In this chapter, the authors discuss about principles of DC generator, variation of voltage induced in the conductor with the position of conductor. After that the authors discuss frequency of induced voltage, magnitude of induced voltage. This chapter ends with discussion of the main parts and construction of a DC machine.
Top2.1 Principle Of D.C. Generator
Faraday’s 2nd law of electro-magnetic induction states that whenever, there is a relative motion between a conductor and a magnetic field, a voltage is induced in the conductor. Now there are two possibilities:
The first is the principle of D.C. generator and the second is the principle of a transformer.
As is obvious from the principle of DC generator that two things are needed
Magenetic Field
This is produced by a permanent magnet or electromagnet. The electromagnet consists of silicon steel core and copper windings.
Rotating Armature
It consists of armature made of silicon steel having slots and teeth. Copper windings are placed in the slots of rotating armature
The voltage induced is given by
where B is flux density
1 is length of conductor
θ is the angle between direction of magnetic field and direction of motion
of conductor.
See Figure 1.
The angles between field and direction of motion of conductor at various points is
180˚ at point 1
90˚ at point 2
0˚ at point 3
-90˚ at point 4
2.2 Variation Of Voltage Induced In The Conductor With The Position Of Conductor
Let us consider an elementary two pole generator as shown in Figure 1. A conductor starts moving in the direction shown from position 1. It is under the influence of North Pole up to position 3 and generates a positive half cycle. Similarly under the influence of South Pole a negative half cycle is generated. So we see that for a two pole machine corresponding to 360˚ mechanical, a complete full cycle is generated. One electrical cycle corresponds to 360˚ electrical. Similarly for a four pole machine corresponding to 360˚ mechanical rotation, two complete electrical cycles i.e. 7200 are generated, so we can write down following relation:
(2.1) where Ө
e are the electrical degrees and Ө
m are the mechanical degrees, P is the number of poles.
Differentiating equation
(2.2)(2.2) where ω
e is the electrical angular velocity and ω
m is the angular mechanical velocity. From the above discussion it is obvious that alternating voltage is being generated in this elementary generator. This alternating voltage is converted to direction voltage by the use of commutator.
2.2.1 Frequency of Induced Voltage
From equation 2.2
where f is the frequency and n is the speed in revolutions per second
(2.3) where N is speed in revolutions per minute