BASIC ELECTRICAL

By :- Rajneesh Kumar

GENERAL INTRODUCTION

What is Electrical Engineering?

Electrical engineering is a Field of Engineering that Deals With Study And Application of Electricity , Electronics And Electromagnetism.

“Electricity is not the flow of electrons, instead it's the electrons and protons themselves.”

Or“A form of energy resulting from the existence of charged particles (such as electrons or protons), either statically as an accumulation of charge or dynamically as a current.”

What is Electricity?

Atom

The atom consist of a positively charged nucleus surrounded by negatively charged electrons, so that the atom as a whole electrically neutral.

Fig 1. Atom

Electrostatic Force: “The attraction or the repulsion of particles or objects because of their electric charge is known as electrostatic force.” For ex: An attractive force between the electron and the nucleus of an atom.

First law of electrostatics: “Unlike charges attracts; like charges repel.”

Electrostatic Field: “A electrostatic field is a electric field produced by static electric charges. The charges are static in the sense of charge amount and their position in space.”

Electric Charge: Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges: positive and negative.

BASIC DEFINITIONS

Coulomb’s Law: According to law, “The force exerted between two point charges is directly proportional to the product of their strengths, and is inversely proportional to the square of distance between them. ” i.e.

N (In Air)Or

N (In Medium)

Permittivity: “The ability of a substance to store electrical energy in an electric field.” Permittivity of free space = = F/m Relative Permittivity of material = Absolute permittivity = Permittivity in a medium =

=

ELECTRICAL TERMINOLOGY

Conductor : The materials in which electron are loosely bound to their atom, or the material which permit free motion of a large no. of electron. Atoms with only one valence electron, like Copper, Silver are good conductor of electricity. Most metals are good conductors.

Insulators : Insulators are materials with electron that are tightly bound to their atoms and require large amount of energy to free them from the influence of the nucleus. Atoms of good insulators have their valence shell half filled with 8 electrons, which means they are half filled, like Rubber, Plastic, Glass etc.

Resistors : Resistors are made of material that conduct electricity, but offer opposition to current flow. These type of material are also called semi-conductors. These have more than 1 or 2 electrons in their valence shell but less than 7 or 8, like Carbon, Silicon, Germanium etc.

Voltage or Potential difference : “Potential difference between two points in a circuit is the work done in moving unit charge (i.e. one coulomb) from one point to the other.” Or, “The Pressure that causes electrons to move in a conductor.”

Electro motive force (E.M.F.) : “Electromotive force, or emf, is better described as the total voltage in an electric circuit generated by the source or battery.” Or, “Electromotive force is the characteristic of any energy source capable of driving electric charge around a circuit.” It is basically the energy required to move a unit positive charge from the negative terminal of the battery to the positive terminal when the circuit is open.

Current : It is defined as, “ The rate of flow of electric charges.” i.e.

C/s or A

Resistance : “The opposition to current flow.” Its unit is ohm (Ω) and represented by ‘R’.

Conductance : The opposite or reciprocal of resistance is called conductance. It can be defined as, “ the ability to conduct the current.” Its unit is mho () and represented by ‘G’.

i.e.

Inductance : It is defined as, “ The ability of a coil to store energy, induce a voltage in itself and oppose changes in current flowing in it.” Its unit is henry (H) and represented by ‘L’.

Capacitance : “The ability to store electric charge.” Its unit is farad (F) and represented by ‘C’.

i.e.

Power : Power is the rate at which work is done. Its unit is watt (W) and represented by ‘P’ . One watt of power equals the work done in one second by one volt of potential difference in moving one coulomb of charge.

i.e. W

Electrical energy : It is the product of total power consumption and time. Its unit is watt-hour (W-h). i.e.

i.e. Wh

1 watt = 1 joule/second1000 watt = 1 kW1 KWh = joules

Active Power : Real or true or active power, that is used to do work on loads.

i.e. W

Reactive Power : The power that is wasted and not used to do work on loads.

i.e. VAR

Apparent Power : The power that is supplied to the circuit.

i.e. VA

• Relation between active, reactive and apparent power:

Basic Law’s

Ohm’s Law : “At a constant temperature, the current flowing through the circuit is directly proportional to the voltage and inversely proportional to the resistance .” i.e.

Law of resistance : The resistance of conductor in a cicruit depends upon the following states-

• It depends upon the material.• Directly proportional to the length of the

conductor.• Inversely proportional to the area of the cross

section of the conductor.• It also depends upon the temperature of the

conductor.

i.e. Ωhere, = specific resistance = The resistance that is offered by 1 cubic cm material.

Temperature coefficient of resistance : “It is defined as the increase in resistance per degree rise in temperature of a material. i.e.

Kirchhoff's Current Law (KCL) : It states that, “ the algebraic or vector summation of all the currents meeting at a junction or node is zero.”

According to KCL,

Or

KCL is based on Law of Conservation of charge.

Kirchhoff's Voltage Law (KVL) : It states that, “ the total potential rise in any closed path is equal to total potential drop in that path.”

According to KVL,

KVL is based on Law of conservation of energy.

When current moves through a conductor a circular magnetic field is induced around the conductor

Magnetic Field LinesDirection of C

onventional Curren

t

Negative Battery TerminalPositive Battery

Terminal

The Direction and Motion of an Induced Magnetic Field in a Conductor

Current Coming Towards You

Current Moving Away From You

X

NS

A Motor Armature in a Fixed Magnetic Field

The magnetic field surrounding a current carrying conductor interacts with an

existing magnetic field.

Direction of Force (Torque) acting to turn the Armature (Conductor)

Fleming's Left Hand (Motor) Rule

Determines the direction of DC current carrying conductor in a fixed magnetic field

Thumb = Direction of Conductor Motion Fore Finger = Direction of

Fixed Magnetic Field (N to S)

Middle Finger = Conventional Current Direction

Fleming's Left Hand (Motor) Rule

N

S

Direction of Rotation

Fixed Magnetic Field Direction

Conventional Current Direction

DC MachinesBasically Dc machine divided in two parts:- 1. DC Generator 2. DC Motor

There are two types of generators, one is AC generator and other is DC generator. Whatever may be the types of generators, it always converts mechanical power to electrical power

DC Motor This DC or direct current

motor works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action. If the direction of electric current in the wire is reversed, the direction of rotation also reverses.

DC Motor

DC Motor When magnetic field and electric field

interact they produce a mechanical force, and based on that the working principle of dc motor established. The direction of rotation of a this motor is given by Fleming’s left hand rule,

DC MotorThe force which is experienced by DC

machine is determined by:-F = BIL

where, B = Flux DensityI = Current flowing in conductorL = Length of conductorConstruction wise a DC motor is exactly

similar to a DC generator, but electrically it is just the opposite.

TYPES OF DC MOTORS• DC motor are of 3 types they are…..

1. DC SHUNT MOTOR

2. DC SERIES MOTOR

3. DC COMPOUND MOTOR

1. DC SHUNT MOTOR

Armature

• The parallel combination of two windings is connected across a common dc power supply.

• The resistance of shunt field winding (Rsh) is always higher than that is armature winding.

• This is because the number of turns for the field winding is more than that of armature winding.

• The cross-sectional area of the wire used for field winding is smaller than that of the wire used for armature winding.

Shunt wound motorsShunt wound motors are used when

starting torque is low and increases with motor

speed. They are particularly useful where

constant speed under varying load conditions

is a requirement. Typical applications in aircraft are

fuel pumps and fans.

2. DC SERIES MOTOR• The field winding is

connected in series with the armature.

• The current passing through the series winding is same as the armature current .

• Therefore the series field winding has fewer turns of thick wire than the shunt field winding.

• Also therefore the field winding will posses a low resistance then the armature winding.

NoteSeries wound motors should

never be allowed to operate without a

mechanical load applied. This is because they are liable to

over- speed, possibly to destruction.

3. DC COMPOUND MOTOR

DC COMPOUND MOTOR

LONG SHUNT COMPOUND

MOTOR

SHORT SHUNT

COMPOUND MOTOR

CUMULATIVE COMPOUND

MOTOR

DIFFERENTIAL COMPOUND

MOTOR

Compound Wound Motors The compound wound motor is

suited to applications where loads may vary

from zero to maximum and where

starting loads may be high. In aircraft they are often used to

drive hydraulic pumps and used as a

starter/ generator.

MOTOR SPEED CONTROL METHOD

armature control field control.