Commutators are rotary electrical switches that are used to reverse the direction of a current between a rotor and an external circuit. Commutators are used in electric motors, electrical generators, and in direct current (DC) rotating machines. In motors, commutators apply power to the best location on the rotor. A steady, rotating force (torque) is produced by reversing the current direction in the moving coil of the motor's armature. In generators, reversing the coil's connection to the external circuit provides a unidirectional (direct) current. As rotary electrical switches, commutators have an exceptionally long life considering the number of circuit makes and breaks that occur during normal operation.
Most commutators consist of metal segments and spring-loaded brushes. The sizing and composition of each of these components should be considered when selecting a commutator.
The segments are fixed around the circumference of the rotor and the brushes are attached to the machine's stationary frame. Typically, the size of the equipment and the requirements of the application determine whether these segments are made of copper or another metal. By using plastic spacers, these conductive segments are insulated from each other. Commutator segments are designed to be replaced on industrial motors but are often discarded with less expensive pieces of equipment. The commutator segments are connected to the armature's coil, where the number of coils depends upon the speed and voltage of the machine.
The brushes in brushed commutators are usually made of either copper or carbon. Carbon brushes wear more evenly, cause less sparking, and do less damage to the conductive metal segments. Copper brushes are better suited for very low voltages and high current, however. The higher electrical resistance of carbon brushes also causes greater voltage drops. Although copper brushes are preferred for small motors and some consumer products, carbon brushes are used in large industrial motors. Friction between the brushes and segments, however, eventually causes both components to wear, regardless of the brush's construction.
Both commutated motors and commutated generators require constant contact between the brush and the commutator. Typically, brush-holders with springs are used. When more than one brush is required, assemblies with multiple brushes may be mounted in parallel. These parallel holders distribute current evenly across all brushes so that a brush can be replaced without stopping the equipment.
Solid-State Commutation Systems
Solid-state commutation utilizes solid-state electronics rather than a mechanical commutator and corresponding brushes. These systems have the advantage of being more efficient, since the elimination of brushes means less friction resistance or system wear. However, because of the controllers and electronics required, these types of motors and generators tend to cost more and are more complex than simple brushed systems.