Re: [CAD_CAM_EDM_DRO] Differences between AC & DC brushless

bitnick78 wrote:

>Except for encoder type, are there really any differences between a
>DC brushless motor and an AC brushless motor? (I'm talking about
>servo motors here, not motors driven by a VFD or the mains.)
>
>If I have understood things correctly, the difference is really in
>the way the motor is driven -
>
>* it's a DC brushless motor if the commutation is trapezoidal (which
>require hall effect sensors or other low-resolution absolute
>feedback).
>
>* it's an AC (brushless) motor if the commutation is sinusoidal
>(which require some kind of higher-resolution absolute or semi-
>absolute feedback device).
>
>If this is correct, then I wonder what is the result of using the
>different drive methods? How much "better", and in what ways, is the
>AC approach?
>
>

I'm not sure this is correct. The so-called DC brushless motor is truly
a synchronous AC motor, with permanent magnets in the rotor.

The so-called AC brushless motor is often an induction motor, with no fixed
pole locations on the rotor. But, it could also be the exact same as a DC
brushless motor. There is a move afoot in the last couple of years to
correct this misuse of motor names. To be strictly correct, the only
motor that can be called DC brushless are the ones on computer fans and
the like, where the commutation function is built INTO the motor, and
it has only two wires coming out. The other general types are (1) permanent
magnet brushless motors, where the rotor poles are always in the exact
same spot on the rotor, hence the encoder provides commutation info
to the drive. And, (2), induction motors which have poles developed by
magnetic induction from stator current, and need AC flux-vector drives
to control the development of the induced poles. There is quite a bit more
difference here than just the encoders.

There are some PM brushless motors that use ordinary A/B/Z encoders,
without specific outputs for commutation. they depend on the brushless
motor drive to figure out when to switch windings by counting encoder
pulses. But, this is still what used to be called a DC brushless motor.

As far as I know, sinusoidal or trapezoidal drive schemes can be used
on the same motors, although the windings of the motors may be
optimized for smoothest torque with one type of drive.

There are also some new motor types such as the written pole motor.
I think these are used more for spindle drives that need to put out near
rated HP over an extremely wide range of speeds.

Jon