DC Generator Working Principle

  • Introduction:

    An electrical generator is a machine which converts mechanical energy in to electrical energy.

    DC Generator Working Principle:

    Electrical generator is based on the principle that whenever a conductor cuts magnetic flux an e.m.f is induced in the conductor. This e.m.f causes a current to flow if the circuit is closed. The direction of induced e.m.f is given by “Fleming’s right hand rule.”

    The important components of d.c generator are,

    1. A magnetic field
    2. Conductor (or) group of conductors
    3. Motion of conductor with respect to magnetic field.

    In D.C generators , a stationary magnetic field is produced by field magnets. The armature consisting of conductor is rotated inside this magnetic field by a prime mover. The prime mover may be a turbine or diesel engine petrol engine. The nature of e.m.f induce in the armature conductors is alternating (A.C). The A.C e.m.f converted into unidirectional e.m.f (D.C) by means commutator. The commutator rotates along with the armature.

    The method of producing emf in a single loop generator is explained with the help of figure.
    Figure (a) & (b) shows a single loop of copper coil ABCD moving in a magnetic field. The two ends of coil are joined to two slip rings ‘a’ and ‘b’. These slip rings are insulated from each other. Two collecting brushes press against the slip rings a, b as shown in the figure. The brushes collect the current induced in the coil and supply it to the external load.

    When the coil rotates inside the magnetic field the flux linked with the coil changes and hence e.m.f is induced in the coil, which is proportional to the rate of change of flux linkages. Imagine the coil to be rotating is the clock- wise direction.

    When the plane of the coil is right angles to flux lines i.e., in position -1(where θ = 0) the flux linked with the coil is maximum, but rate of change flux linkages is minimum. Because in this position the coil side AB and CD do not cut or share the flux. Hence no e.m.f is induced in the coil at position-1.
    Now the coil moves to at position-3 from position-1. When the coil reached the position-3 (where θ = 90˚), the coil plane horizontal to the flux lines the flux linked with coil is minimum, but the rate of change of flux linkages is maximum. Therefore maximum e.m.f is reduced in the coil position 3.
    In the next quarter revolution from position 3 position 5 (90˚ to 180˚) the flux linked with coil is gradually increases but the rate of change of flux linkages is decreases. Therefore maximum e.m.f is reduced is zero position 5.

    Now the coil moves from position 5 to position 7 (180˚ to 270˚) the e.m.f induced in the coil is in the reverse direction. Therefore at position 7, the e.m.f induced is negative maximum. Then the coil moves from position 7 to position 1, the flux the linked with the coil gradually increases, but the rate of change of flux linkages decreases the e.m.f induced is zero at position 1. Thus the emf induced in the coil is an alternating e.m.f as shown in figure.

    If the slip rings are replaced by split rings, the alternating e.m.f will become unidirectional current (D.C). The split rings are made out of a conducting cylinder which is cut into two segments insulated from each other by thin sheet of mica. The coil ends are joined to this segment. Carbon brushes rest on the segments.

    Figure (a) shows the connection of coil ends with split rings ‘a’ and b. in the first half revolution current flows along ABLMCD in the brush No: 1 is contact with segment ‘a’ acts the positive end of e.m.f and ’b’ act as the negative end.

    In the next half revolution the direction of current in the coil has reversed as shown in figure (b). But at the same time the positions of segment a and b have also been reversed. The segment ‘a’ is coming in contact with brush no: 2 and becomes negative end of induced e.m.f. Again the current in the load resistance flows in the same direction is from L to M. The current is unidirectional current due to rectifying action of split rings (also called as commutator).
    The unidirectional current is shown in the fig (c). To minimize the ripple in D.C current , the number of coil in the armature should be increased.

    Fleming’s Right Hand rule

    Keep the thump, fore finger and middle finger of the right hand mutually perpendicular to each other. If the fore finger points the direction of magnetic flux lines and the thump points the direction of motion of conductor, then the middle finger points the direction of induced e.m.f (or) current.

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