Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department


the process by which alternating current in the rotating coil of a DC machine is converted to unidirectional current. Commutation is accomplished via a set of stationary electrical contacts (brushes) sliding over multiple, shaft-mounted electrical contacts that turn with the machine rotor. The contacts are the connection points in a series-connected loop of the coils that make up the rotor winding. The brushes, sliding over these contacts, continually divide the loop into two parallel electrical paths between the brushes.
The brushes are positioned such that they make contact with those commutator segments that are connected to coils that are
moving through a magnetic neutral point between poles of the machine’s field flux. As a result, all coils making up one parallel path are always moving under a north magnetic pole, and the others are always moving under
a south magnetic pole. The movement of the commutator contacts underneath the brushes automatically switches a coil from one path to the other as it moves from a north pole region to a south pole region. Since the coils in both paths move in the same direction, but through opposite flux regions, the voltages induced in the two paths are opposite. Consequently, the positive and negative ends of each path occur at the same points in the series loop, which are at the points where the
brushes contact the commutator. The brush positions, thus, represent a unidirectional (or DC) connection to the rotating coil.

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