This is a question from one of the sheets which my physics teacher gave me:A student is experimenting with a 6V DC motor. He connects it accidentally to a 12V 'AC' power supply. The motor does not burn out.Can someone answer what will happen to the DC motor when it is connected to the 12V AC power supply via the split ring commutator. Thanks in advance.
Lots of different things could happen. I think your teacher is probably thinking about just one of those things, where the DC motor goes round as though it was connected to 6V DC : have a look at various starting positions for the rotor and work out which direction forces act for different polarity of the AC for a start.
A DC motor does -- its velocity is relative to the voltage (assuming you do no longer pass over the optimum voltage). An AC motor does not -- increasing the voltage to an AC motor will advance its torque (the rigidity it could turn whilst linked to a load), yet no longer its velocity.
The D/C supply contains a rectifier and regulator. To increase the A/C would load the rectifier or regulator in which case the rectifier or the regulator would be overloaded and burn out leaving the D/C motor intact.
