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SNS COLLEGE OF ENGINEERING
Kurumbapalayam(Po), Coimbatore – 641 107
Accredited by NAAC-UGC with ‘A’ Grade
Approved by AICTE & Affiliated to Anna University, Chennai
INTERNAL ASSESSMENT EXAMINATIONS - I
COURSE: B.E.CIVIL & MECH
GE6252- Basic Electrical and Electronics Engineering
Class: I CIVIL and I MECH / II Sem.
SET A
2016(FN)
Duration: 2 Hours
Answer ALL questions
Date:
9th
March
Maximum: 66 Marks
PART A - (9 X 2 = 18 marks)
1.
2
List the significance of Back EMF in DC motor.
2
Ans: When the armature of a d.c. motor rotates under the influence of the driving
torque, the armature conductors move through the magnetic field and hence e.m.f. is
induced in them as in a generator. The induced e.m.f. acts in opposite direction to the
applied voltage V (Lenz's law) and in known as back or counter e.m.f.
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Classify the transformer by its construction.
Ans: Core- Type Transformer , Shell-Type Transformer, Berry Type
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4
5
6
Induction Motor is not a self starting. Justify.
2
According to double field revolving theory, any alternating quantity can be resolved
into two components, each component have magnitude equal to the half of the
maximum magnitude of the alternating quantity and both these component rotates in
opposite direction to each other.
Enumerate the Properties of semiconductor.
2
Semiconductor materials are nominally small band gap insulators. The defining
property of a semiconductor material is that it can be doped with impurities that alter its
electronic properties in a controllable way.
Define Depletion region in PN junction diode.
2
Depletion region or depletion layer is a region in a P-N junction diode where no mobile
charge carriers are present. Depletion layer acts like a barrier that opposes the flow of
electrons from n-side and holes from p-side.
Differentiate Zener Breakdown and Avalanche Breakdown.
2
Zener breakdown
Avalanche breakdown
This is observed in Zener diodes having This is observed in Zener diodes having
Vz 5 to 8 volts.
Vz greater than 8 volts.
The valence electrons are pulled into The valence electrons are pushed into
conduction due to very intense electric conduction band due to the energy
field appearing across the narrow imparted by colliding accelerated
depletion region.
minority carries.
The V-I characteristics with the
V-I characteristics with Zener the break
avalanche
breakdown
increases
gradually. It is not as sharp as that with
down is very sharp.
the Zener breakdown
The breakdown voltage decreases with The breakdown voltage increases with
increase in temperature
increase in temperature.
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8
9
Define Transformer utilization factor.
Ans: Transformer utilization factor (TUF) of a rectifier circuit is defined as the ratio of 2
the DC power available at the load resistor and the AC rating of the secondary coil of a
transformer. T
What is Voltage Regulator?
(𝑉𝑛𝑙 − 𝑉𝑓𝑙) ∗ 100
𝑉𝑅𝑖𝑛 % =
𝑉𝑓𝑙
where Vnll is voltage at no load and Vfl is voltage at full load
2
BJT is a current controlled devic. Justify.
output current is a function of input current and hence BJT is called as a is a Current 2
Controlled Device
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PART B - (3 X 16 = 48 marks)
8.
(a)
(i)
Outline different Types of Single phase Induction Motor with neat diagram 16
(any three types).
Split Phase Induction Motor
In addition to the main winding or running winding, the stator of single phase
induction motor carries another winding called auxiliary winding or starting
winding. A centrifugal switch is connected in series with auxiliary winding.
The purpose of this switch is to disconnect the auxiliary winding from the
main circuit when the motor attains a speed up to 75 to 80% of the
synchronous speed. We know that the running winding is inductive in nature.
Our aim is to create the phase difference between the two winding and this is
possible if the starting winding carries high resistance. Let us say Irun is the
current flowing through the main or running winding, Istart is the current
flowing in starting winding, and VT is the supply voltage. split phase
induction motor We know that for highly resistive winding the current is
almost in phase with the voltage and for highly inductive winding the current
lag behind the voltage by large angle. The starting winding is highly resistive
so, the current flowing in the starting winding lags behind the applied voltage
by very small angle and the running winding is highly inductive in nature so,
the current flowing in running winding lags behind applied voltage by large
angle. The resultant of these two current is IT. The resultant of these two
current produce rotating magnetic field which rotates in one direction. In
split phase induction motor the starting and main current get splitted from
each other by some angle so this motor got its name as split phase induction
motor.
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Applications of Split Phase Induction Motor
Split phase induction motors have low starting current and moderate starting
torque. So these motors are used in fans, blowers, centrifugal pumps,
washing machine, grinder, lathes, air conditioning fans, etc. These motors are
available in the size ranging from 1 / 20 to 1 / 2 KW.
Capacitor Start IM and Capacitor Start Capacitor Run IM
capacitor start run induction motor The working principle and construction of
Capacitor start inductor motors and capacitor start capacitor run induction
motors are almost the same. We already know that single phase induction
motor is not self starting because the magnetic field produced is not rotating
type. In order to produce rotating magnetic field there must be some phase
difference. In case of split phase induction motor we use resistance for
creating phase difference but here we use capacitor for this purpose. We are
familiar with this fact that the current flowing through the capacitor leads the
voltage. So, in capacitor start inductor motor and capacitor start capacitor run
induction motor we are using two winding, the main winding and the starting
winding. With starting winding we connect a capacitor so the current flowing
in the capacitor i.e Ist leads the applied voltage by some angle, φst.
The running winding is inductive in nature so, the current flowing in running
winding lags behind applied voltage by an angle, φm. Now there occur large
phase angle differences between these two currents which produce an
resultant current, I and this will produce a rotating magnetic field. Since the
torque produced by these motors depends upon the phase angle difference,
which is almost 90°. So, these motors produce very high starting torque. In
case of capacitor start induction motor, the centrifugal switch is provided so
as to disconnect the starting winding when the motor attains a speed up to 75
to 80% of the synchronous speed but in case of capacitor start capacitors run
induction motor there is no centrifugal switch so, the >capacitor remains in
the circuit and helps to improve the power factor and the running conditions
of single phase induction motor.
Application of Capacitor Start IM and Capacitor Start Capacitor Run IM
These motors have high starting torque hence they are used in conveyors,
grinder, air conditioners, compressor, etc. They are available up to 6 KW.
Permanent Split Capacitor (PSC) Motor
It has a cage rotor and stator. Stator has two windings – main and auxiliary
winding. It has only one capacitor in series with starting winding. It has no
starting switch. Advantages and Applications No centrifugal switch is
needed. It has higher efficiency and pull out torque. It finds applications in
fans and blowers in heaters and air conditioners. It is also used to drive office
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machinery.
Shaded Pole Single Phase Induction Motors
\
shaded pole single phase induction motor The stator of the shaded pole single
phase induction motor has salient or projected poles. These poles are shaded
by copper band or ring which is inductive in nature. The poles are divided
into two unequal halves. The smaller portion carries the copper band and is
called as shaded portion of the pole.
ACTION: When a single phase supply is given to the stator of shaded pole
induction motor an alternating flux is produced. This change of flux induces
emf in the shaded coil. Since this shaded portion is short circuited, the
current is produced in it in such a direction to oppose the main flux. The flux
in shaded pole lags behind the flux in the unshaded pole. The phase
difference between these two fluxes produces resultant rotating flux.
We know that the stator winding current is alternating in nature and so is the
flux produced by the stator current. In order to clearly understand the
working of shaded pole induction motor consider three regionsWhen the flux changes its value from zero to nearly maximum positive
value.
When the flux remains almost constant at its maximum value.
When the flux decreases from maximum positive value to zero.
REGION 1: When the flux changes its value from zero to nearly maximum
positive value – In this region the rate of rise of flux and hence current is
very high. According to Faraday's law whenever there is change in flux emf
gets induced. Since the copper band is short circuit the current starts flowing
in the copper band due to this induced emf. This current in copper band
produces its own flux. Now according to Lenz's law the direction of this
current in copper band is such that it opposes its own cause i.e rise in current.
So the shaded ring flux opposes the main flux, which leads to the crowding
of flux in non shaded part of stator and the flux weaken in shaded part. This
non uniform distribution of flux causes magnetic axis to shift in the middle of
the non shaded part.
REGION 2: When the flux remains almost constant at its maximum value- In
this region the rate of rise of current and hence flux remains almost constant.
Hence there is very little induced emf in the shaded portion. The flux
produced by this induced emf has no effect on the main flux and hence
distribution of flux remains uniform and the magnetic axis lies at the center
of the pole.
REGION 3: When the flux decreases from maximum positive value to zero In this region the rate of decrease in the flux and hence current is very high.
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According to Faraday's law whenever there is change in flux emf gets
induced. Since the copper band is short circuit the current starts flowing in
the copper band due to this induced emf. This current in copper band
produces its own flux. Now according to Lenz's law the direction of the
current in copper band is such that it opposes its own cause i.e decrease in
current. So the shaded ring flux aids the main flux, which leads to the
crowding of flux in shaded part of stator and the flux weaken in non shaded
part. This non uniform distribution of flux causes magnetic axis to shift in the
middle of the shaded part of the pole. This shifting of magnetic axis
continues for negative cycle also and leads to the production of rotating
magnetic field. The direction of this field is from non shaded part of the pole
to the shaded part of the pole.
OR
(b)
(i)
Enlighten the Principle and Construction of Transformer with neat sketch.
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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.
The figure at right shows the simplest form of a transformer. 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
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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).
Basic Construction Of Transformer
Basically a transformer consists of two inductive windings and a laminated
steel core. The coils are insulated from each other as well as from the steel
core. A transformer may also consist of a container for winding and core
assembly (called as tank), suitable bushings to take our the terminals, oil
conservator to provide oil in the transformer tank for cooling purposes etc.
The figure at left illustrates the basic construction of a transformer.
In all types of transformers, core is constructed by assembling (stacking)
laminated sheets of steel, with minimum air-gap between them (to achieve
continuous magnetic path). The steel used is having high silicon content and
sometimes heat treated, to provide high permeability and low hysteresis loss.
Laminated sheets of steel are used to reduce eddy current loss. The sheets are
cut in the shape as E,I and L. To avoid high reluctance at joints, laminations
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are stacked by alternating the sides of joint. That is, if joints of first sheet
assembly are at front face, the joints of following assemble are kept at back
face.
Types Of Transformers
Transformers can be classified on different basis, like types of construction,
types
of
cooling
etc.
(A) On the basis of construction, transformers can be classified into two
types as; (i) Core type transformer and (ii) Shell type transformer, which are
described
below.
(I) Core Type Transformer
In core type transformer, windings are cylindrical former wound, mounted
on the core limbs as shown in the figure above. The cylindrical coils have
different layers and each layer is insulated from each other. Materials like
paper, cloth or mica can be used for insulation. Low voltage windings are
placed nearer to the core, as they are easier to insulate.
(Ii) Shell Type Transformer
The coils are former wound and mounted in layers stacked with insulation
between them. A shell type transformer may have simple rectangular form
(as shown in above fig), or
it may have a distributed form.
(B) On the basis of their purpose
1. Step up transformer: Voltage increases (with subsequent decrease in
current) at secondary.
2. Step down transformer: Voltage decreases (with subsequent increase in
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current) 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 in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively
lower rating than that of power transformers.
3. Instrument transformer: Used in relay and protection purpose in
different instruments in industries

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)
9.
(a)
(i)
Narrate the Characteristics of PN Junction Diode with relevant diagram.
16
-N junction diode is the most fundamental and the simplest electronics
device. When one side of an intrinsic semiconductor is doped with acceptor
i.e, one side is made p-type by doping with n-type material, a p-n junction
diode is formed. This is a two terminal device. It appeared in 1950’s. P-N
junction can be step graded or linearly graded. In step graded the
concentration of dopants both, in n - side and in p - side are constant up to
the junction. But in linearly graded junction, the doping concentration varies
almost linearly with the distance from the junction. When the P-N diode is in
unbiased condition that is no voltage is applied across it, electrons will
defuse through the junction to p-side and holes will defuse through the
junction to n-side and they combine with each other. Thus the acceptor atom
near the p-side and donor atom near n-side are left unutilized. An electron
field is generated by these uncovered charges. This opposes further diffusion
of carriers. So, no movement of region is known as space charge or depletion
region.
P-N Junction Diode Characteristics
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Let's a voltage V is applied across a p-n junction and total current I, flows
through the junction. It is given as. I = IS[exp(eV/ɳKBT) - 1] Here, IS =
reverse saturation current e = charge of electron ɳ = emission co-efficient KB
= Boltzmann constant T = temperature The current voltage characteristics
plot is given below. The current voltage characteristics. characteristics of p n
junc When, V is positive the junction is forward biased and when V is
negative, the junction is reversing biased. When V is negative and less than
VTH, the current is very small. But when V exceeds VTH, the current
suddenly becomes very high. The voltage VTH is known as threshold or cut
in voltage. For Silicon diode VTH = 0.6 V. At a reverse voltage
corresponding to the point P, there is abrupt increment in reverse current.
The PQ portion of the characteristics is known as breakdown region.
OR
(b)
Explain the working of Half wave Rectifier with neat sketch and necessary 16
diagram
Working of a Half wave rectifier
The half-wave rectifier circuit using a semiconductor diode (D) with a load
resistance RL but no smoothing filter is given in figure. The diode is
connected in series with the secondary of the transformer and the load
resistance RL. The primary of the transformer is being connected to the ac
supply mains.
The ac voltage across the secondary winding changes polarities after every
half cycle of input wave. During the positive half-cycles of the input ac
voltage i.e. when upper end of the secondary winding is positive w.r.t. its
lower end, the diode is forward biased and therefore conducts current. If the
forward resistance of the diode is assumed to be zero (in practice, however, a
small resistance exists) the input voltage during the positive half-cycles is
directly applied to the load resistance RL, making its upper end positive
w.r.t. its lower end. The waveforms of the output current and output voltage
are of the same shape as that of the input ac voltage.
During the negative half cycles of the input ac voltage i.e. when the lower
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end of the secondary winding is positive w.r.t. its upper end, the diode is
reverse biased and so does not conduct. Thus during the negative half cycles
of the input ac voltage, the current through and voltage across the load
remains zero. The reverse current, being very small in magnitude, is
neglected. Thus for the negative half cycles no power is delivered to the load.
Thus the output voltage (VL) developed across load resistance RL is a series
of positive half cycles of alternating voltage, with intervening very small
constant negative voltage levels, It is obvious from the figure that the output
is not a steady dc, but only a pulsating dc wave. To make the output wave
smooth and useful in a DC power supply, we have to use a filter across the
load. Since only half-cycles of the input wave are used, it is called a half
wave rectifier.
Half Wave Rectifier with Capacitor Filter – Circuit Diagram & Output
Waveform
Half Wave Rectifier Analysis
The following parameters will be explained for the analysis of Half Wave
Rectifier:1.
Peak Inverse Voltage (PIV)
Peak Inverse Voltage (PIV) rating of a diode is important in its design stages.
It is the maximum voltage that the rectifying diode has to withstand, during
the reverse biased period.
When the diode is reverse biased, during the negative half cycle, there will
be no current flow through the load resistor RL. Hence, there will be no
voltage drop through the load resistance RL which causes the entire input
voltage to appear across the diode. Thus VSMAX, the peak secondary
voltage, appears across the diode. Therefore,
Peak Inverse Voltage (PIV) of half wave rectifier = VSMAX
2.
Average and Peak Currents in the diode
By assuming that the voltage across the transformer secondary be sinusoidal
of peak values VSMAX, instantaneous value of the voltage given to the
rectifier can be written as
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Instantaneous value of voltage applied to Half Wave Rectifier
Assuming that the diode has a forward resistance of RF ohms and infinite
reverse resistance value, the current flowing through the output load
resistance RL is
Current flowing through the diode
IMAX = VSMAX/(RF + RL)
3.
DC Output Current
The dc output current is given as
DC Output Current of Half Wave Rectifier
Substituting the value of IMAX
for the equation IMAX =
VSMAX/(RF + RL), we have
Idc = VSMAX/ = VSMAX/ RL if RL >> RF
4.
DC Output Voltage
Dc value of voltage across the load is given by
Vdc = Idc RL = VSMAX/pi(RF + RL)X RL = VSMAX/{1+RF/RL }
If RL >> RF, Vdc = VSMAX/pi
5.
Root Mean Square (RMS) Value of Current
RMS value of current flowing through the diode is given as
RMS value of current flowing through diode in half wave rectifier
6.
Root Mean Square (RMS) Value of Output Voltage
RMS value of voltage across the load is given as
VLrms = Irms RL = VSMAX RL /2(RF + RL) = VSMAX/2{1+RF/RL }
If RL >> RF, VLrms = VSMAX/2
7.
Rectification Efficiency
Rectification efficiency is defined as the ratio between the output power to
the ac input power.
Efficiency, Ƞ = DC power delivered to the load/AC input power from the
transformer = Pdc/Pac
DC power delivered to the load, Pdc = I2dc RL = (Imax/pi)2 RL
AC power input to the transformer, Pac = Power dissipated in diode junction
+ Power dissipated in load resistance RL
= I2rms RF + I2rms RL = {I2MAX/4}[ RF + RL]
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So, Rectification Efficiency, Ƞ = Pdc/Pac = {4/2}[ RL/ (RF + RL)] =
0.406/{1+ RF/RL }
The maximum efficiency that can be obtained by the half wave rectifier is
40.6%. This is obtained if RF is neglected.
8.
Ripple Factor
Ripple factor is in fact a measure of the remaining alternating components in
a filtered rectifier output. It is the ratio of the effective value of the ac
components of voltage (or current) present in the output from the rectifier to
the dc component in output voltage (or current).
The effective value of the load current is given as
I2 =I2dc+I21+I22+I24 = I2dc +I2ac
Where, I1,I2, I4 and so onare the rms values of fundamental, second, fourth
and so on harmonics and I2acis the sum of the squares if the rms values of
the ac components.
So, ripple factor, γ = Iac/ Idc = I2 – I2dc)/ Idc = {( Irms/ Idc2)-1} = Kf2 –
1)
Where Kf is the form factor of the input voltage. For half wave rectifier,
form factor is given as
Kf = Irms /Iavg = (Imax/2)/ (Imax/pi) = pi/2 = 1.57
So, ripple factor, γ = (1.572 – 1) = 1.21
9.
Regulation
The variation of the output voltage as a function of dc load current is called
regulation. Percentage regulation is given as
% Regulation = {(Vno-load – Vfull-load)/ Vfull-load}* 100
Fror an ideal power supply, the output voltage should be independent of load
current and the percentage regulation should be equal to zero.
10.
(a)
With a help of neat diagram discuses the characteristics of BJT in Common 16
Base Configuration.
The Common Base (CB) Configuration
As its name suggests, in the Common Base or grounded base configuration,
the BASE connection is common to both the input signal AND the output
signal with the input signal being applied between the base and the emitter
terminals. The corresponding output signal is taken from between the base
and the collector terminals as shown with the base terminal grounded or
connected to a fixed reference voltage point.
The input current flowing into the emitter is quite large as its the sum of both
the base current and collector current respectively therefore, the collector
current output is less than the emitter current input resulting in a current gain
for this type of circuit of “1” (unity) or less, in other words the common base
configuration “attenuates” the input signal.
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The Common Base Transistor Circuit
common base configuration
This type of amplifier configuration is a non-inverting voltage amplifier
circuit, in that the signal voltages Vin and Vout are “in-phase”. This type of
transistor arrangement is not very common due to its unusually high voltage
gain characteristics. Its input characteristics represent that of a forward
biased diode while the output characteristics represent that of an illuminated
photo-diode.
Also this type of bipolar transistor configuration has a high ratio of output to
input resistance or more importantly “load” resistance ( RL ) to “input”
resistance ( Rin ) giving it a value of “Resistance Gain”. Then the voltage
gain ( Av ) for a common base configuration is therefore given as:
Common Base Voltage Gain
Av = (Vout/Vin ) = Ic/Ie
common base transistor gain
Where: Ic/Ie is the current gain, alpha ( α ) and RL/Rin is the resistance gain.
The common base circuit is generally only used in single stage amplifier
circuits such as microphone pre-amplifier or radio frequency ( Rf ) amplifiers
due to its very good high frequency response.
(b)
OR
With a help of neat diagram discuses the characteristics of BJT in Common 16
Emitter Configuration.
The Common Emitter (CE) Configuration
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In the Common Emitter or grounded emitter configuration, the input signal is
applied between the base and the emitter, while the output is taken from
between the collector and the emitter as shown. This type of configuration is
the most commonly used circuit for transistor based amplifiers and which
represents the “normal” method of bipolar transistor connection.
The common emitter amplifier configuration produces the highest current
and power gain of all the three bipolar transistor configurations. This is
mainly because the input impedance is LOW as it is connected to a forward
biased PN-junction, while the output impedance is HIGH as it is taken from a
reverse biased PN-junction.
Ie= Ic + Ib
Ai + Ie/Ib = (Ic +Ib)/Ib
Ai = β+1
This type of bipolar transistor configuration is a non-inverting circuit in that
the signal voltages of Vin and Vout are “in-phase”. It has a voltage gain that
is always less than “1” (unity). The load resistance of the common collector
transistor receives both the base and collector currents giving a large current
gain (as with the common emitter configuration) therefore, providing good
current amplification with very little voltage gain.
This type of bipolar transistor configuration is a non-inverting circuit in that
the signal voltages of Vin and Vout are “in-phase”. It has a voltage gain that
is always less than “1” (unity). The load resistance of the common collector
transistor receives both the base and collector currents giving a large current
gain (as with the common emitter configuration) therefore, providing good
current amplification with very little voltage gain.
*****
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