Therefore modern boards are running on 24V.
No, that’s not why modern boards are running 24V - they do it to support 200mm/s and faster X/Y movements where the Back EMF can reache 8V or more (with the rule of thumb being that you should provide a power supply with 3x the maximum Back EMF expected). If you are using NEMA 17s, keep under 150mm/s and don’t have any significant binding in your system, you can run at 12V very comfortably. In this situation, 24V runs out of steam at speeds of 500mm/s or more.
I can go into the science of this or you can look it up but the reason why 24V is most popular now is because it better supports high speed operation and, in most people’s minds, 24V has to be better than 12V.
I had never heard of wiring steppers in series until this discussion and I did a bit of research on it.
First off, there doesn’t seem to be any authoritative references on how to wire multiple steppers for different situations. There was debate about the issue in the community starting around 2006 and continued until 2014 or so. I couldn’t find any research papers or driver/stepper manufacturer’s papers on the issue.
The general consensus is that wiring stepper motors in series is best for low speed operation where lots of torque is required and the Back EMF the driver experiences (which will be the sum of the Back EMF of the two motors) will not be problematic. Parallel wiring is best for higher speed applications where low-end torque is not required (and the Back EMF will be the same as for one motor).
Series wiring for Z Axis steppers is probably optimal but it’s something of an odd duck so, unless you know your board has the Z Axis stepper connectors wired this way and have jumpered any unused connectors, you’re going to be wondering why your Z Axis stepper isn’t moving like what is seen here.