Re: Differences between AC & DC brushless

Jon, Arvid, from what I know about motors, there are AC & DC
brushless motors and both have permanent magnets on the rotor. From
what I have heard, they are wound different. The DC brushless gives a
trapezoidal back EMF and the AC gives a sinusoidal back EMF. The DC
brushless is commutated by powering only two of the three leads at a
time with a PWM signal that averages to the desired voltage for the
commutation rate. The undriven lead is often used for feedback in
sensorless applications. The AC is commutated by by driving all three
leads with PWM signals that average to 3 phase sinusoidal voltages of
the desired frequency. I'm not sure what the performance diferences
are, but the drive types are not compatible.

~petev

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "bitnick78"
<arvidpublic@h...> wrote:

> --- 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