Build Log: Servo Drive and VFD with vector control

[Jonathan]

Re. stepper motor use.
If you are splitting the power section onto a separate board maybe you could use a common DC link psu to power several controllers. I suppose this is what the current stepper drivers do.

Currently the design has a rectifier so it accept an AC input, but I also put connection to the DC-link so you can power from a DC source, add more capacitance or share between drives. One interesting consequence of connecting the DC links in parallel is energy can transfer from one motor (regenerating) to another, which is a regular occurrence on a CNC machine as often when one axis is decelerating another will be accelerating.

If instead of adding an extra leg I connect the stepper motor to the 3-leg inverter, as mentioned in my previous post, the only obvious disadvantage is you end up with 'poor' utilization of the DC-bus voltage, V. Depending on the output voltage vector angle, you either get V or V/sqrt(2). See this paper, fig 4. I don't think that's an issue though, as we can simply supply the motor from a higher voltage to compensate. The current controller bandwidth will have to be a little higher to compensate and the insulation resistance of the motor also must be greater, but I don't think the difference in voltage is a big enough factor to matter in either case. Maybe there's something I've missed, as it seems strange that all commercial drives seem to use 8 mosfets when this method only needs 6.

Also if you fit encoders maybe it would be worth including a system for homing a master /slave without external intervention.

Yes, why not ... it's just software. When I've got the code tidy enough I'll release it as open source and hardware, then people can add things like that if they like. Also the encoder I'm currently using is absolute, so homing is trivial.

I've started soldering five of the PCBs. It's very time consuming though, maybe I should make a pick and place machine next.