Commutator (electric)

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Conventional continuous current flows from the battery. The commutator itself is the red and blue curved segments. The brushes are dark gray and contacting the commutator contacts, and the rotor winding is violet. As the motor rotates, the commutator contacts will turn through 180° and the current flowing in the winding will reverse. The reverse in coil current compensates for the fact that the coil has rotated 180° relative to the fixed magnetic field (not shown). By varying the relative angle between the two fields a torque on the coil can be produced and maintained by the commutator allowing work to be extracted from the coils rotation.

A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator. The setting of the switch is tied to the motors axis of rotation which allows the coil to maintain a magnetic field whose direction is roughly fixed while still allowing the coil to rotate freely. The operation of DC motors relies on the fact that in the presence of a properly aligned fixed external magnetic field the coil will feel a roughly constant torque in one direction. The end result of this type of operation and its converse, an un-electrified coil rotating in an external magnetic field, enables motors to run on and generators to produce direct current, as opposed to the alternating current involved without the presence of the commutator.

[edit] Construction

A commutator typically consists of a set of copper contacts, fixed around the circumference of the rotating part of the machine (the rotor), and a set of spring-loaded carbon brushes fixed to the stationary part of the machine (the stator) that complete the electrical circuit from the rotor's windings to the outside of the machine. Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster and are designed to be replaced easily without dismantling the machine. The copper contacts are usually inaccessible and, on small motors, are not designed to be repaired. On large motors the commutator may be re-surfaced with abrasives. Each conducting segment on the armature of the commutator is insulated from adjacent segments and any number of segments is theoretically possible, with some large motors containing hundreds of segments--each pair connected to a unique coil of wire on the armature. In the lower limit of a single conducting segment the commutator behaves like a slip ring and no directional alignment of the created magnetic field is provided. By increasing the number of segments the direction of the electrically created magnetic field and consequently the induced torque is held more constant, smoothing the operation of the motor. Furthermore, certain configurations of segments help mitigate "dead spots" in the rotation where the applied torque is lower than average making starting the motor difficult if it stops in one of these positions.

[edit] Longevity

The mechanical system employed by the brushes creates three problems. First, brushes tend to wear out requiring occasional maintenance, although the task can be made very simple by making the brushes removable and new brushes are inexpensive, applications where maintenance is not possible like robotic spacecraft need to consider the life span of the brushes. Second, brushes for use on very rapidly rotating armatures must either be made of more durable materials or face more rapid decay. Finally, the friction constitutes a source of energy loss in the motor. The rotary transformer avoids the wear and other problems associated with the mechanical coupling while still allowing for the free rotation of the armature.

[edit] See also

• Slip ring
• Rotary transformer

[edit] Patents

• Nikola Tesla - U.S. Patent 334823  - Commutator for Dynamo Electric Machines - 1886 January 26 - Elements to prevent sparking on dynamo-electric machines; Drum-style with brushes.
• Nikola Tesla - U.S. Patent 382845  - Commutator for dynamo electric machines - 1888 May 15 - Relates to dynamo-electric machines or motors; Improvements on devices to collect or communicate currents; Avoids destruction and wear of machine; Avoid adjustments due to destruction and wear; Enable practical construction of very large dynamo electric machines or motors with the minimum number of communicator segments; Increases safety and efficiency.

[edit] External articles

• "Commutator and Brushes on DC Motor". HyperPhysics, Physics and Astronomy, Georgia State University.