Explain how using a commutator rather than slip rings allows dc rather then ac to be used to drive a motor.
A coil rotating past a magnetic field generates Alternating Current (AC), so when DC (unidirectional current) is required, it has to be rectified in some way. Slip rings are used to extract electrical power from the armature when the armature is the rotor. There may be 2 for a single phase generator, and 3 or 6 for three phases. A brush makes the stationary contact with the rotating slip ring. Slip rings are a continuous connection between the stationary and the rotating parts of the machine. The alternating voltage from the armature is the output, but this can be converted to DC by using rectifier diodes. Slip rings are also used to feed DC to a rotating field in either an AC or a DC machine. A commutator, like slip rings, provides a connection between the fixed and rotating parts of the machine and is used to extract electrical power from the armature when the armature is the rotor. However the commutator is broken up into segments, so that it can act as a rotating switch. It is arranged so that as the rotor rotates, the connections to the armature coils are reversed just as the voltage being generated is reversed. It is a mechanical rectifier. The AC voltage with current in both directions is converted to DC voltage with current in one direction through the circuit.
The commutator changes the armature's winding power connections as it rotates, so that the pole pieces are always energized to the right polarity for the motor to spin. Obligatory wiki link below has a nifty diagram.
