Re: stepper reluctance

Bob,

I think you are confusing apples with oranges here. A high frequency
switching drive simply regulates motor phase current at low to
moderate speeds. As speed increases, so does inductive reactance
until it becomes large enough to limit motor current. At that point a
switching drive ceases switching and simply applies the full power
supply voltage across the winding for the entire period, (2 full
steps).

The switching algorithm goes like this:

(1) Apply full voltage to the winding. Current builds up at the rate
of Volts / henries (V/L) amps per second.

(2) Has the current built up to the command reference value? If no,
repeat (1) and (2); if yes, goto (3)

(3) Apply reverse voltage on the motor. Current now decays at a V/L
rate.

(4) Has the switching time period expired (typically 20 to 25uS)? If
no, repeat (3) and (4); if yes, goto (1).

As you can see, at high speeds you would never get to (3) and (4)
before it was time to reverse current in the winding because the step
sequence demands it.

There is no down side to it. The up side is a tremendous heat saving
at low speeds when compared to a linear amplifier drive.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "echnidna" <echnidna@y...> wrote:
> Hi group,
> given that motor reluctance governs maximum motor speed.
> what is the advantage of chopper drive at high frequency which
means higher impedance?
> wouldnt a simple linear current control be more effective by
avoiding very high chopper frequencies?
>
> Regards
> bob Thomas