Quote
yoyobob19
The existing Pololu boards found in common RepRap 3D printers are at their limits when driving the 2 Nema17 z-axis stepper motors in parallel. Continuous z-axis movement can cause the board to overheat. These boards hardly drive stepper motors bigger than a Nema17. To avoid overheating or to drive larger motors a more powerful driver board is needed.
Connecting two Z motors in parallel and driving them from a single Pololu driver is a very stupid thing to do. People only do it because they don't know any better and RAMPS makes it easy to do. The types of motor generally used in RepRaps should be connected in series instead. The better designs do this, the cheap kits don't.
But you are right in that Pololu drivers don't handle bigger motors well, so more powerful drivers are needed in those applications.