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Hi all,

Most of people use either steppingmotors or DC-servo, I noticed that
the big manufactures (fadal, haas, dmg etc.) all use AC-servo. The
benefits are more dynamic, more compact, higher peak torque. Are
there any step and direction AC-drives out there, or is the price too
high.
AC-servo motors are synchroonmotors while a regular AC-motor (like
the spindle motor) is asynchronic, right? Could it be possible to use
a regular AC-motor as a drive motor, these motor are very cheap and
deliver a lot of power.
Just an idea,

Hugo

hugo_cnc wrote:

Hi all,

Most of people use either steppingmotors or DC-servo, I noticed that the big manufactures (fadal, haas, dmg etc.) all use AC-servo. The benefits are more dynamic, more compact, higher peak torque. Are there any step and direction AC-drives out there, or is the price too high. AC-servo motors are synchroonmotors while a regular AC-motor (like the spindle motor) is asynchronic, right? Could it be possible to use a regular AC-motor as a drive motor, these motor are very cheap and deliver a lot of power.

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. The major differences are in cooling provisions and
to reduce rotor inertia.

If you had a flexible (ie. programmable) AC servo drive, you could connect a standard induction
3-phase motor. It may not perform quite as well as a proper servo motor, but probably would
do fine. You'd need to pull out the internal fan and supply a "boxer" fan to provide cooling.
A long, narrow motor would probably be better for the inertia situation. You'd need to provide
an encoder with the exact characteristics needed for the servo drive. Most of them just need an
incremental encoder with index, though.

Jon

--- 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. 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.

How would you describe that type of motor?

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.

Regards Peter

--- In CAD_CAM_EDM_DRO@y..., "Peter Seddon" <peter@s...> wrote:

AC servo and DC brushless motors are I believe one and the same

for all

intents and purposes.

That's the only conclusion I can come to.

With permanent magnets on the rotor (ie your ac servo motor) the
rotor flux is permanently established.

Yes, and being smaller diamter, lighter and longer means it's much
lower inertia. Also means that if you drive the input shaft with no
power to the stator, then it generates, so making it easy to adjust
the commutation timing without phase shift inducing currents and
losses to allow for.

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

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

Thanks all,

It's a little clearer now, the conversion of a vfd to flex-drive
sounds interresting, hope to try it some day.
Thanks

Hugo

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

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

I think they're referred to as 'written pole' motors.
You're virtually 'writing' every pole with a pulse of
current.

RayHex

----------

From: gittt2000 <gitlang@...>



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. 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.

How would you describe that type of motor?

hugo_cnc wrote:

Thanks all,

It's a little clearer now, the conversion of a vfd to flex-drive sounds interresting, hope to try it some day.

N

No, I wasn't in any way suggesting an end user make the conversion! I was saying that
for the manufacturer, there really isn't that much difference between a plain VFD and
a flux-vector servo drive. The amount of time needed to do such a thing really makes it
very impractical for an individual to try it.

Jon

My apologies - shows how out of date I am.

Regards Peter

Raymond Heckert wrote:

I think they're referred to as 'written pole' motors. You're virtually 'writing' every pole with a pulse of
current.

This is yet another new type of motor. the idea is it has a variable number
of rotor poles (and therefore, stator poles, too).

Jon