our supply is use to start of a small motor.. which wouldnt consume too much. 3V should be sufficient for a motor to roll on and generate DC power from dynamo.
check
Bicycle Dynamos for a 6V dynamo and a small example,where they checking power output of a dynamo
and btw, to start a motor after switch off, maybe you can just create a circuit full of capacitors. then connect to a power supply and motor so what you need to do is turn on power supply, then wait a few sec or minute to charge up all capacitors then switch off, motor will still continue to rotate and you could turn mechanical power of the motor to any form of generator.
example i given above is to turn electrical --> mechanical --> magnetic --> electrical
just did a circulation since you wan the power supply to switch off ^_^