Stepper motor 101 was Re: Are steppers considered low frequency motors?

Rick,

As with the previous two replies; I'm not sure what you're asking...

But I can say that if you hook up a stepper to a 60 cycles A/C line--
at reduced voltage!--and use a capacitor to offset the phase of one
coil in relation to the other, you will have an AC syncronous motor
turning at 72 rpm's (assuming a typical 1.8° motor; 200 SPR)

Superior Electric sold them marked this way for many years.

Is this perhaps what you're asking? (Can the motor "rated" at 72RPM
go faster than 72 RPM?)

If so, the answer is yes. And microstepping is not needed; but it may
be useful.

A stepper motor is as Mariss said, an AC syncronous motor. When we
use if for stepping; we drive it with two "trains" of simulated sine
waves, offset by 90 degrees.

At one end of the scale these sine waves are SO poorly approximated
that they are not sine waves at all; but rather square waves that go
full-on to full-off. This gives our little AC synchronous motor
its "full step" rotation and is the simplest driver for us to make.

Because of all the extra harmonics(as Steve mentioned) this is a
very "jumpy" motor, and cannot go as fast without locking up. (Jumpy
means resonant. Because the sine wave approximation is so poor, there
is a lot of "extra" energy being wasted in the motor as it tries to
follow the huge steps--changes in current.)

In half-stepping we do a little better job of approximating the sine
wave, because we turn one OR two coils on at a time. This puts
a "step" in the simulated sine wave, so it looks a little more like a
complete sine wave. (Very little actually in appearance; but quite a
bit better in performance)

Microstepping is simply adding more and more "levels" of definition
to the simulated sine waves that all stepper drivers use to create
discrete steps of movement from these motors. And as the simulated
sine wave gets better; so does motor performance at slow speeds.

If you look at the Allegro A3977 data sheet, you can see graphs of
the various "simulated" sine waves that this chip provides.

Now simulating the SHAPE of the sine wave is one thing; but we also
can change the FREQUENCY of the simulated sine wave. By doing this we
can move the motor faster or slower than the 72RPM speed we get when
it has only the 60hz input first described above.

It's a synchronous motor. Which means it will "go" at a speed that is
relative to the FREQUENCY of the sine--or simulated sine--waves fed
to it.
The closer to a "real" sine wave we get, the smoother the motor runs.
Which is why microstepping is useful at slower speeds.

Side note: The relationship between input frequency and final speed
is determined by the motor construction. Our "typical" hybrid 1.8°
motors have a 48/50 pole construction. (so they go 72 RPM with a 60Hz
input frequency.)

At higher speeds, motor inductance (the motor coil's resistance to
change) turns the sine wave into something that looks more like a
triangle, no matter what "shape" we feed it with. So some advanced
drivers will change back to the square waves. Others will continue
sending the simularted sine wave. Either is turned into a triangle by
the motor inductance, and the motor still "goes" in relation to the
frequency of the two waves it is being "fed"...

Hope this helps,

Ballendo


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "rllalonde2000"
<alalonde@...> wrote:

>
> To anyone who may know: Do stepper motors become medium frequency
> motors when microstepping at higher pulse rates?
>
> Put another way, does applying high stepping rates have any bearing

on

> how the motor itself could be looked at in terms of its frequency
> rating?
>
> Thanks in advance,
>
> Rick L.
>