Principle of Operation of a D.C. Generator

Principle of Operation of a D.C.Generator

AU: Dec.-16, May-10,15

• All the generators work on a principle of dynamically induced e.m.f. This principle is nothing but the Faraday's law of electromagnetic induction. It states that, 'Whenever the number of magnetic lines of force i.e. flux linking with a conductor or a coil changes, an electromotive force is set up in that conductor or coil.' The change in flux associated with the conductor can exist only when there exists a relative motion between a conductor and the flux. The relative motion can be achieved by rotating conductor with respect to flux or by rotating flux with respect to a conductor. So a voltage gets generated in a conductor, as long as there exists a relative motion between conductor and the flux.

• Such an induced e.m.f. which is due to physical movement of coil or conductor with respect to flux or movement of flux with respect to coil or conductor is called dynamically induced e.m.f.

Key Point : So a generating action requires following basic components to exist,

i) The conductor or a coil ii) The flux iii) The relative motion between conductor and flux.

• In a practical generator, the conductors are rotated to cut the magnetic flux, keeping flux stationary. To have a large voltage as the output, the number of conductors are connected together in a specific manner, to form a winding. This winding is called armature winding of a d.c. machine. The part on which this winding is kept is called armature of a d.c. machine. To have the rotation of conductors, the conductors placed on the armature are rotated with the help of some external device. Such an external device is called a prime mover. The commonly used prime movers are diesel engines, steam engines, steam turbines, water turbines etc. The necessary magnetic flux is produced by current carrying winding which is called field winding. The direction of the induced e.m.f. can be obtained by using Fleming's right hand rule.

1. Fleming's Right Hand Rule

• If three fingers of a right hand, namely thumb, index finger and middle finger are outstretched so that everyone of them is at right angles with the remaining two and if in this position index finger is made to point in the direction of lines of flux, thumb in the direction of the relative motion of the conductor with respect to flux then the outstretched middle finger gives the direction of the e.m.f. induced in the conductor. Visually the rule can be represented as shown in the Fig. 3.2.1.

• This rule mainly gives direction of current which induced e.m.f. in conductor will set up when closed path is provided to it.

• The magnitude of the induced e.m.f. is given by,

E = B × l × v

where l = Active length of conductor in m

 v = Relative velocity component of

conductor in m/s in the direction

perpendicular to direction of the flux.

• The active length means the length of conductor which is under the influence of magnetic field. In all the cases above, direction of motion of conductor is perpendicular to the plane of the flux.

• But if it is not perpendicular then the component of velocity which is perpendicular to the plane of the flux, is only responsible for inducing e.m.f. in the conductor. This is shown in the Fig. 3.2.2 (a). In this Fig. 3.2.2 (a), though the velocity is v, its component v' which is perpendicular to the flux lines is only responsible for the induced e.m.f.

• If the plane of the rotation oof conductor is parallel to the plane of the flux, there will not be any cutting of flux and hence there cannot be any induced e.m.f. in the conductor. This is shown in the Fig. 3.2.2(b)

Key Point: So to have an induced e.m.f. in the conductor not only the relative motion between the conductor and the flux is necessary but plane of rotation and plane of flux should not be parallel to each other.

• If angle between the plane of rotation and the plane of the flux is '0' as measured from the axis of the plane of flux then the induced e.m.f. is given by,

E = Bl (v sin θ) volts

• where v sin θ is the component of velocity which is perpendicular to the plane of flux and hence responsible for the induced e.m.f. This is shown in the Fig. 3.2.3.

• From the equation of the induced e.m.f., it can be seen that the basic nature of the induced e.m.f. in a d.c. generator is purely sinusoidal i.e. alternating. To have d.c. voltage, a device is used in a d.c. generator to convert the alternating e.m.f. to unidirectional e.m.f. This device is called commutator.

Review Questions

1. Explain the principle of operation of a d.c. not generator?

AU: Dec.-16, Marks 8

2. Discuss the basic concepts of e.m.f. generation in a d.c. machines in detail.

AU May-10, Marks 8