Electrical Technology by nanajee

Electrical Technology Page 1
Electrical Technology
(Very simple manner for ECE/ME/CE/EEE)
Author:
Nanajee Karri M.Tech. MISTE, (Ph.D)
Assistant Professor,
Dept. of Electrical Engineering,
Sasi Institute of Technology & Engineering,
Tadepalligudem-534101,
Andhra Pradesh, India.
nanajee.karri@gmail.com
Contents:
DC machines:
 DC Generator
 DC Motor
AC machines:
 Transformers
 Alternator
 Induction Motor

Principle of DC Generator
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. An AC generator produces alternating
power. A DC generator produces direct power. Both of these generators
produce electrical power, based on same fundamental principle of Faraday's
law of electromagnetic induction.
According to this law, when a conductor moves in a magnetic field it
cuts magnetic lines of force, due to which an emf is induced in the
conductor. The magnitude of this induced emf depends upon the rate of
change of flux (magnetic line force) linkage with the conductor. This emf will
cause a current to flow if the conductor circuit is closed.
Hence the most basic two essential parts of a generator are
1. Magnetic field
2. Conductors which move inside that magnetic field.
Basic Construction and Working of A DC Generator.
A dc generator is an electrical machine which converts mechanical energy
into direct current electricity. This energy conversion is based on the
principle of production of dynamically induced emf. This article
outlines basic construction and working of a DC generator.
1. Yoke: The outer frame of a dc machine is called as yoke. It is made up
of cast iron or steel. It not only provides mechanical strength to the

whole assembly but also carries the magnetic flux produced by the field
winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of
bolts or welding. They carry field winding and pole shoes are fastened to
them. Pole shoes serve two purposes; (i) they support field coils and (ii)
spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former
wound and placed on each pole and are connected in series. They are
wound in such a way that, when energized, they form alternate North
and South poles.
4. Armature core: Armature core is the rotor of the machine. It is
cylindrical in shape with slots to carry armature winding. The armature
is built up of thin laminated circular steel disks for reducing eddy
current losses. It may be provided with air ducts for the axial air flow
for cooling purposes. Armature is keyed to the shaft.
5. Armature winding: It is usually a former wound copper coil which
rests in armature slots. The armature conductors are insulated from
each other and also from the armature core. Armature winding can be
wound by one of the two methods; lap winding or wave winding. Double
layer lap or wave windings are generally used. A double layer winding
means that each armature slot will carry two different coils.
6. Commutator and brushes: Physical connection to the armature
winding is made through a commutator-brush arrangement. The
function of a commutator, in a dc generator, is to collect the current
generated in armature conductors. Whereas, in case of a dc motor,
commutator helps in providing current to the armature conductors. A
commutator consists of a set of copper segments which are insulated
from each other. The number of segments is equal to the number of
armature coils. Each segment is connected to an armature coil and the
commutator is keyed to the shaft.
Brushes are usually made from carbon or graphite. They rest on
commutator segments and slide on the segments when the commutator
rotates keeping the physical contact to collect or supply the current.
Working Principle Of A DC Motor
A motor is an electrical machine which converts electrical energy into
mechanical energy. The principle of working of a DC motor is that
"whenever a current carrying conductor is placed in a magnetic field,
it experiences a mechanical force".
The direction of this force is given by Fleming's left hand rule and it's
magnitude is given by F = BIL.
Where,
B = magnetic flux density,
I = current and

L = length of the conductor within the magnetic field.
Fleming's left hand rule: If we stretch the first finger, second finger and
thumb of our left hand to be perpendicular to each other AND direction of
magnetic field is represented by the first finger, direction of the current is
represented by second finger then the thumb represents the direction of the
force experienced by the current carrying conductor.
Classifications of DC Machines : (DC Motors and DC
Generators)
 Each DC machine can act as a generator or a motor.
 Hence, this classification is valid for both: DC generators and DC
motors.
 DC machines are usually classified on the basis of their field
excitation method.
This makes two broad categories of dc machines;
(i) Separately excited and
(ii) (ii) Self-excited.
 Separately excited: In separately excited dc machines, the field
winding is supplied from a separate power source. That means the field
winding is electrically separated from the armature circuit. Separately
excited DC generators are not commonly used because they are
relatively expensive due to the requirement of an additional power
source or circuitry. They are used in laboratories for research work, for
accurate speed control of DC motors with Ward-Leonard system and in
few other applications where self-excited DC generators are
unsatisfactory. In this type, the stator field flux may also be provided
with the help of permanent magnets (such as in the case of
a permanent magnet DC motors). A PMDC motor may be used in a
small toy car.
 Self-excited: In this type, field winding and armature winding are
interconnected in various ways to achieve a wide range of performance
characteristics (for example, field winding in series or parallel with the
armature winding).
In self-excited type of DC generator, the field winding is energized by the
current produced by themselves. A small amount of flux is always
present in the poles due to the residual magnetism. So, initially, current
induces in the armature conductors of a dc generator only due to the
residual magnetism. The field flux gradually increases as the induced
current starts flowing through the field winding.
Self-excited machines can be further classified as –
 Series wound – In this type, field winding is connected in series
with the armature winding. Therefore, the field winding carries
whole load current (armature current). That is why series winding

is designed with few turns of thick wire and the resistance is kept
very low (about 0.5 Ohm).
 Shunt wound – Here, field winding is connected in parallel with the
armature winding. Hence, the full voltage is applied across the field
winding. Shunt winding is made with a large number of turns and
the resistance is kept very high (about 100 Ohm). It takes only
small current which is less than 5% of the rated armature current.
 Compound wound – In this type, there are two sets of field
winding. One is connected in series and the other is connected in
parallel with the armature winding. Compound wound machines
are further divided as -
 Short shunt – field winding is connected in parallel with only
the armature winding
 Long shunt – field winding is connected in parallel with the
combination of series field winding and armature winding

3 Point Starter | Working Principle and Construction of
Three Point Starter
A 3 point starter in simple words is a device that helps in the starting and
running of a shunt wound DC motor or compound wound DC motor. Now
the question is why these types of DC motors require the assistance of the
starter in the first case. The only explanation to that is given by the presence
of back emf Eb, which plays a critical role in governing the operation of the
motor. The back emf, develops as the motor armature starts to rotate in
presence of the magnetic field, by generating action and counters the supply
voltage. This also essentially means, that the back emf at the starting is
zero, and develops gradually as the motor gathers speed.
The general motor emf equation E = Eb + Ia.Ra,
at starting is modified to E = Ia.Ra as at starting Eb = 0.
Thus we can well understand from the above equation that the current will
be dangerously high at starting (as armature resistance Ra is small) and
hence its important that we make use of a device like the 3 point starter to
limit the starting current to an allowable lower value.

Construction of 3 Point Starter
Construction wise a starter is a variable resistance, integrated into number
of sections as shown in the figure beside. The contact points of these
sections are called studs and are shown separately as OFF, 1, 2, 3, 4, 5,
RUN. Other than that there are 3 main points, referred to as
1. 'L' Line terminal. (Connected to positive of supply.)
2. 'A' Armature terminal. (Connected to the armature winding.)
3. 'F' Field terminal. (Connected to the field winding.)
And from there it gets the name 3 point starter. Now studying the
construction of 3 point starter in further details reveals that, the point 'L'
is connected to an electromagnet called overload release (OLR) as shown in
the figure. The other end of 'OLR' is connected to the lower end of
conducting lever of starter handle where a spring is also attached with it
and the starter handle contains also a soft iron piece housed on it. This
handle is free to move to the other side RUN against the force of the
spring. This spring brings back the handle to its original OFF position
under the influence of its own force. Another parallel path is derived from
the stud '1', given to the another electromagnet called No Volt Coil (NVC)
which is further connected to terminal 'F'. The starting resistance at
starting is entirely in series with the armature. The OLR and NVC acts as
the two protecting devices of the starter.
Working of Three Point Starter
Having studied its construction, let us now go into the working of the
3 point starter. To start with the handle is in the OFF position when the
supply to the DC motor is switched on. Then handle is slowly moved against
the spring force to make a contact with stud No. 1. At this point, field
winding of the shunt or the compound motor gets supply through the
parallel path provided to starting resistance, through No Voltage Coil. While
entire starting resistance comes in series with the armature. The high
starting armature current thus gets limited as the current equation at this
stage becomes Ia = E/(Ra+Rst). As the handle is moved further, it goes on
making contact with studs 2, 3, 4 etc., thus gradually cutting off the series
resistance from the armature circuit as the motor gathers speed. Finally
when the starter handle is in 'RUN' position, the entire starting resistance is
eliminated and the motor runs with normal speed. This is because back emf
is developed consequently with speed to counter the supply voltage and
reduce the armature current. So the external electrical resistance is not
required anymore, and is removed for optimum operation. The handle is
moved manually from OFF to the RUN position with development of speed.

Electrical Transformer - Basic Construction, Working And Types
Electrical transformer is a static electrical machine which transforms
electrical power from one circuit to another circuit, without changing the
frequency. Transformer can increase or decrease the voltage with
corresponding decrease or increase in current.
Working Principle of Transformer
The basic principle behind working of a transformer is the phenomenon of
mutual induction between two windings linked by common magnetic

flux. Basically a transformer consists of two inductive coils; primary winding
and secondary winding. The coils are electrically separated but magnetically
linked to each other.
When, primary winding is connected to a source of alternating voltage,
alternating magnetic flux is produced around the winding. The core provides
magnetic path for the flux, to get linked with the secondary winding. Most of
the flux gets linked with the secondary winding which is called as 'useful
flux' or main 'flux', and the flux which does not get linked with secondary
winding is called as 'leakage flux'. As the flux produced is alternating (the
direction of it is continuously changing), EMF gets induced in the secondary
winding according to Faraday's law of electromagnetic induction. This emf is
called 'mutually induced emf', and the frequency of mutually induced emf is
same as that of supplied emf. If the secondary winding is closed circuit, then
mutually induced current flows through it, and hence the electrical energy is
transferred from one circuit (primary) to another circuit (secondary)
Types Of Transformers
Transformers can be classified on different basis, like types of construction,
types of cooling etc.
(A) On the basis of construction
[1] Core type transformer
[2] Shell type transformer
(B) On the basis of their purpose
[1] Step up transformer: Voltage increases at secondary.
[2] Step down transformer: Voltage at secondary.
(C) On the basis of type of supply
[1] Single phase transformer
[2] Three phase transformer
(D) On the basis of their use
[1] Power transformer: Used
[2] Distribution transformer:
[3] Instrument transformer:
 Current transformer (CT)
 Potential transformer (PT)
(E) On the basis of cooling employed

[1] Oil-filled self cooled type
[2] Oil-filled water cooled type
[3] Air blast type (air cooled)
Losses in Transformer
(I) Core Losses or Iron Losses
Eddy current loss and hysteresis loss depend upon the magnetic properties
of the material used for the construction of core. Hence these losses are also
known as core losses or iron losses.
 Hysteresis loss in transformer:
Hysteresis loss is due to reversal of magnetization in the transformer
core. This loss depends upon the volume and grade of the iron,
frequency of magnetic reversals and value of flux density.
It can be given by, Steinmetz formula:
Wh= Kh Bmax1.6 f V (watts)
where, Kh = Steinmetz hysteresis constant
V = volume of the core in m3
 Eddy current loss in transformer:
In transformer, AC current is supplied to the primary winding which
sets up alternating magnetizing flux. When this flux links with
secondary winding, it produces induced emf in it. But some part of this
flux also gets linked with other conducting parts like steel core or iron
body or the transformer, which will result in induced emf in those parts,
causing small circulating current in them. This current is called as eddy
current. Due to these eddy currents, some energy will be dissipated in
the form of heat.
(II) Copper Loss in Transformer
Copper loss is due to ohmic resistance of the transformer windings. Copper
loss for the primary winding is I12R1 and for secondary winding is I22R2.
Where, I1 and I2 are current in primary and secondary winding respectively,
R1 and R2 are the resistances of primary and secondary winding
respectively. It is clear that Cu loss is proportional to square of the current,
and current depends on the load. Hence copper loss in transformer varies
with the load.

Efficiency of Transformer
Just like any other electrical machine, efficiency of a transformer can be
defined as the output power divided by the input power.
That is Efficiency = Output / Input
Transformers are the most highly efficient electrical devices.
Most of the transformers have full load efficiency between 95% to 98.5%.
As a transformer being highly efficient, output and input are having nearly
same value, and hence it is impractical to measure the efficiency of
transformer by using output / input.
A better method to find efficiency of a transformer is using,
Efficiency = (Input - Losses) / Input = 1 - (Losses / Input).
All day efficiency of a transformer is always less than ordinary efficiency of
it.

Condition For Maximum Efficiency
Hence, efficiency of a transformer will be maximum when copper loss and
iron losses are equal.
That is Copper loss = Iron loss.

Constructional details of Alternator(AC Generator)
Construction of Alternators:
An alternator has 3,-phase winding on the stator and a d.c. field winding on
the rotor.
a. Stator It is the stationary part of the machine and is built up of silicon
steel laminations having slots on its inner periphery. A 3-phase winding is
placed in these slots and serves as the armature winding of the alternator.
The armature winding is always connected in star and the neutral is
connected to ground.
b. Rotor The rotor carries a field winding which is supplied with direct
current through two slip rings by a separate d.c. source. This d.c. source
(called exciter) is generally a small d.c. shunt or compound generator
mounted on the shaft of the alternator. Rotor construction is of two types,
namely;
1. Salient (or projecting) pole type
2. Non-salient (or cylindrical) pole type
Salient pole type:

In this type, salient or projecting poles are mounted on a large circular steel
frame which is fixed to the shaft of the alternator as shown in Fig. (1). The
individual field pole windings are connected in series in such a way that
when the field winding is energized by the d.c. exciter, adjacent poles have
opposite polarities.
Non-salient pole type:
In this type, the rotor is made of smooth solid forged-steel radial cylinder
having a number of slots along the outer periphery. The field windings are
embedded in these slots and are connected in series to the slip rings
through which they are energized by the d.c. exciter. The regions forming
the poles are usually left un slotted as shown in Fig. (2). It is clear that the
poles formed are non-salient i.e., they do not project out from the rotor
surface.

Induction Motor | Working Principle | Types of Induction
Motor
One of the most common electrical motor used in most applications which is
known as induction motor. This motor is also called as asynchronous
motor because it runs at a speed less than its synchronous speed(Ns).
Synchronous speed is the speed of rotation of the magnetic field in a rotary
machine and it depends upon the frequency and number poles of the
machine.
An induction motor always runs at a speed less than synchronous speed
because the rotating magnetic field which is produced in the stator will
generate flux in the rotor which will make the rotor to rotate, but due to the
lagging of flux current in the rotor with flux current in the stator, the rotor
will never reach to its rotating magnetic field speed i.e. the synchronous
speed.
Working Principle of Induction Motor
We need to give double excitation to make a machine to rotate. For example
if we consider a DC motor, we will give one supply to the stator and another
to the rotor through brush arrangement. But in induction motor we give
only one supply. It is very simple, from the name itself we can understand
that induction process is involved. Actually when we are giving the supply to
the stator winding, flux will generate in the coil due to flow of current in the
coil. Now the rotor winding is arranged in such a way that it becomes short
circuited in the rotor itself. The flux from the stator will cut the coil in the
rotor and since the rotor coils are short circuited, according to Faraday's law
of electromagnetic induction, current will start flowing in the coil of the
rotor. When the current will flow, another flux will get generated in the rotor.
Now there will be two flux, one is stator flux and another is rotor flux and
the rotor flux will be lagging w.r.t to the stator flux. Due to this, the rotor
will feel a torque which will make the rotor to rotate in the direction of
rotating magnetic flux. So the speed of the rotor will be depending upon the
ac supply and the speed can be controlled by varying the input supply.
There are basically two types of induction motor that depend upon the
input supply –
[1] Single phase induction motor
a. Split phase induction motor
b. Capacitor start induction motor
c. Capacitor start capacitor run induction motor
d. Shaded pole induction motor
[2] Three phase induction motor
a. Squirrel cage induction motor
b. Slip ring induction motor

Single phase induction motor is not a self starting and three phase
induction motor is a self-starting motor.
A three phase induction motor runs on a three phase AC supply. 3 phase
induction motors are extensively used for various industrial applications.
Advantages:
 They have very simple and rugged (almost unbreakable) construction
 They are very reliable and having low cost
 They have high efficiency and good power factor
 Minimum maintenance required
 3 phase induction motor is self starting hence extra starting motor or
any special starting arrangement is not required
Disadvantages:
 Speed decreases with increase in load, just like a DC shunt motor
If speed is to be varied, we have sacrifice some of its efficiency

Torque-SlipCharacteristics ofInduction Motor
As the induction motor is located from no load to full load, its speed
decreases hence slip increases. Due to the increased. load, motor has to
produce more torque to satisfy load demand. The torque ultimately depends
on slip as explained earlier. The behaviour of motor can be easily judged by
sketching a curve obtained by plotting torque produced against slip of
induction motor.
The curve obtained by plotting torque against slip from s = 1 (at start)
to s = 0 (at synchronous speed) is called torque-slip characteristics of the
induction motor. It is very interesting to study the nature of torque-slip
characteristics.
We have seen that for a constant supply voltage, E2 is also constant. So we
can write torque equations as,
Now to judge the nature of torque-slip characteristics let us divide the slip
range (s = 0 to s = 1) into two parts and analyse them independently.
i) Low slip region :
In low slip region, 's' is very very small. Due to this, the term (s X2)2 is
so small as compared to R22 that it can be neglected.
Hence in low slip region torque is directly proportional to slip. So as load
increases, speed decreases, increasing the slip. This increases the torque
which satisfies the load demand.
Hence the graph is straight line in nature.
At N = Ns , s = 0 hence T = 0. As no torque is generated at N = Ns, motor
stops if it tries to achieve the synchronous speed.
Torque increases linearly in this region, of low slip values.
ii) High slip region :
In this region, slip is high i.e. slip value is approaching to 1. Here it can
be assumed that the term R22 is very very small as compared to (s X2)2.
Hence neglecting from the denominator, we get

So in high slip region torque is inversely proportional to the slip. Hence its
nature is like rectangular hyperbola.
Now when load increases, load demand increases but speed decreases.
As speed decreases, slip increases. In high slip region as T α1/s, torque
decreases as slip increases.
But torque must increases to satisfy the load demand. As torque
decreases, due to extra loading effect, speed further decreases and slip
further increases. Again torque decreases as T α1/s hence same load acts
as an extra load due to reduction in torque produced. Hence speed further
drops. Eventually motor comes to standstill condition. The motor can not
continue to rotate at any point in this high slip region. Hence this region is
called unstable region of operation.
So torque - slip characteristics has two parts,
1. Straight line called stable region of operation
2. Rectangular hyperbola called unstable region of operation.
In low slip region, as load increases, slip increases and torque also
increases linearly. Every motor has its own limit to produce a torque. The
maximum torque, the motor can produces as load increases is Tm which
occurs at s = sm. So linear behaviour continues till s = sm.
If load is increased beyond this limit, motor slip acts dominantly
pushing motor into high slip region. Due to unstable conditions, motor
comes to standstill condition at such a load. Hence i.e. maximum torque
which motor can produce is also called breakdown torque or pull out torque.
So range s = 0 to s = sm is called low slip region, known as stable region of
operation.
Motor always operates at a point in this region. And range s = sm to s = 1 is
called high slip region which is rectangular hyperbola, called unstable
region of operation. Motor can not continue to rotate at any point in this
region.
At s = 1, N = 0 i.e. start, motor produces a torque called starting torque
denoted as Tst.
The entire torque - slip characteristics is shown in the Fig. 1.

Applications of Three PhaseWoundRotorInduction Motors
[1] Wound rotor motors are suitable for loads requiring high starting
torque and where a lower starting current is required.
[2] The Wound rotor induction motors are also used for loads having high
inertia, which results in higher energy losses.
[3] Used for the loads which require a gradual build up of torque.
[4] Used for the loads that requires speed control.
[5] The wound rotor induction motors are used in conveyors, cranes,
pumps, elevators and compressors.
[6] The maximum torque is above 200 percent of the full load value while
the full load slip may be as low as 3 percent. The efficiency is about 90
%.
Applications of Three Phase Cage Rotor Induction Motors
Many polyphase cage induction motors are available in the market to meet
the demand of the several industrial applications and various starting and
running condition requirement. They are classified according to the Class.
Class A Motors
Class A motors have normal starting torque, high starting current and low
operating slip (0.005-0.015). The design has low resistance single cage
rotor. The efficiency of the motor is high at full load. Applications of Class A
motors are fans, blowers, centrifugal pumps, etc.
Class B Motors
Class B motors have normal starting torque, low starting current and low
starting current and low operating slip. The motor is designed, in such a
way to withstand the high leakage reactance; as a result, the starting
current is reduced. The starting torque is maintained by use of a double
cage or deep bar rotor.
The Class B motors are most commonly used motor and used for full voltage
starting. The applications and the starting torque are same as that of Class
A motors.
Class C Motors
The class C motors have high starting torque and low starting current. Such
motors are of the double cage and deep bar and has higher rotor resistance.
The loads are compressors, conveyors, reciprocating pumps, crushers,
etc.

Class D Motors
Class D motors have the highest starting torque as compared to all the other
class of motors. The bars of the rotor cage are made up of brass. These types
of motors have low starting current and high operating slip. The value of full
load operating slip varies between 8 to 15%. Thus, the efficiency of the
motor is low.
These motors are suitable for driving intermittent loads which require
frequent acceleration and high loads. For example – punch presses,
bulldozers and die stamping machines. When the motor is driving the high
impact loads, it is coupled to a flywheel to provide kinetic energy.

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