best stepper driver???????

[Jonathan]

However I have had clamp ammeters fitted to power supply leads and running a system under rapids and loads i have never seen these figures, they have always been far less.

In fact in many cases the figures have been very small, far smaller than I would have imagined so the 50% is a far more acceptable figure.

Part of the reason for your low readings is the sample frequency of the clamp meters is low relative to the frequency of the current the motors are drawing, so it tends to miss the peaks and generally not be reliable, especially if the meter is not true-rms. An oscilloscope reveals a lot more.

However, it is still true to say the rms current, and therefore power drawn, is substantially lower than the 2/3rds 'rule' implies. It's easy to show why - just look at the motor ratings to work out the power. For example the common 3Nm motors require 4.2A and 2.73V. Power is current multiplied by voltage, so P=4.2*2.73=11.466W. That's per phase though, and it's a two phase motor, so 2 times that which gives 23W. You're driving it from about 70V, so the current is pulsed such that the mean power does not exceed this rated power. Same formula again, so the mean current is I=P/V=23/70=0.33A.

That's why you see a low current (about 0.33A) per motor, not 4.2A or some significant fraction of that. The reason it helps to have the power supply rated for quite a lot more (i.e. 2/3*4.2) than this anyway is that the peak currents can be substantially greater, so you need a transformer capable of delivering that. That's not the only way to solve it though.