Motor Commutator

A commutator typically consists of a set of copper contacts, fixed around part of the circumference of the rotating part of the machine (the rotor), and a set of spring-loaded carbon brushes fixed to the stationary frame of the machine. The external source of current (for a motor) or electrical load (for a generator) is connected to the brushes.

A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator. A commutator is a common feature of direct current rotating machines. By reversing the current in the moving coil of a motor's armature, a steady rotating force torque is produced. Similarly, in a generator, reversing of the coil's connection to the external circuit produces unidirectional current in the circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832, based on a suggestion by Ampere.

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 may be designed to be replaced easily without dismantling the machine. The copper contacts on small motors (say, less than a kilowatt rating) are not designed to be repaired. On large motors the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal lathe, and the commutator resurfaced by cutting it down to a smaller diameter.

Each conducting segment on the armature of the commutator is insulated from adjacent segments. Initially when the technology was first developed, mica was used as an insulator between commutation segments. Later materials research into polymers brought the development of plastic spacers which are more durable and less prone to cracking, and have a higher and more uniform breakdown voltage as with mica.

The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment, using insulating wedges around the perimeter of each commutation segment. Due to the high cost of repairs, for small appliance and tool motors the segments are typically crimped permanently in place and cannot be removed; when the motor fails it is simply discarded and replaced. On very large industrial motors it is economical to be able to replace individual damaged segments, and so the end-wedge can be unscrewed and individual segments removed and replaced.

Commutator segments are connected to the coils of the armature, with the number of coils (and commutator segments) depending on the speed and voltage of the machine. Large motors may have hundreds of segments.

A practical commutator must contain more than two segments to avoid a "dead spot" where the brushes span both segments, resulting in a short-circuit between them.