Re: 8-wire stepper: series is 1/2 the amps.Strange but True

First off, it's pi/4 or .785 (instead of 71%) of the area under the
curve for a sine function compared to a square wave.

To compensate for that a good drive changes its reference from sine
to square wave (full-step sequence) once the speed is high enough (4
revs/sec) and the benefits of microstepping has disappeared.

The second part: Torque begins to "drop off" past the speed where
inductive reactance, not drive set current current begins to limit
phase current.

Inductive reactance (Xl) = 2*pi*F*L
Inductive current (torque) = V / Xl

Solving for F: F = V / 2pi*I*L

Because the step rate is 4 times F (coil frequency),

Full steps per sec = 4*V/2pi*I*L

This is agreeably close with dyno data taken from "square" step
motors. See the file "MS234A24V.wmf" in the files section.

Mariss






--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp" <xylotex@h...>
wrote:

> Hi Dave,
>
> > so, the question how does one determine where the torque starts

to

> > drop off when on is not using a single coil (on an 8 wire) but is
> > using less than nameplate on a series wired motor ?
> >
> > Dave
>
> I think torque will start to drop off when there isn't enough

time

> to charge the coil to required amperage before the next step pulse
> comes in.
>
> A microstepping drive, when set to full step mode, will generally
> power each coil at about 71% of rated power. For full stepping you
> would get the following (4 steps shown):
> Ph.A Ph.B
> +71% +71%
> +71% -71%
> -71% -71%
> -71% +71%
> and so on. As you can see, at each step, one of the coils is
> reversing it's polarity. You can figure out how long it will take
> the coil to go from + to - (or - to +).
>
> I think the formula is going to be something like:
> 1 / (((Coil Amp *.71) / (Supply Voltage / Inductance in H))) * 2)
>
> So a 4 amp motor with 4mH inductance driven with 40V would be:
> 1/(((4 amp motor *.71) / (40V/.004H)) * 2)
> 1/((2.84/10000)*2)
> 1/(.000284 * 2)
> 1/.000568
> 1760 steps per second
> before it starts loosing torque.
>
> I'm sure one of the other members out there can (and hope they

will)

> scrutinize this and let us know if this turns out to be to case,
> close, or way off.
>
> Jeff
> http://www.xylotex.com