Although a far greater percentage of the electrical machine in service are AC machines, the DC machines are of considerable industrial importance. The principle advantage of DC machine, Particularly the DC motor, is that it provides a fine control of speed.
However, DC generators are not as common as they used to be, because direct current, when required, is mainly obtained from a AC supply by the use of rectifiers. Nevertheless, an understanding of DC generator is important because it represents a logical introduction to the behavior of DC motors. In this chapter, we shall deal with various aspects of DC generators.
Principle of operation of DC Generator-
An electrical generator is a machine that converts mechanical energy into electrical energy.
All the generators work based upon the principle of dynamically induced e.m.f. This principle is nothing but the Faraday’s law of electromagnetic induction. It states that whenever the numbers of magnetic lines of force i.e. flux linking with a conductor or a coil changes, an electromotive force setup in the 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.
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.
So for generating action following basic components must exist,
- The conductor or a coil
- The flux
- The relative motion between conductor and flux
In a practical generator the magnetic flux is cut by the conductor, keeping flux stationery. To have a large voltage as the output, the numbers of conductors are connected together in a specific manner, to form a winding. This winding is called DC machine’s armature winding.
On the armature of a DC machine the winding is kept. To have the rotation of conductors, the conductors placed on the armature area are rotated with the assistance of some external device. Such an external device is called prime mover. The commonly used prime movers are steam engines, diesel engines, water turbines, steam turbines etc. The necessary magnetic flux is produced by field winding( current carrying winding). The direction of the induced e.m.f. can be obtained by using Fleming’s right hand rule.
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 angle with 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 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 following Fig.
This rule mainly gives direction of current which induced e.m.f. in conductor will set up when closed path is provided to it.
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