run 6V DC motor @ 12V w/ PWM ?

PWM frequency should be high enough(50khz-1mhz), AND DUTYCYCLE < 50% , the induction of the motor should provide enough smoothing.

Depends on how hot it gets. How hot it gets depends on load, time at load aka duty cycle, ambient temp, etc. etc.

If your drive circuit limits the current to the rated current of the motor, you'll be fine, except you'll be dissipating some power in your drive by regulating the current.

I'd have qualms about hitting a 6V motor with a 12V supply, even if you're absolutely sure that you'll never go over 50% duty cycle - I'd worry about the instantaneous currents.

If you know the spec - the motor is 6V at X amps, that's 6/X ohms - put a 6/X ohm power resistor in series, and you'll be safe for sure, but you'll waste a lot of power.

The last time I did a PWM controller, I used a current sink at a regulated current and a supply that could be anywhere from 24-36V; I had to heatsink the output Xsistor because it was dissipating some watts.

Good Luck! Rich

depends on the mechanical load, the capabiites of the commutator, etc.

PWM: 50% 12V,50% shorted should, give similar mechanical perfromance to a 6V supply, but the load on the commutator will be higher etc...

the easiest way to kill a motors is too much heat in the windings due to much current. this can be caused by too much mechanical load, over or under voltage etc.

The frequency range can go lower, maybe 500 Hz. You can easily add a series choke (and maybe a capacitor to shunt commutator noise) or, if you're really energetic, can use switchmode power supply control circuits with current-overload-sense features. The important thing, is that there be an absolute current limit (because at 12V, the motor winding resistance no longer performs that function).