Re: Differences between AC & DC brushless

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

>
>
> 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

Let's limit our discussion to only 3-ph PM motors.

Here there are still motors called "3-ph AC" and "Brushless DC". But
what you're saying is that, as long as they have the same type of
encoders and current and voltage ratings, they are interchangeable
from the driver's point of view, no matter if it uses trapezoidal or
sinusoidal commutation?

This is interesting bescause there are quite a few high performance
used or surplus 3-ph PM motors out there, but very few drivers that a
hobbyist can afford (The Rutex is the only one that comes to mind).
So if one could just buy some 3-ph PM motors and hook up to some
Rutex drivers, this could be an interesting high-performance solution
for a hobby machine or retrofit. One would just have to make sure the
commutation feedback were compatible.

Arvid