Tuesday, June 19, 2012
GE 2151 BASIC ELECTRICAL AND ELECTRONICS ENGINEERING(BEEE) IMPORTANT QUESTIONS WITH ANSWERS ( 2 , 6, 10 MARKS QUESTIONS ) (FOR FIRST YEAR STUDENTS ) -by R.Anirudhan
BASIC ELECTRICAL AND ELECTRONICS ENGINEERING
DOWNLOAD LINK: BEEE Important Questions 2marks, 16marks with Answers
2 MARKS
1. What is meant by charge?
Charge is an electrical property of the atomic
particles which matter
consists. The charge of an electron is so small.
Charge in motion represents
current. The unit of charge is coulomb.
2. What is meant by Current?
The flow of free electrons in a conductor is called
current. Unit is ampere
(A).
I = Q/t
3. What is meant by Voltage?
The poterntial difference between two points is called
as voltage. Unit is
Volts (V).
V=W/Q , W=work done in joules & Q = charge in
coulombs
4. State Ohm’s Law.
The potential difference across any two ends of a
conductor is directly
proportional to the current flowing between the two
ends provided the
temperature of the conductor remains constant.
5. State Krichoff’s Voltage Law
KVL states that the algebraic sum of voltages in a
closed path is zero.
6. State
Krichoff’s current Law.
KCL states that the algebraic sum of currents in a
node is zero.
7. Give
notes on Nodal Analysis.
• KCL is
used.
• No: of
equations = n-1, n=no: of nodes
8. Give notes on Mesh Analysis.
• KVL is used
• Here mesh
currents are found.
9. Give short notes on resistor.
It is a property of a substance3 which opposes the
flow of electrons. It is
denoted by R and its unit is Ohm (_)
10. Distinguish between a Branch and a node of a
circuit.
A pair of network which connects the various points of
the network is
called branch
A point at which two or more elements are joined
together is called node.
11. Distinguish between a mesh and a loop of a circuit.
A mesh is a loop that does not contain other loops.
All meshes are loop,
but all loops are not meshes.
A loop is any closed path of branches
12. Write down the formula for a star connected network
is converted into a
delta network?
RA=( R1 R2)/(
R1 +R2+ R3)
RB=( R1 R3)/(
R1 +R2+ R3)
RC=( R2 R3)/(
R1 +R2+ R3)
13. Write down the formula for a delta connected
network is converted into a
star network?
R1=( RARB+RBRC+RCRA)/RC
R2=( RARB+RBRC+RCRA)/RB
R3=( RARB+RBRC+RCRA)/RA
14. Define line currents and phase currents?
• The
currents flowing in the lines are called as line currents
• The
currents flowing through phase are called phase currents
15. Define line voltage and phase voltage?
The voltage across one phase and neutral is called
line voltage & the
voltage between two lines is called phase voltage
16. Give the phase value & Line value of a star
connected system.
VL= 3Vph
17. Give the phase value and line valued of a delta
connected system.
IL= 3Iph
18. What is the power equation for a star connected
system?
P= 3I V cosΦ
L L W
19. What is the power equation for a delta connected
system?
P= 3I V cosΦ
L L W
20. What is meant by Real power?
Real power means the useful power transfer from source
to load. Unit is
watts.
21. What is meant by apparent power?
Apparent power is the product of voltage and current
and it is not true
power. Unit is VA
22. What is reactive power?
If we consider the circuit as purely inductive the
output power is reactive
power. Its unit is VAR
23. Define Instrument.
Instrument is defined as a device for determining the
value or magnitude
of a quantity or variable.
24. Mention the two main differences between an ammeter
and a voltmeter.
Ammeter Voltmeter
It is a current measuring device it is a voltage
measuring deveice
Always connected in series with circuit Always connected in parallel with
circuit
The resistance is very small The resistance is very
high
25. Give short notes on resistor.
It is a property of a substance3 which opposes the
flow of electrons. It is
denoted by R and its unit is Ohm (_)
26. What is control system?
A system consists of a number of components connected
together to
perform a specific function . In a system when the
output quantity is controlled by
varying the input quantity then the system is called
control system.
27. What are the two major types of control system?
The two major types of control system are open loop
and closed loop
28. .Define open loop control system.
The control system in which the output quantity has no
effect upon the
input quantity are called open loop control system.
This means that the output is
not feedback to the input for correction.
29. .Define closed loop control system.
The control system in which the output has an effect
upon the input
quantity so as to maintain the desired output value
are called closed loop control
system
30. Mention the errors in Moving iron instruments.
• Hysteresis
error
• Temperature
error
• Stray
magnetic field error
• Frequency
error
• Eddy
current error
31. Mention any two precautions to be taken while using
an Ammeter.
• It should
never be connected across any source.
• The
polarity must be observed correctly.
• First use
the highest range and then decrease the voltage range until
the sufficient deflection is obtained.
32. Give
some applications of DC motor.
Shunt : driving constant speed, lathes, centrifugal pumps, machine tools,
blowers and fans, reciprocating pumps
Series : electric locomotives, rapid transit systems, trolley cars, cranes
and
hoists, conveyors
Compound : elevators, air compressors, rolling mills, heavy
planners
33. Define slip.
S = Ns
– Nr
Ns
Where, Ns
= synchronous speed in rpm.
Nr
= rotor speed in rpm
S = Slip
34. Define synchronous speed.
It is given by Ns =
120f / p rpm.
Where Ns
= synchronous speed, p = no. of stator
poles, f = supply frequency in
Hz
35. Why a single phase induction motor does not self
start?
When a single phase supply is fed to the single phase
induction motor. Its
stator winding produces a flux which only alternates
along one space axis. It is
not a synchronously revolving field, as in the case of
a 2 or 3phase stator winding,
fed from 2 or 3 phase supply.
36. Is Induction motor runs with synchronous speed or
not.
Induction motor never runs with synchronous speed. It
will stop if it tries
to achieve synchronous speed.
37. Define Form factor and Crest factor.
Form factor= RMS value
Average Value
Crest(peak) factor=Maximum Value
RMS value
38. . Which type of instrument is called as universal
instrument?
The moving iron instrument are known as universal
instruments, because
these instruments can be used for AC and DC.
39. What are the applications of MI instruments?
i) Used as multirange ammeters and voltmeters.
ii) Used as in expensive indicators such as charging
and discharging
current indicators in automobiles.
iii)Extensively used in industries for measurement of
AC voltage and
current where errors of the order of 5% to 10% are
accepetable.
40. What is meant by eddy current damping?
When the conductor moves in a magnetic field an emf is
induced in it and
if a closed path is provided ,a current flows known as
eddy current. This current
intersect with the magnetic field to produce an
electromagnetic torque , which
opposes the deflecting torque.
41. .How is electrical power measured?
i) Using Voltmeter-ammeter method for DC circuits.
ii)Using Watt meters for AC circuits.
42. .What do you mean by compensation coil in a
wattmeter?
By connecting a compensating coil in series with a
p[ressure coil ,The
error caused by the pressure coil flowing in the current
coil can be neutralized.
43. What are the three types of power used in a a.c
circuit?
i) Real power or active power P=EI cosq
ii) Reactive power Q=EI sinq
iii) Apparent power,S=EI
44. Define average value.
The average value of an alternating current is that
value of steady direct
current which transfers the same charge as the
alternating current flowing for the
same time.
45. Define RMS value.
The effective value of an alternating current is that
value of steady ,direct
current which produces the same heat as that produced
by the alternating current
when passed which produces the same heat as that
produced by the alternating
current when passed through the same resistance for
the same interval of time.
46. Define reactive power.
The power consumed by a pure reactance (XL or
Xc ) in a a.c circuit is
called reactive power. The unit is VAR. Q=EIsinq.
47. What is the basic principle of a dc generator?
Basic principle of a dc generator is Faraday’s law of
electromagnetic
induction.That is whenever a conductor is moved in
amagnetic field dynamically
induced emf is produced in that conductor.
48. .What is the purpose of interpoles in modern d.c
machine?
In modern d.c machines commutating poles or interpoles
are provided to
improve commutation.
49. What is the use of commutator and brush in a d.c
machine?
The commutator converts the alternating emf into
unidirectional or direct emf.
The brushes are mainly used to collect current from
the commutator.
50. What is a d.c series motor?
In a d.c series motor,the field winding is connected
in series with the
armature.The field winding should have less number of
turns of thick wire.
51. Why a series motor cannot be started without any
load?
Series motor cannot be started without any load
because under no load
condition the starting torque is less and motor
rotates at dangerous speed and may
be damaged.
52. What is meant by transformer?
The transformeris a static piece of apparatus by means
of which electrical
energy is transformed from one circuit to another with
desired change in voltage
and current , without any change in the frequency.It
works on the principle of
mutual induction.
53. What are the different types of single phase motor?
i)Single phase induction motor
ii)Single phase synchronous motor.
iii)Single phase series motor
54. What are the two types of rotors of an induction
motor?
i) Squirrel cage rotor
ii)Slip ring or wound rotor
6 MARKS
1. A 3 φ 4
pole 50 hz induction motor runs at 1460 r.p.m. find its % of slip.
Solution
N s
= 120f/p
= 120*50/4
= 1500r.p.m.
Running speed of motor
n= 1460r.p.m.
Slip S=( N s–N)/ N s*100
=(1500-1460) x 100 / 1500
= 2.667%
2. Explain the working principle of Transformer.
A Transformer is a device that transfers electrical
energy from one circuit to
another by electromagnetic induction (transformer
action). The electrical energy is
always transferred without a change in frequency, but
may involve changes in
magnitudes of voltage and current. Because a
transformer works on the principle of
electromagnetic induction, it must be used with an
input source voltage that varies in
amplitude. There are many types of power that fit this
description; for ease of explanation
and understanding, transformer action will be
explained using an ac voltage as the input
source.
The amount of power used by the load of an electrical
circuit is equal to the
current in the load times the voltage across the load,
or P = EI. If, for example, the load in
an electrical circuit requires an input of 2 amperes
at 10 volts (20 watts) and the source is
capable of delivering only 1 ampere at 20 volts, the
circuit could not normally be used
with this particular source. However, if a transformer
is connected between the source
and the load, the voltage can be decreased (stepped
down) to 10 volts and the current
increased (stepped up) to 2 amperes. Notice in the
above case that the power remains the
same. That is, 20 volts times 1 ampere equals the same
power as 10 volts times 2
amperes.
A Transformer consists of the following parts
• A primary
coil or winding.
• A secondary
coil or winding.
• A core that
supports the coils or windings.
The primary winding is connected to a 50 hertz ac
voltage source. The magnetic
field (flux) builds up (expands) and collapses
(contracts) about the primary winding. The
expanding and contracting magnetic field around the
primary winding cuts the secondary
winding and induces an alternating voltage into the
winding. This voltage causes
alternating current to flow through the load. The
voltage may be stepped up or down
depending on the design of the primary and secondary
windings.
3. Calculate the amount of resistance (R) in a circuit, given
values of voltage (E) and
current (I):
the amount of resistance (R) offered by the lamp
4. calculate the amount of voltage supplied by a battery,
given values of
current (I) and resistance (R):
the amount of voltage provided by the battery
5. Calculate the electric power in the given circuit
and discuss the effect of
increasing the battery voltage.
The formula for determining the power in an electric
circuit: by multiplying the
voltage in "volts" by the current in
"amps" we arrive at an answer in "watts." Let's apply
this to the given circuit.
In the above circuit, we know we have a battery
voltage of 18 volts and a
lamp resistance of 3 _. Using
Ohm's Law to determine current, we get:
Now that we know the current, we can take that value
and multiply it by
the voltage to determine power:
6. What is meant by DEFLECTING TORQUE ?
The deflecting torque is produced by making use of one
of the magnetic,
chemical, electrostatic and electromagnetic induction
effects of current or voltage
and causes the moving system of the instrument to move
from its zero position
when the instrument is connected in an electrical
circuit to measure the electrical
quantity. The method of producing this torque depend
upon the type of
instrument. In attracting the type of instrument, this
torque to equal to
Td
= 1/2 I2 dL/dθ
Whereas in Pmmc instruments
Td
= Bilur
Where B - magnetic density
i - current flowing
l - length of coil
u - number of turn
r - radius of coil
7. Find the voltage across each resistors in the
following circuit.
8. The effective resistance of two resistors connected
in series is 100 .
When connected in parallel, then effective value in 24
ohm’s.
Determine the value of two resistors
Series R1+R2=100 => R2 =100 - R1
R1R2/R1+R2 = 24
R1R2/100 = 24
R1R2 =2400
R1 (100-R1) = 2400
100 R1-R1^2-2400 = 0
R1^2-100 R1 + 2400 = 0
(R1-60)(R1-40) = 0
There Fore R1 = 60; R1 = 40
When R1 = 60 ; R2 = 100 – 60 = 40
When R1 = 40 ; R2 = 100 - 40 = 60
9. Find the Req between two points A & B.
1/Req = ½+1/3+1/3 = 1.17 (Req = 1/1.17= 0.8547)
1/Req = 2+.85+4
Req = 7.2
10. Explain about Krichoffs voltage and current laws.
Kirchhoff’s Current Law
The sum of current flowing towards a function is equal
to the current
flowing away from it.
Consider a function formed by 6 conductors. The
current in these
conductors are i1, i2, .i6.Some of these currents are
flowing towards a 8
other’s away from A
According to Kirchhoff’s Law,
i1+i4+i5+i6 = i2+i3
(Flowing towards) (Flowing away from A)
Kirchhoff’s Voltage Law (II Law)
In a closed circuit, the sum of the potential drops is
equal to the
sum of the potential resistance
ABCDA forms a closed circuit.
From A -> B, We have a potential drop of IR1.
From D -> A, We have a potential drop of V.
Sum of potential drops = IR1+IR2+IR3
Potential rise from D -> A =V
IR1+IR2+IR3 = V
11. Explain the principle of operation of DC Motor.
When a current passes through a conductor, lines of
magnetic force
(flux) are generated around the conductor. The
direction of the flux is
dependent on the direction of the current flow. In
terms of conventional
current flow (positive to negative) then, using your
right hand point your
thumb in the direction of the current flow and your
fingers will wrap around
the conductor in the same direction of the flux lines.
On the side of the conductor where the lines of flux
oppose each other, the
magnetic field will be made weaker. On
the side of the conductor where the lines of flux
are not opposing each other, the magnetic field will
be made stronger. Because of the
strong field on one side of the conductor and a weak
field or, the other side, the conductor
will be pushed into the weaker field.
The armature is connected to the commutator which
rides along the brushes which
are connected to a DC power source. The
current from the DC power source flows from
the positive lead, through the brush labeled A1
through one commutator section, through
the armature coil, through the other commutator
section, through the brush labeled A2
and back to the negative lead.This
current will generate lines of flux around the armature
and affect the lines of flux in the air gap. On
the side of thecoil where the lines of flux
oppose each other, the magnetic field will be made
weaker. On the side of the coil where
the lines of flux are riot opposing each other, the
magnetic field is made stronger.
Because of the strong field on one side of the coil
and the weak field on the other side,
the coil will be pushed into the weaker field and,
because the armature coil is free to
rotate, it will rotate.
The torque available at the motor shaft (turning
effort) is determined by the
magnetic force (flux) acting on the armature coil and
the distance from the renter of
rotation that force is. The
flux is determined by the current flowing through the armature
coil and strength of the field magnets.
12. Explain
the working principle of three phase induction motor.
In three phase induction motor, the magnetic field
generated by the stator rotates
in the ac case. Three electrical phases are introduced
through terminals, each phase
energizing an individual field pole. When each phase
reaches its maximum current, the
magnetic field at that pole reaches a maximum value.
As the current decreases, so does
the magnetic field. Since each phase reaches its
maximum at a different time within a
cycle of the current, that field pole whose magnetic
field is largest is constantly changing
between the three poles, with the effect that the
magnetic field seen by the rotor is
rotating. The speed of rotation of the magnetic field,
known as the synchronous speed,
depends on the frequency of the power supply and the
number of poles produced by the
stator winding. For a standard 60 Hz supply, as used
in the United States, the maximum
synchronous speed is 3,600 rpm.
In the three phase induction motor, the windings on
the rotor are not connected to
a power supply, but are essentially short circuits.
The most common type of rotor
winding, the squirrel cage winding, bears a strong
resemblance to the running wheel used
in cages for pet gerbils. When the motor is initially
switched on and the rotor is
stationary, the rotor conductors experience a changing
magnetic field sweeping by at the
synchronous speed.
From Faraday's law, this situation results in the
induction of currents round the
rotor windings; the magnitude of this current depends
on the impedance of the rotor
windings. Since the conditions for motor action are
now fulfilled, that is, current carrying
conductors are found in a magnetic field, the rotor
experiences a torque and starts to turn.
The rotor can never rotate at the synchronous speed
because there would be no relative
motion between the magnetic field and the rotor
windings and no current could be
induced. The induction motor has a high starting
torque.
13. Explain the working principle of single phase
induction motor.
Single phase induction motor has only one stator
winding (main winding) and
operates with a single-phase power supply. In all
single-phase induction motors, the rotor
is the squirrel cage type. The single-phase induction
motor is not self-starting. When the
motor is connected to a single-phase power supply, the
main winding carries an
alternating current. This current produces a pulsating
magnetic field. Due to induction,
the rotor is energized. As the main magnetic field is
pulsating, the torque necessary for
the motor rotation is not generated. This will cause
the rotor to vibrate, but not to rotate.
Hence, the single phase induction motor is required to
have a starting mechanism that can
provide the starting kick for the motor to rotate.
The starting mechanism of the single-phase induction
motor is mainly an
additional stator winding (start/ auxiliary winding)
as shown in Figure. The start winding
can have a series capacitor and/or a centrifugal
switch. When the supply voltage is
applied, current in the main winding lags the supply
voltage due to the main winding
impedance. At the same time, current in the start winding
leads/lags the supply voltage
depending on the starting mechanism impedance.
Interaction between magnetic fields
generated by the main winding and the starting
mechanism generates a resultant magnetic
field rotating in one direction. The motor starts
rotating in the direction of the resultant
magnetic field. Once the motor reaches about 75% of
its rated speed, a centrifugal switch
disconnects the start winding. From this point on, the
single-phase motor can maintain
sufficient torque to operate on its own.
14. Explain the working principle of single phase
Energy Meter.
An electric meter or energy meter is a device that
measures the amount of
electrical energy supplied to or produced by a
residence, business or machine.
The most common type is a kilowatt hour meter. When
used in electricity
retailing, the utilities record the values measured by
these meters to generate an invoice
for the electricity. They may also record other
variables including the time when the
electricity was used.
Modern electricity meters operate by continuously
measuring the instantaneous
voltage (volts) and current (amperes) and finding the
product of these to give
instantaneous electrical power (watts) which is then
integrated against time to give
energy used (joules, kilowatt-hours etc). The meters
fall into two basic categories,
electromechanical and electronic.
The energy meter operates by counting the revolutions
of an aluminium disc
which is made to rotate at a speed proportional to the
power. The number of revolutions
is thus proportional to the energy usage. It consumes
a small amount of power, typically
around 2 watts.
The metallic disc is acted upon by two coils. One coil
is connected in such a way
that it produces a magnetic flux in proportion to the
voltage and the other produces a
magnetic flux in proportion to the current. The field
of the voltage coil is delayed by 90
degrees using a lag coil. This produces eddy currents
in the disc and the effect is such that
a force is exerted on the disc in proportion to the
product of the instantaneous current and
voltage. A permanent magnet exerts an opposing force
proportional to the speed of
rotation of the disc - this act as a brake which
causes the disc to stop spinning when
power stops being drawn rather than allowing it to spin
faster and faster. This causes the
disc to rotate at a speed proportional to the power
being used.
The type of meter described above is used on a
single-phase AC supply. Different
phase configurations use additional voltage and
current coils.
The aluminium disc is supported by a spindle which has
a worm gear which
drives the register. The register is a series of dials
which record the amount of energy
used. The dials may be of the cyclometer type, an
odometer-like display that is easy to
read where for each dial a single digit is shown
through a window in the face of the
meter, or of the pointer type where a pointer
indicates each digit. It should be noted that
with the dial pointer type, adjacent pointers
generally rotate in opposite directions due to
the gearing mechanism.
10 MARKS
1. DETERMINE THE EQUIVALENT RESISTANCE BETWEEN
TERMINALS A & B
SOLUTION
50 & 12.5 ARE PARALLEL
50*12.5 / 50+12.5 = 10
STEP – I
STEP – II
B
20*30/20+30 = 12
STEP – III
B
35 8
60
A
60*20/60+20 = 15
STEP – I V
STEP – V
RAB = 50
2. Find the mesh currents in the following network
Solution:
The solution of -1 amp for I2 means
that our initially assumed direction of current was
incorrect. In actuality, I2 is
flowing in a counter-clockwise direction at a value of
(positive) 1 amp:
3. Explain the working of a moving iron type
instruments.
These instruments are widely used in laboratories and
switch board at
commercial frequencies because these are cheaper in
cost, robust in construction and
can be manufactured with required accuracy.
These are generally of two types:-
1. The attraction type.
2. The repulsion type.
The attraction type instrument operate on the
principle of attraction of a single piece of
soft iron into a magnetic field and repulsion type
instrument operate on the principle of
repulsion of two adjacent iron pieces magnified by the
same magnetic field. Repulsion
type instrument are more sensitive than attraction
type instrument as in repulsion type
instrument large separating torque is developed by
having two iron element positional
class together inside the field coil where the
magnetizing effect is maximum.
In both type of these instruments, the current under
measurement is passed through a coil
of wire. This current carrying coil set up the
necessary field depending on the magnitude
of the current to be measured. The coil may be of a
few turns of very heavy conductor or
of many turns of fine wire. The instrument to be used
as an ammeter is provided with a
coil of few turns of thick wire in order to have low
resistance and carry large current and
that to be used as a voltameter is provided with a
coil of large number of turns of wire in
order to have high resistance and draw as small
current as possible.
4. Derive the expression for torque produced in moving
iron instrument.
Let L be the self inductance corresponding to a total
angular deflection of
q radians and change in inductance be dL correponding
to small change in
deflection angel dq due to small change in current.
The change in energy of magnetic field,
dw = Td
dθ
Since change in energy dE = workdone, dw
Td
dθ = ½ I2dL
Td
= ½ I2dL/dθ
where I is in amperes, L is in Henry and θ is
in Radians.
Thus toruqe is proportional to the square of the
instrument current and to the rate
of change of inductance with deflection.
5. An energy meter revolves 10 revolutions of disc for
unit of energy. Find
the number of revolutions made by it during an hour
when connected
across when connected 20A at 210V and 0.8 power factor
leading. If
energy meter revolves 350 revolutions, find the %
error.
Answer.
Energy consumed in one hour = VI cos φ /
1000
= 210 x 20 x 0.8 / 1000
= 3.360 kwh.
The number of revolution the meter should make it is
correct = 3.360 x
registration const in revolution per kwh
= 3.360 x 100
= 336
Number of revolution actually made = 350
% error = (350-336) x 100 / 350
% error = 0.1466 %
6. Explain how following torque are produced in pmmc
instrument and
attracted type moving iron instruments
1. Deflecting torque
2. Control torque
3. Damping torque
1.
DEFLECTING TORQUE:-
The deflecting torque is produced by making
use of one of the magnetic, chemical, electrostatic
and electromagnetic
induction effects of current or voltage and causes the
moving system of the
instrument to move from its zero position when the
instrument is connected in
an electrical circuit to measure the electrical
quantity. The method of
producing this torque depend upon the type of
instrument. In attracting the
type of instrument, this torque to equal to
Td
= 1/2 I2 dL/dθ
Whereas in Pmmc instruments
Td
= Bilur
Where B - magnetic density
i - current flowing
l - length of coil
u - number of turn
r - radius of coil
2. CONTROLLING
TORQUE:- The magnitude of the movement to
the
moving system would be somewhat indefinite under the
influence of
deflecting torque unless some controlling torque
exist. This torque opposes
the deflecting torque and increases with increase in
deflection of the moving
system without controlling system the irrespective
magnitude of current and
moreover, once deflected it would not return to its
zero position on removing
the current.
In attraction type instrument it is produced by spring
control and in PMMC
too it would be produced by spring control.
3. DAMPING
TORQUE:- This torque is also necessary
to avoid oscillation
of the moving system about it's final deflected
position owing to the inertia of
the moving parts and to bring the moving system to
rest in it's final deflected
position quickly.
7. Briefly discuss the construction and working of a
PMMC instrument.
How range is extended in moving coil ammeter and
voltmeter?
Answer.
CONSTRUCTION:- In a permanent magnet with soft iron pole pieces, a
cylindrical iron core is mounted between the two poles
of the magnet giving
very narrow air gap in which the sides of a pointted
light rectangular coil lies.
The rectangular coil is wound of many turns of coil.
The purpose of using
core is to make the field uniform and to reduce the
reluctance of the magnetic
circuit. A low reluctance helps to retain permeance of
magnet for a longer
period.
WORKING OF TORQUE EQUATION:- When the current to be measured
is passed through the coil, say in the direction as
shown in fig. (a), deflecting
torque is produced. On account of relation between
permanent magnetic field
and coil magnetic field, the direction of deflecting
torque can be determined
by applying Fleming's left hand rule.
It is the current in amperes flowing through the coil
of turns N and length l
meters B is flux density in test as in air gap:-
Then, deflecting force F = BilN newtons.
If r is the distance in meters in between the centers
of the coil and force F.
Then deflecting torque Td =
F x r = BilNr Nm
From the above expression it is obvious that if flux
density B in the air gap is
constant, then
deflecting torque Td = i
, Tc= Td
So, θ = i
And since q is directly proportional to current, the
scale of the basic dc
PMMC instrument are usually linearly spaced. Hence
scale is linear.
8. Discuss the construction and working of an
electrodynamic wattmeter
with the help of diagram?
Answer.
This type of instrument is similar in design and
principle to the dynanometer
type ammeter and voltameter.
WORKING AND CONSTRUCTION:- When the instrument of this
type is
used as wattmeter, the fixed coil which is divided
into two equal portions in
order to provide them uniform field , is employed as
current coil and moving
coil is used as pressure coil. The fixed coil which is
divided into two equal
portion in order to provide them uniform field, is
employed as current coil and
the moving coil is used as pressure coil, i.e the
fixed coil carries the current
proportional to the voltage across the circuit. A high
non inductive resistance
is connected in series with the moving coil in order
to limit current. The
magnetic field of the fixed and moving coil react on
one another causing the
moving coil to turn about it's axis. The movement is
controlled by hair springs
which also leads the current into and out of the
moving element. Damping is
provided by light aluminium moving in an air dash pot.
The pointer is fixed to
the moving coil spindle and moves over a suitable
caliberated scale.
THEORY:- Let us be the supply voltage, i the load
current and R the load
resistance of the moving coil circuit.
Current through fixed coil, if =
i
Current through moving coil, im =
V/R
deflecting torque, Td ∝ if im ∝ V/R
For a DC circuit the deflecting torque is thus
proportional to the power and for
any circuit with fluctuating torque. The instantaneous
to the instantaneous
power.
9. Compare merits and demerits of moving iron type
instruments and
dynamometer type instruments. Which one is superior
why?
Answer.
1. TORQUE HEIGHT RATIO:- Dynamometer type instruments
have equal small
torque height ratio.
2. FRICTION ERROR:- Dynamometer type instruments have
considerable
friction error.
3. FRICTION LOSS:- Owing to heavy moving system,
dynamometer type
instruments have more friction losses.
4. COST AND SENSITIVITY TO OVERLOAD:- As a result of
measures to
reduce the frictional error, the dynamometer type
instruments are more sensitive
to overloads and mechanical impactsis in comparison to
moving iron type
instruments.
5. SENSTIVITY:- The senstivity of dynamometer
instrument is typically very
poor due to poor deflecting torque.
6. POWER CONSUMPTION:- Dynamometer type instrument
have
comparatively higher power consumption.
7. EFFECT OF STRAY MAGNETIC FIELD:- There is no effect
of stray
magnetic field on moving iron type while dynamometer
type are most sensitive
towards it.
8. HYSTERISIS AND EDDY CURRENT ERRORS:- Dynamometer
type
instruments are free from these erors while moving
iron have these errors.
9. EFFECT OF WAVE FORM:- Dynamometer type instruments
are very useful
for accurate measurement of runs voltage while
frequency change serious e rrors
in AC measurement in moving iron type instruments.
10. CALIBRATION:- Dynamometer type instruments have
same calibration for
AC and DC measurements while moving iron type have a
difference between AC
and DC calibration.
10. Why shunt is usually used voltmeter and ammeter? A
moving coil
instrument has a resistance of 5 _ and gives full deflection of 100mv.
Show how the instrument may be used to measure:-
1. voltage upto 50V
2. current upto 10A
Answer.
Shunt is usually used in voltmeter and ammeter to
extend the range of
voltmeter and ammeters.
Rm
= 5_
Vm
= 100mv
Im
= Vm/Rm =
100mv/5_ = 20mA
1. For measuring voltage upto 50V.
Series resistance is used with the instrument whose
resistance is
R = V/Im
- Rm =
50/(20 x 10-3) - 5
R = 2.5 x 10-3 - 5 = R =
2495 _
2. Such resistance of resistance Rf is
used to be connected
Rf
= Rm/[I/Im -
1]
= 5/[10/20 x 10-3 -1]
= 5 x 2/998
Rf
= 0.01002004 _
11. Explain the principle of operation of attraction
type moving iron
instruments and explain how the controlling and damping
forces are
obtained?
Answer. The earliest and simplest form of attraction moving iron
instruments
uses a solenoid and moving oval shaped soft iron
pinoted eccentrically. To
this iron a pointer is attached so that it may deflect
along with the moving iron
over a graduate scale. The iron is made of sheet metal
specially shaped to give
a scale as nearby uniform as possible. The moving iron
is drawn into field of
solenoid when current flows through it. The movement
of the iron always
from weaker magnetic field outside the coil into the
stronger field inside the
coil regardless the direction of flow of current. When
the current to be
measured is passed through the solenoid, a magnetic
field is set up inside the
solenoid, which in turn magnetises the iron. Thus the
iron is attached into the
coil causing the spindle and the pointer to rotate.
So much instruments normally have spring control and
pneumatic damping
forces.
12. 3-φ 4
pole induction motor is supplied from 3φ 50Hz ac supply. Find
(1) synchronous speed
(2) rotor speed when slip is 4%
(3) the rotor frequency when runs at 600r.p.m.
Sulution
1) Ns
=120f/p
=120*50/4
= 1500 r.p.m.
2) speed when slip is 4% or .04
N=Ns
(1-s)
=1500(1-0.04)
= 1440 r.p.m.
3) slip when motor runs at 600 r.p.m.
S’=(Ns
–N)/Ns
=(1500-600)/1500
= 0.6
13. Explain
the method of temperature control in open loop and closed loop
systems.
Temperature controllers are needed in any situation
requiring a given
temperature be kept stable. This can be in a situation
where an object is required
to be heated, cooled or both and to remain at the
target temperature (set point),
regardless of the changing environment around it.
There are two fundamental
types of temperature control; open loop and closed
loop control. Open loop is the
most basic form and applies continuous heating/cooling
with no regard for the
actual temperature output. It is analogous to the
internal heating system in a car.
On a cold day, you may need to turn the heat on to
full to warm the car to 75°.
However, during warmer weather, the same setting would
leave the inside of the
car much warmer than the desired 75°.
Open loop control block diagram
Closed loop control is far more sophisticated than
open loop. In a closed
loop application, the output temperature is constantly
measured and adjusted to
maintain a constant output at the desired temperature.
Closed loop control is
always conscious of the output signal and will feed
this back into the control
process. Closed loop control is analogous to a car
with internal climate control. If
you set the car temperature to 75°, the climate
control will automatically adjust
the heating (during cold days) or cooling (during warm
days) as required to
maintain the target temperature of 75°.
A temperature controller is a device used to hold a
desired temperature at a
specified value. The simplest example of a temperature
controller is a common
thermostat found in homes. For instance, a hot water
heater uses a thermostat to control
the temperature of the water and maintain it at a
certain commanded temperature.
Temperature controllers are also used in ovens. When a
temperature is set for an oven, a
controller monitors the actual temperature inside of
the oven. If it falls below the set
temperature, it sends a signal to activate the heater
to raise the temperature back to the set
point. Thermostats are also used in refrigerators. So
if the temperature gets too high, a
controller initiates an action to bring the
temperature down.
14. Explain about open loop and closed loop control
system.
Figure 1 shows an open loop system. A computed force
is applied to the system
which is expected to respond based on the
specifications. If the system fails to respond
correctly (because your estimates were off) or an
unanticipated disturbance acted on it,
then there is no way to correct the course. On the
other hand, figure 2 shows a feed-back
system. The response C(s) is measured using the sensor
H(s) and the resultant is
compared with the input R(s). The resultant difference
(error) is acted upon by the
controller which works on the actuator. The actuator
then applies the required force on
the system.
The closed loop thus contains the sensor dynamics, the
controller dynamics, the
actuator dynamics in addition to the system we are
interested in. It should be noted that
all measurements have to be done or converted if
necessary into one unit so that
comparison with the target signal is possible.
Usually, measurements result in currents
and voltages. Hence, this conversion from a mechanical
input to an electrical output is
also included in the sensor, controller and actuator
dynamics.
In designing the full control system the dynamics of
all the components need to be
accounted for. If the controller is very slow compared
to the system, it will not send the
right input at the right time. In this class, we will
assume perfect sensor and actuator
dynamics, i.e., what goes into the sensor (it is
commonly denoted by H(s)) and the
actuator comes out unmodified instantaneously. So we
replace them with unity transfer
functions.
15. Explain the relation between voltage, current and
resistance
An electric circuit is formed when a conductive path
is created to allow free
electrons to continuously move. This continuous
movement of free electrons through
the conductors of a circuit is called a current, and it is often referred to
in terms of
"flow," just like the flow of a liquid
through a hollow pipe.
The force motivating electrons to "flow"
in a circuit is called voltage.
Voltage
is a specific measure of potential energy that is
always relative between two points.
When we speak of a certain amount of voltage being
present in a circuit, we are
referring to the measurement of how much potential energy exists to move electrons
from one particular point in that circuit to another
particular point. Without reference
to two particular
points, the term "voltage" has no meaning.
Free electrons tend to move through conductors with
some degree of
friction, or opposition to motion. This opposition
to motion is more properly
called resistance.
The amount of current in a circuit depends on the amount of
voltage available to motivate the electrons, and
also the amount of resistance in
the circuit to oppose electron flow. Just like
voltage, resistance is a quantity
relative between two points. For this reason, the
quantities of voltage and
resistance are often stated as being
"between" or "across" two points in a circuit.
To be able to make meaningful statements about these
quantities in
circuits, we need to be able to describe their
quantities in the same way that we
might quantify mass, temperature, volume, length, or
any other kind of physical
quantity. For mass we might use the units of
"kilogram" or "gram." For
temperature we might use degrees Fahrenheit or
degrees Celsius. Here are the
standard units of measurement for electrical
current, voltage, and resistance:
16. Explain the construction of DC machine with neat
diagram.
A D.C. machine consists mainly of two part the
stationary part called stator and
the rotating part called stator.
The stator consists of main poles used to produce
magnetic flux ,commutating
poles or interpoles in between the main poles to avoid
sparking at the commutator but in
the case of small machines sometimes the interpoles
are avoided and finally the frame or
yoke which forms the supporting structure of the
machine.
The rotor consist of an armature a cylindrical
metallic body or core with slots in it
to place armature windings or bars, a commutator and
brush gears
The magnetic flux path in a motor or generator is show
below and it is called the
magnetic structure of generator or motor.
Cross sectional view of a DC Machine
Frame
Frame is the stationary part of a machine on which the
main poles and
commutator poles are bolted and it forms the
supporting structure by connecting the
frame to the bed plate. The ring shaped body portion
of the frame which makes the
magnetic path for the magnetic fluxes from the main
poles and interpoles is called Yoke.
Yoke
In early days Yoke was made up of cast iron but now it
is replaced by cast steel.
This is because cast iron is saturated by a flux
density of 0.8 Wb/sq.m where as saturation
with cast iron steel is about 1.5 Wb/sq.m.So for the
same magnetic flux density the cross
section area needed for cast steel is less than cast
iron hence the weight of the machine
too. If we use cast iron there may be chances of blow
holes in it while casting. so now
rolled steels are developed and these have consistent
magnetic and mechanical properties.
End Shields or Bearings
If the armature diameter does not exceed 35 to 45 cm
then in addition to poles end
shields or frame head with bearing are attached to the
frame. If the armature diameter is
greater than 1m pedestral type bearings are mounted on the machine bed plate outside
the frame. These bearings could be ball or roller type
but generally plain pedestral
bearings are employed. If the diameter of the armature
is large a brush holder yoke is
generally fixed to the frame.
17. Explain the Working of dynamometer type wattmeter
The principle of operation of the
electrodynamometer-type wattmeter is the same
as that for dynamo-electric machines. The deflection
torque is produced by the
interaction of two magnetic fluxes. One of the fluxes
is produced by a fixed coil which
carries a current proportional to the load current and
therefore called the current coil. The
other flux is created by a movable coil which carries
a current proportional to the load
voltage and thus called the voltage or potential coil.
A high non-inductive resistance is
connected to the potential coil so that its current is
almost in phase with the load voltage.
The control torque is provided by a control spring.
In a dynamometer type wattmeter the fixed coil
(current coil) is connected in
series with the load.This coil is divided in to two
parts and they are kept parallel to each
other. The coil is thick in cross section and has less
number of turns.The moving coil
(pressure coil) is connected across the load. It is
thin in cross - section and has hundreds
of turns. It has a non - inductive high resistance in
series with it
The wattmeter is an electrodynamic instrument for
measuring the electric power
or the supply rate of electrical energy of any given
circuit. The device consists of a pair of
fixed coils, known as current coils, and a movable
coil known as the potential coil.
The current coils are connected in series with the
circuit, while the potential coil is
connected in parallel. Also, on analog wattmeters, the
potential coil carries a needle that
moves over a scale to indicate the measurement. A
current flowing through the current
coil generates an electromagnetic field around the
coil. The strength of this field is
proportional to the line current and in phase with it.
The potential coil has, as a general
rule, a high-value resistor connected in series with
it to reduce the current that flows
through it. The result of this arrangement is that on
a dc circuit, the deflection of the
needle is proportional to both the current and the
voltage, thus conforming to the equation
W=VA or P=EI. On an ac circuit the deflection is
proportional to the average
instantaneous product of voltage and current, thus
measuring true power, and possibly
(depending on load characteristics) showing a
different reading to that obtained by simply
multiplying the readings showing on a stand-alone
voltmeter and a stand-alone ammeter
in the same circuit. The two circuits of a wattmeter
are likely to be damaged by excessive
current. The ammeter and voltmeter are both vulnerable
to overheating - in case of an
overload, their pointers will be driven off scale -
but in the wattmeter, either or even both
the current and potential circuits can overheat
without the pointer approaching the end of
the scale! This is because the position of the pointer
depends on the power factor, voltage
and current. Thus, a circuit with a low power factor
will give a low reading on the
wattmeter, even when both of its circuits are loaded
to the maximum safety limit.
Therefore, a wattmeter is rated not only in watts, but
also in volts and amperes.
18. Explain the construction of transformer with neat
diagram.
A transformer is an electrical device used to convert
AC power at a certain
voltage level to AC power at a different voltage, but
at the same frequency.
The construction of a transformer includes a
ferromagnetic core around which
multiple coils, or windings, of wire are wrapped. The
input line connects to the 'primary'
coil, while the output lines connect to 'secondary'
coils. The alternating current in the
primary coil induces an alternating magnetic flux that
'flows' around the ferromagnetic
core, changing direction during each electrical cycle.
The alternating flux in the core in
turn induces an alternating current in each of the
secondary coils. The voltage at each of
the secondary coils is directly related to the primary
voltage by the turns ratio, or the
number of turns in the primary coil divided by the
number turns in the secondary coil.
For instance, if the primary coil consists of 100
turns and carries 480 volts and a
secondary coil consists of 25 turns, the secondary
voltage is then:
secondary voltage = (480 volts) * (25/100) = 120 volts
Two coils of wire (called windings) are wound on some
type of core material. In
some cases the coils of wire are wound on a
cylindrical or rectangular cardboard form. In
effect, the core material is air and the transformer
is called an AIR-CORE
TRANSFORMER. Transformers used at low frequencies,
such as 50 hertz and 400 hertz,
require a core of low-reluctance magnetic material,
usually iron. This type of transformer
is called an IRON-CORE TRANSFORMER. Most power
transformers are of the ironcore
type. The principle parts of a transformer and their
functions are:
• The CORE,
which provides a path for the magnetic lines of flux.
• The PRIMARY
WINDING, which receives energy from the ac source.
• The SECONDARY
WINDING, which receives energy from the primary
winding and delivers it to the load.
• The
ENCLOSURE, which protects the above components from dirt,
moisture, and mechanical damage.
A soft-iron-core transformer is very useful where the
transformer must be
physically small, yet efficient. The iron-core
transformer provides better power transfer
than does the air-core transformer. A transformer
whose core is constructed of laminated
sheets of steel dissipates heat readily; thus it
provides for the efficient transfer of power.
The majority of transformers you will encounter in
Navy equipment contain laminatedsteel
cores. These steel laminations are insulated with a
non conducting material, such as
varnish, and then formed into a core. It takes about
50 such laminations to make a core an
inch thick. The purpose of the laminations is to
reduce certain losses.
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