A rotating electrical machine, such as an aircraft starter-generator, that may be operated in either a motor mode or an generator mode. The machine includes a main rotor that is selectively configurable as an M-pole rotor or an N-pole rotor. The machine can also include DC brushes that are selectively moveable into, and out of, electrical contact the main rotor, to thereby electrically couple and decouple a DC power source to and from, respectively, the rotor windings.