m_c,

Looking at the datasheet, the smallest (0.9 and 2.0Nm) motors are 3 phase but the 3, 4 and 8Nm are 2 phase.

It looks like I was drafting my reply when you replied. I think where my misunderstanding lies and what I'm trying to get at is: if it takes a finite time to 'fill' and then 'empty' the coil and if this is the minimum time for one step and gives us a speed limit, how do the stepper drivers get around it to drive the motors faster?

Is it (or related to) the morphing you mention: perhaps driving the stepper more like an AC motor than a stepper motor at higher speeds with a sinusoidal wave and not quite filling/emptying the coil?

I'm struggling with it because I'd like a performance increase over 700RPM motor-speed rapids but don't want to move away from 23's if the drop in RPM is as severe as the calculations above would suggest (and so maybe I have to accept this as the limit of direct-coupled motors and ballscrews).