

>--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>
>
>
>>DC Brushless motors are really misnamed. THEY are truly
>>
>>
>synchronous
>
>
>>motors. AC servo motors
>>are VERY similar to ordinary AC induction motors, and they ARE
>>asynchronous, as they have to be
>>for the induction principle to excite the rotor.
>>
>>
>
>I've been confused between DC Brushless and AC synchronous Servo
>motors and thought I'd sorted this out, but now I'm confused again.
>
>The MAC AC Servo motors by Indramat (now Bosch Rexroth) are described
>as synchronous but they are not similar to ordinary ac induction
>motors. They have a wound stator, electronic commutation, and a rotor
>comprised of 6 (or another multiple of 3 presumably) permanent
>magnets, rather than the squirrel cage of conductors which form the
>normal induction motor rotor.
>

Well, this is the problem when an industry uses incorrect terminology
for so long that nobody
recognizes it as being wrong. When somebody breaks the 'rules' and uses
CORRECT terminology,
people are confused! Bosch is using the correct terminology. As you
describe their motors,
they are truly synchronous permanent magnet motors, which most people
call "DC brushless".

And, now, I've caused confusion by overgeneralizing. If it has
permanent magnets or some other
means of being synchronous, then it is not an induction motor. If it is
asynchronous, and requires
slip between the stator field and rotor to magnetize the rotor, then it
is obviously an induction
motor.

> As far as I can see there is no
>induction involved - the stator produces a rotating magnetic field
>and the magnetic rotor follows it. What makes it synchronous is the
>commutation which signals the servo drive to supply the correctly
>phased stator drive.
>
>

No, what makes it synchronous, BY DEFINITION, is that the stator and
rotor fields follow each
other at all times. An induction motor always has some 'slip' between
these fields, and that is
what magnetizes the rotor.

The commutation is required to make the stator poles rotate. Very
similar schemes are used in
BOTH synchronous and asynchronous motors. The difference is that in an
induction motor, the
slip has to be accounted for. In a torquing application, where the
motor is delivering torque at
zero speed, a permanent magnet motor would have the stator fields held
constant. But, an
induction motor in the same situation would require the fields to
constantly rotate very
slowly, about 5 - 25 RPM.

Jon