Manufacturer provided torque/speed curves are often slightly misleading because many are given for “rated” voltage. That is the voltage that can be directly applied to windings w/o any current limiting. With drivers, steppers usually run at much higher voltages so rated voltage graphs are not super relevant. E.g. 17HS19-2004S1 has winding resistance of 1.4 ohm and rated current of 2A which would give us a rated voltage of a whopping 2.8V.
RepRap Firmware’s EMF calculator is the easiest way to compute the effect of supply voltage, I can’t recommend it enough. It will tell you what voltage is needed for what speed. It even has a preset for 17HS19-2004S
So given this stepper, max current (2A) and the standard lead screws we use on MPCNC, RepRap calculator claims the torque “starts to drop” at
- 28mm/s @ 12V
- 59mm/sec @ 24V
- 100mm/sec @ 40V
Pretty steep differences I’d say.
Probably the best article I’ve seen so far that takes a shot at explaining the theory of elevated voltage is this one from servotecnica. The key takeway from a graph in that article is that even a fairly modest increase from 24V to 35V can significantly “flatten” the curve.
Ok, this is theory, what about actual measurements?
This guy reports torque increase of almost 40% at 500rpm by increasing voltage from 24V to 50V. This of-course applies to his specific steppers.
I also took another look at manuf. information and I’m glad that some vendors are starting to publish torque curves at higher voltages. Specifically our beloved Stepper Online’s 17HS19-2004S1 has published a torque curve at 24V assuming a current limiting (chopping) driver of-course.
This is great because it tells us that given a 24V supply, the motor will not lose much torque all the way to 450 rpm. Given I calculated this correctly, it should be about 60 mm/sec for Z given the “standard” lead screws we use on MPCNCs - exactly as the RRF calculator predicts.
Being able to refer to this graph may be considered an excuse to upgrade to 24V
I also recall the original author of “smart stepper” boards (the design that S42 family is based on) mr. MisfitTech claiming speeds of around 700rpm.
So where are the limits? Well, mechanical limits aside, windings will be ruined if driven over their dielectric strength which is usually around 500V. However stepper drivers and control boards that can do voltages higher than 42V are not that common and truth to be told, I’d be very uncomfortable with driving my steppers at more than 50V anyway, the wires are just not insulated well enough.
EDIT: Re-ran RRF calculator at 2A to make results comparable to manuf. published curve at 2A