project ideas in power electronics
You Can make a DC-DC Converter with a H-Bridge and get a variable Voltage in the output (and you can control motor's velocity).
The hardest on this proyect is the control circuit for the transistors (Power MOSFETs are the best candidates) but you can use a pic microcontroller for this control, a example is PIC 16F877 (or other witn PWM) but you can do this with a PIC16F84 with the appropiate code, but the PIC16F877 you can variatel the voltege with a control signal using A/D Converter .
One Alternative for this control is based in op-amps for do a sawtooth wave generator with a comparator to make a PWM signal.
Other alternative is make a DC-AC converter, basically is the same circuit that in the DC-DC converter with H-Bridge.
In this web there are a lot of books of power electronics...Rashid and Mohan per example
I hope this can help you.