Wednesday, January 6, 2016
Commutation in DC Machine or Commutation in DC Generator or Motor
The commutator is a split ring of larger size with large number of splits (commutator segments). It is called a mechanical, rectifier in generator and an inverter in motor. The connections to the commutator depends upon the type of armature windings. These are made of hard copper so as to withstand the brush forces which are placed upon the commutator segments. The position of the brushes is generally based on the winding. Commutator consists of number of segments or bars insulated from each other and are combined together tightly to form a cylinder as shown in figure, and fitted on the insulated Shaft of the armature. This is known as commutator.
To insulate the segments of commutator thin layers of (0.5 to 1 mm) mica is used. The ends of the coils wound on the armature are soldered on the segment of commutator. The insulating mica sheet is usually M or V shaped so as to prevent the segments from flying out due to centrifugal forces.
Importance of Commutator in a D.C Machine
1. As the voltage build up in the armature conductors is A.C form, to convert it into D.C voltage commutator is used in the external circuit in generator operation, whereas in D.C motor it produces unidirectional torque.
2. It facilitates the collection of current from armature.
3. It helps in connecting the armature with the external circuit.
4. It converts alternating quantity into a direct quantity.(i.e., voltage or current) and vice—versa.
5. It keeps rotor or armature M.M.F stationary in space even though the armature is rotating.
Commutator Working | Commutation in DC Machine
The induced e.m.f in the armature conductors of a DC generator can be made unidirectional with the help of commutator or slip rings and brushes.
The principle behind this can be understood with the help of following explanation,
What is the function of a commutator?
The function of commutator is to collect alternating current induced in armature conductor and convert it into unidirectional current.
Constructionally it is cylindrical in shape built with hard drawn high conductivity copper segments. In figure two segments E and F are shown. They are separated by a thin layer of mica insulation usually of V—shape. Two brushes X and Y are mounted on the segments.
In figure (Case 1) the first half rotation i.e., from 0 to 180° the coil position, with the segments are shown. The direction of current flow is from point A to B.
In the next half rotation as in figure (Case 2) the segments are interchanged. Thus, the direction of current in this case is from B to A, similarly though the segments. i.e., 0 to 180°. is from A to B and 180 to 360° is from B to A.
By considering the total revolution 0 to 180° is taken as positive and 180 to 360° as negative. This current is passed through the load resistance R in the external circuit and develops the unidirectional or DC voltage.