Principle of DC Generator

GENERATOR PRINCIPLE

An electrical generator is a rotating machine, which converts mechanical energy into electrical energy. It operates on the principle based on Faraday's law of electromagnetic induction.

According to Faraday's Law of electromagnetic induction, when a conductor is rotated in a magnetic field to cut the magnetic lines of flux, dynamically induced emf is produce in the conductor. This emf causes a current to flow if the conductor circuit is closed. The direction of the current is found by Fleming's Right Hand Rule.

Hence, the basic requirements for the dynamically induced emf to exist are the following

(i) A steady magnetic field.

(ii) A conductor or coils.

(iii) Relative motion between the magnetic field and the conductors.

Simple Loop Generator

Consider a single‒turn rectangular copper coil ABCD rotating clockwise in a magnetic field provided by permanent magnet or electromagnets as shown in Fig. 2.2 (a). The two ends of the coil are joined to two Slip‒rings 'a' and 'b' which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip rings. Their function is to collect the current induced in the coil and to convey it to the external load resistance.

The rotating coil may be called 'armature' and the magnets as "field magnets". It is rotated in clockwise direction at an uniform magnetic field.

At position 1 (θ=0°), the plane of the coil is perpendicular to the direction of lines of flux. Now, the flux linked with the coil is maximum. But the rate of change of flux linkages is minimum. So, no emf is induced in 'the coil. That is at the starting position emf induced is zero.

When the coil is rotated further, the rate of change of flux linkage increases upto position, 3 (θ=90°). At this position, the plane of the coil is parallel to the lines of flux. Now, the flux linked with the coil is minimum, but rate of change of flux linkages is maximum. Therefore, at this position emf induced in the coil is maximum.

On further rotation of the coil from position 3(θ = 90°) to position 5 (θ = 180°) the rate of change of flux linkages decreases, and the emf induced is gradually decreased.

At position 5 (θ = 180°), it is reduced to zero. The magnitude of emf with respect to the coil position is indicated in Fig. 2.2 (b). From position 5 to position 7 (that is 180° to 270°), the induced emf value starts again from zero to maximum and from position 7 to position 1 (from 270° to to 360°) maximum to zero in opposite direction.

Note that emf generated in the loop is alternating one. It is because any coil side, say AB has emf in one direction when under the influence of N‒pole and CD in the other direction when under the influence of S‒pole. If a load is connected across the ends of the loop, then alternating current will flow through the load. The alternating voltage generated in the loop can be converted into direct voltage by a device called commutator.