Q. Explain the working principle of a d.c. generator. Explain clearly the function of commutator in d.c. machines.

Sol. Working principle of a D.C. generator: For the 2- poles d.c. machine, one full pitched N - turn coil.

One copper ring is split into two portions, which are insulted from each other and also from the shaft on which these are mounted. The two ends of the coil are connected to those two copper segments, on which two stationary carbon brushes are resting under suitable spring pressure. For an actual d.c. machine, there are a large no. of coils and accordingly a copper ring will have to be cut in a large no. of copper segments. These copper segments as a whole are called a commutator.

For understanding the process of rectification by means of commutator, consider end view of the coil, seen from the commutator side. The variation of field flux density along the air gap periphery is depicted. For the present, only fundamental sine component  of the flux - density wave is considered. The rotor is assumed to revolve at a clockwise speed of w_r rad/ second. For the position of the coil, the emf induced in a, a^' is zero, since the instantaneous velocity of the coil is parallel to the magnetic flux and the rate of flux cutting is zero. After 90^o travel of the coil, the emf induced is maximum, since each coil side lies in the peak of flux density wave and the rate of flux - cutting is maximum.

The direction of emf in coil side a - a^', as determined by right - hand rule, is indicated by dot and cross respectively. Further coil a and a^' are connected via the commutator segment to brushes B₁ and B₂ respectively. If output terminals are connected to the load, current comes out from brush B₁ and enters at brush B₂.

In the positive, the emf induced is zero and after further travel of 90^o, the emf induced in the coil is maximum, but of reversed polarity, the brushes B₁ is in contact with coil side a which is under south pole, again the brush B₁ is in contact with that coil - side which comes under the north - pole. Consequently the polarities if the brushes B₁ and B₂ remain positive and negative. The emf induced in the coil is alternating but the voltage available across the brushes B₁ and B₂ is unidirectional.