Confirming PSU spec for steppers.

[Jonathan]

Hi Jonathan, thanks for chipping in again. I'm still aiming to finish this before you finish at Uni so I'll need you to work towards a PhD, should give me enough time.

That's the plan, however at work they seem to want me to do the PhD whilst working them, in which case you've only got a year left!

My question was whether it's better to run the steppers at their max or give them a bit of a margin. If your answer still applies (which logic suggests it does) then it seems reasonable to set the drivers for a little under 4.2A.

Yes my answer does still apply. As you increase the current in the motor windings, the copper losses increase by the square of the current, so the motor will get hotter. The degradation of the insulation on the motor windings is dependent on the temperature, as with most chemical reactions, so according to the Arrhenius equation we can say (very approximately) that if the motors run 10° hotter, their lifetime will be halved (assuming they fail due to insulation breakdown). This sounds bad, but we have to put it in perspective - if the motors last 20 years then halving this to 10 years probably isn't a big deal.
I'd set the driver current as near as you can to the motor rating without exceeding it. You might be able to go lower, but I think it's better to ensure the machine will not stall by having a good safety margin, rather than worrying about the lifetime of the motors.

Going back to me previous point about the power supply, I just ran the simulation again but this time with both configurations. The first graph is with the windings connected in parallel on the DC side and the second graph has them connected in parallel before the rectifier:



I realise that there's not much point discussing this further, but it's interesting to note that the waveforms do look slightly better for the DC coupled version. There's still not much in it.