Quote Originally Posted by Neale View Post
Not quite, Jazz - the problem is reading the Gecko manual. Their kit of current-set resistors includes the 3.48K resistor specifically for this drive. Why they use this daft value is anyone's guess, though. But then, Gecko are also responsible for the widely-quoted max voltage based on motor inductance formula which at best leads people astray - or perhaps it suits their drivers better? I also run Nema23 at 65V via a Chinese digital drive and I'm very happy with it. And a linear/toroidal PSU... And I have complete freedom to choose my own micro-step ratios and motor current at the flick of a DIP switch. Americans buy Gecko because they know that "American (products) are great". The rest of the world is forced to buy on price/performance/reliability alone. Oh, pity the rest of the world.
I'm sure they only supply that resistor to minimise people asking how they set 3.5A if they've not got a resistor.

The max voltage based on inductance is a valid figure. You can run higher voltage, however unless you're running the stepper motors at speed, it has no benefit and will do no harm.
Any problem will arise when the motor is spinning fast enough that back emf increases the required voltage to drive the set motor current through the motor windings, to the point where you get diminishing returns for the voltage applied.
The key thing to remember is just because you have a 65V supply, is the motor will rarely see that applied. While stationery the motor should only see the rated stepper motor coil voltage, as there is no back emf to require a higher voltage to get the rated current flowing through the motor windings. It's only as the motor starts turning and back emf increases, that the required voltage increases.
Combine that with the laws of physics, where by power = volts time current, and with a stepper using a couple amps running flat out at maximum voltage, will be getting over one hundred watts of power applied. How long do you think your Nema23 could handle that before overheating and demagnetising?

As I said, that Gecko guide is pretty good. I can remember Marris's posts over on CNC zone where he explained a lot about stepper motors, drives, and the theory behind the figures. He didn't build a very successful business by plucking figures out of no where. Which is essentially what you've done by saying your 65V supply hasn't given you any problems, despite you not understanding the theory behind why it probably really isn't a good idea.