In turbogenerators, the winding producing the magnetic field is made of a number of coils, single-circuit, energized with dc power fed via the shaft from the collector rings riding on the shaft and positioned outside the main generator bearings. In self-excited generators, shaft-mounted exciter and rectifier (diodes) generate the required field current. The shaft-mounted exciter is itself excited from a stationary winding. The fact that unlike the stator, the rotor field is fed from a relatively low power, low voltage circuit has been the main reason why these machines have the field mounted on the rotating member and not the other way around. Moving high currents and high power through the collector rings and brushes (with a rotating armature) would represent a serious technical challenge, making the machine that much more complex and expensive.
Older generators have field supplies of 125 volts dc. Later ones have supplies of 250 volts and higher. Excitation voltages of 500 volts or higher are common in newer machines.
