Electric motor



The commutator is a component of an electric motor formed by strips of hard electrolytic copper with insulating material in the middle (formerly micanite, nowadays plastic). The assembly is insulated on the rotor shaft and the copper blades are connected to the anchor windings. Permanently installed carbon brushes make contact with the collector and provide power to the anchor windings.

A commutator is a rotary electrical switch in certain types of electric motors and electric generators that periodically changes the direction of the current between the rotor and the external circuit. In an engine, it provides the energy to the best location in the rotor, and in a generator, it receives the energy in a similar way. As a switch, it has an exceptionally long service life, taking into account the number of openings and closures that occur in normal operation.

The switch was first used by Hippolyte Pixii in 1833, based on an idea by André-Marie Ampère. Antonio Pacinotti built in Pisa in 1859 a prototype of a DC machine for induction with a radial ring and a collector or radial collector. He improved the switch by building it from multiple segments instead of the two in Pixii. This design is known as "Pacinotti's Ring".

Around 1869 Zénobe Gramme introduced a series of important improvements. He managed to produce a model that can deliver a large enough constant current to a relatively large capacity for use in laboratories.

Switch objective

The switch is an important part in the operation of a DC machine. This one provides two things:

As with a slip ring, it provides an electrical connection between the stationary carbon brushes and the rotating shaft with the rotor windings.

The alternating current generated when rotating the rotor coils beyond the fixed field magnets becomes a DC voltage (alternator) or reverses the direction of the current in the rotor windings so that the armature continues to rotate (motor) .

A switch is actually a mechanical rectifier and a rotary switch in one. Reversing the direction of flow in the winding is called switching.

Principle of operation

A switch consists of a set of contact bars fixed to the rotating shaft of a machine, and connected to the armature windings. As the shaft rotates, the switch reverses the current flow in a winding. For an individual armature winding, when the shaft has completed half a turn, the winding is now connected so that current flows through it in the opposite direction to the initial one.

In an engine, the armature current causes the fixed magnetic field to exert a rotational force or a twisting torque on the winding to cause it to rotate. In a generator, the mechanical torque applied to the shaft maintains the movement of the armature winding through the stationary magnetic field, inducing a current in the winding. Both in the case of the motor and the generator, the commutator periodically inverts the direction of the current flow through the winding, so that the current flow in the external circuit to the machine continues in only one direction.

Simplified practical switch

Practical switches have at least three contact segments, to avoid a "dead" point where two brushes simultaneously bridge only two segments of the collector. The brushes are made wider than the insulated space, to ensure that the brushes are always in contact with an armature coil.

For switches with at least three segments, although the rotor can potentially stop at a position where two segments of the switch touch a brush, this only deactivates one of the arms of the rotor while the others will continue to function correctly. With the remaining rotor arms, an engine can produce enough torque to start rotating the rotor, and a generator can provide useful power to an external circuit.


Published: August 27, 2018
Last review: August 27, 2018