Quote Originally Posted by m_c View Post
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.
Moray what you say is correct upto to point.? Can't argue with Laws of Pyhisics and like You I've got Massive respect for Marris.

However what your saying about 65V Killing steppers isn't really valid because takes many years to have this affect and the fact they are cheap as chips make it none starter. I've been running my machine for nearly 8yrs with same motors running 70+Vdc and built many many machines using same setup without any issues. In fact never had single stepper fail or go weak.

Now where it gets fuzzy is when to use the higher voltage.? For milling machine or lathe then it's wasted and like you say not benificial. However for Router then the extra voltage makes all the difference. . . Horses for courses.!!