Electrical Engineering

The Structure of Matter

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In the understanding of fundamentals of electricity, the knowledge of the structure of matter plays an important role. The matter which occupies the space may be solid, liquid or gaseous. The molecules and atoms, of which all substances are composed are not at elelemental, but are themselves made up of simpler entities. We know this because we, up to certain extent, are successful in breaking atoms and studying the resulting products. For instance, such particles are obtained by causing ultraviolet light to fall on cold metal surfaces, such particles are spontaneously ejected from the radioactive elements. So these particles are obtained from many different substances under such widely varying conditions. It is believed that such particles are one of the elemental constituents of all matter , called electrons .

Infact, according to the modern electron theory, atom is composed of the three fundamental particles , which are invisible to bare eyes. These are the neutron, the proton and the electron. The proton is defined as positively charged while the electron is defined as negatively charged. The neutron is uncharged i.e. neutral in nature possessing no charge.

The mass of neutron and proton is same while the electron is very light, almost 1/1840th the mass of the neutron and proton. The following table gives information about these three particles.

1.1 Introduction

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In practice, the electrical circuits may consist of one more sources of energy and number of electrical parameters, connected in different ways. The different electrical parameters or elements are resistors, capacitors and inductors. The combination of such elements alonghith various source of energy gives rise to complicated electrical circuits,generally referred as networks. The terms circuit and network are used synonymously in the electrical literature. The d.c. circuits consist of only resistances and d.c. sources of energy. And the circuit analysis means to find a current through or voltage acrross any branch of the circuit.

1.Introduction

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The study of the electrical engineering, basiccally involves the analysis of the energy transfer from to another. An electrical machine, deals with the energy transfer either from mechanical form or from electrical to mechanical form.

This process is called electromechanical energy conversion.

An electrical machine which converts mechanical energy into an electrical energy is called an electric generator. While an electrical machine which converts an electrical energy into the mechanical energy is called an electrical motor.

Such electrical machines may be related to an electrical energy of an alternating type called a.c. machines or may be related to an electrical energy of direct type called d.c. machines.

The d.c. machines are classified as d.c. generator and d.c. motors. The construction of a d.c. machines basically remains same whether it is a generators or a. motors. But before beginnig the study of the d.c. machines, it is necessary to revise the basic concept of magnetism and electromagnetism.

1.4 Principle of Operation of a D.C.generator

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All the generators work on a principle of dynamically induced e.m.f. This principle is nothing but the Farady'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 whith respect to flux or by rotating flux whith respect to a conductor. So a voltage gets generated in a conductor, as long as there exist a relative motin between conductor and the flux.
Such an induced e.m.f. which id 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 component to exist,
i) The conductor or a coil ii)The flux iii) The relative motion between conductor and flux.
In a practical generator, the conductor 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 spicific manner, to from 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 manetic flux is produced by current carying winding which is called field winding. The direction of the induced e.m.f. can be obtained by using fleming's right hand rule.

1.2.3 Manetic Field due to Circular Conductor

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Consider an arrangement in which a long conductor is wound with number of turns on a core, close together to form a coil. This is called a solenoid as shown in the Fig. 1.4

When such a conductor carries a current, the magnetic field gets produced around the core.

Identifying the direction of flux and hence identifying the two ends of the core as N pole or S pole is important in understanding the principle of d.c. machine. The right hand thumb rule can be modified for such case as started below.

The right hand thumb rule : Hold the solenoid in the right hand such that curled fingers point in the direction of the current though the curled conductor, then the outstretched thumb along the axis of the solenoid points to the North pole of the solenoid or points in the direction of flux lines inside the core.

Whith this background, let us start the detail study of a d.c. generator.

key point : The direction of flux can be reversed either by changing direction of current though the conductor by reversing the polarities of the supply ore by changing the direction of winding of the conductors around the core.

1.3 Revision of Electromagnetism

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When a conductor carries a current, it creates a magnetic field around it. The direction of such magnetic field depends on the direction of the current passing through the conductor.

So electric current and magnetism are very closely related to each other. This relatoinship plays an important role in the d.c. machines.

Let us see in brief, the rule to determine the direction of the flux prouduced by a current carrying conductor.
1.3.1 Right Hand Thumb Rule
It states that "Hold the current carrying conductor in the right hand such that the thumb is pointing in the direction of current and parallel to the conductor, then curled fingers point in the direction of the magnetic field or flux around it".
The Fig. 1.2 explains the rule.

      Conventionally, such conductors are observed, assuming them to be placed perpendicular to the plane of the paper. So current moving away from the observer is denoted by a 'cross' while current coming towards the observer is dentoted in the direction of current denoted as 'cross' i.e. going into the paper, then curled fingers indicate the

direction of flux as clockwise, as show in the Fig. 1.3 (a). While if thumb of right hand is adjusted in the direction of the current shown as 'dot' i.e. coming out of paper, then curled fingers indicate the direction of flux as anticlockwise as shown in the Fig. 1.3 (b).

1.2.2 Magnetic Field and flux

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The region around a magnet within which the influence of the magnet can be expresented

is called its magnetic field. The presence of magnetic field is represented by imaginary lines around a magnet. These are called magnetic lines of force.

The total number of lines of force existing in a particular magnetic field is called magnetic flux, denoted by a symbol . It is measured in a unit weber.

Key Point : The lines of force never intersect each other and are like stretched rubber bands and always try to contract in length.

The properties of force play an importan role in the understanding of the working principle of the d.c. machines.

The lines of lines of flux have a fixed direction. The flux lines start at N-pole and terminate at S-pole, extemal to the magnet.While the direction of flux lines is from S-pole to N-pole, intemal to the magnet. The distribution and direction of such flux lines for a bar magnet is show in the fig.1.1.

The lines always form a closed loop and never intersect each other.

1.2.1 Laws of Magnetism

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Law 1: It states that 'like' magnetic poles repel and 'unlike' poles attract each other. When the two magnts are brought near each other, such that two like poles i.e.N and N or S are S are facing each other, then the two magnets experience a force of repulsion. As against this, if two unlike poles i.e.N and S or S and N are facing towards each other, then they experience a force of attraction and try to attract each other.

Law 2: This law is experimentally proved by Scientist Coulomb and hence also known as Coulomb's Law.

The force (F) exerted by one pole on the other pole is,

1- Directly proportional to the product of the pole strengths.

2- Inversely proportional to the square of the distance between them and

3- Dependent on the nature of medium surrounding the poles.

Mathematically this law can be expressed as,

where M1 and M2 are the pole strengths of the poles while 'd' is the distance between the two poles.

where K is constant which depends on the nature of the surrounding.

1.2 Revision of Magnetism

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Magnetismis a property by virtue of which a piece of solid body attracts iron pieces and pieces of some other metals. Such a piece of solid body is called a natural magnet.

The two ends of a magnet are called its poles. When such a magnet is suspended freely by a piece of a silk fiber, it turns and adjusting itself in the direction of North and South of the earth. The end adjusting itself in the direction of North is called N pole while other is called S pole. When such two magnets are brought near each other, their behaviour is governed by some laws called laws of magnetism.

1.Introduction

2012-10-19T13:08:00.000-07:00

The study of the electrical engineering, basically involves the analysis of the energy transfer from one form to another. An electrical machine, deals with the energy transfer either from mechanical to electrical form or from electrical to michanecal form. This process is called electromechanical energy conversion.

An electrical machine which converts mechanical energy into an electrical energy is called an electric generator.While an electrical machine which converts an electrical energy into mechanical energy is called an electrical motor.

Such electrical machines may be related to an electrical energy of an alternating type called a.c. machines or may be related to an electrical energy of direct type called d.c. machines.

The d.c. machines are classified as d.c. generators and d.c. motors. The construction of a d.c. machine basically remains same whether it is a generator or a motor. In this chapter constructional features of a d.c. machine, working principle and types of d.c. generator are discussed. But befor beginning the study of the d.c. machines, it is necssary to revise the basic concepts of magnetism and electromagnetisme.