How Strict is the Max Vsupply Calculation for Steppers?

[Muzzer]

The max motor temperature is generally limited by the varnish on the winding wires, along with the other insulating tapes and sleeving used in their construction. These are generally either Class B (130C max) or Class F (155C). That's for the hottest spot within the whole motor. You'd be doing well to get them that hot and the corresponding external case temp of the motor would likely be over 100C, even for the lower Class B rating. The magnets in the rotor shouldn't be self heating and their demagnetisation ("Curie") temperature is going to be over 300C.

In terms of the max voltage, the limitations will be partly the motor insulation (as above) which isn't going to suddenly break down if you increase the supply by a few tens of volts; the max voltage rating of the power semiconductors (FETs in this case); the max voltage rating of the controller IC; and the max voltage rating of the passive components, most notably the electrolytic caps on the supply rails. You'd need to look at the internal design of the controller (driver) to know what the weakest links are - best to simply respect the limits suggested by the manufacturer though. However, sudden deceleration of the load by the motor can cause a regenerative transient surge back into the supply, so it's sensible to leave a decent safety margin on the steady state supply voltage.

Apart from overheating the copper windings, exceeding the current rating of the motor could lead to saturation of the magnetic steel laminations. This would cause a rapid and uncontrollable rise in current which could stress or blow the driver. This is less likely than overheating but can be done.