www.rejinpaul.com by anirudhan RBASIC ELECTRICAL AND ELECTRONICS
ENGINEERING
2 MARKS1. What is meant by charge?Charge is an electrical
property of the atomic particles which matterconsists. The charge
of an electron is so small. Charge in motion representscurrent. 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/t3. What is meant by Voltage?The poterntial
difference between two points is called as voltage. Unit isVolts
(V).V=W/Q , W=work done in joules & Q = charge in coulombs4.
State Ohms Law.The potential difference across any two ends of a
conductor is directlyproportional to the current flowing between
the two ends provided thetemperature of the conductor remains
constant.5. State Krichoffs Voltage LawKVL states that the
algebraic sum of voltages in a closed path is zero.6. State
Krichoffs 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 nodes8. Give notes on Mesh
Analysis.KVL is usedHere mesh currents are found.9. Give short
notes on resistor.It is a property of a substance3 which opposes
the flow of electrons. It isdenoted 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 iscalled
branchA 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 branches12. Write down the formula for a star connected
network is converted into adelta 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 astar
network?R1=( RARB+RBRC+RCRA)/RCR2=( RARB+RBRC+RCRA)/RBR3=(
RARB+RBRC+RCRA)/RA14. Define line currents and phase currents?The
currents flowing in the lines are called as line currentsThe
currents flowing through phase are called phase currents15. Define
line voltage and phase voltage?The voltage across one phase and
neutral is called line voltage & thevoltage between two lines
is called phase voltage16. Give the phase value & Line value of
a star connected system.VL= 3Vph17. Give the phase value and line
valued of a delta connected system.IL= 3Iph18. What is the power
equation for a star connected system?P= 3I V cos L L W19. What is
the power equation for a delta connected system?P= 3I V cos L L
W20. What is meant by Real power?Real power means the useful power
transfer from source to load. Unit iswatts.21. What is meant by
apparent power?Apparent power is the product of voltage and current
and it is not truepower. Unit is VA22. What is reactive power?If we
consider the circuit as purely inductive the output power is
reactivepower. Its unit is VAR23. Define Instrument.Instrument is
defined as a device for determining the value or magnitudeof a
quantity or variable.24. Mention the two main differences between
an ammeter and a voltmeter.Ammeter VoltmeterIt is a current
measuring device it is a voltage measuring deveiceAlways connected
in series with circuit Always connected in parallel with circuitThe
resistance is very small The resistance is very high25. Give short
notes on resistor.It is a property of a substance3 which opposes
the flow of electrons. It isdenoted by R and its unit is Ohm (_)26.
What is control system?A system consists of a number of components
connected together toperform a specific function . In a system when
the output quantity is controlled byvarying 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 loop28. .Define open loop control system.The
control system in which the output quantity has no effect upon
theinput quantity are called open loop control system. This means
that the output isnot feedback to the input for correction.29.
.Define closed loop control system.The control system in which the
output has an effect upon the inputquantity so as to maintain the
desired output value are called closed loop controlsystem30.
Mention the errors in Moving iron instruments.Hysteresis
errorTemperature errorStray magnetic field errorFrequency errorEddy
current error31. 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 untilthe sufficient deflection is
obtained.32. Give some applications of DC motor.Shunt : driving
constant speed, lathes, centrifugal pumps, machine tools,blowers
and fans, reciprocating pumpsSeries : electric locomotives, rapid
transit systems, trolley cars, cranes andhoists, conveyorsCompound
: elevators, air compressors, rolling mills, heavy planners33.
Define slip.S = Ns NrNsWhere, Ns = synchronous speed in rpm.Nr =
rotor speed in rpmS = Slip34. Define synchronous speed.It is given
by Ns = 120f / p rpm.Where Ns = synchronous speed, p = no. of
stator poles, f = supply frequency inHz35. Why a single phase
induction motor does not self start?When a single phase supply is
fed to the single phase induction motor. Itsstator winding produces
a flux which only alternates along one space axis. It isnot 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 triesto achieve synchronous
speed.37. Define Form factor and Crest factor.Form factor= RMS
valueAverage ValueCrest(peak) factor=Maximum ValueRMS value38. .
Which type of instrument is called as universal instrument?The
moving iron instrument are known as universal instruments,
becausethese 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
dischargingcurrent indicators in automobiles.iii)Extensively used
in industries for measurement of AC voltage andcurrent 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 andif a closed path is provided ,a current flows
known as eddy current. This currentintersect with the magnetic
field to produce an electromagnetic torque , whichopposes 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 ,Theerror 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
cosii) Reactive power Q=EI siniii) Apparent power,S=EI44. Define
average value.The average value of an alternating current is that
value of steady directcurrent which transfers the same charge as
the alternating current flowing for thesame time.45. Define RMS
value.The effective value of an alternating current is that value
of steady ,directcurrent which produces the same heat as that
produced by the alternating currentwhen passed which produces the
same heat as that produced by the alternatingcurrent 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 iscalled reactive power. The unit is VAR.
Q=EIsin.47. What is the basic principle of a dc generator?Basic
principle of a dc generator is Faradays law of
electromagneticinduction.That is whenever a conductor is moved in
amagnetic field dynamicallyinduced 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 toimprove 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 thearmature.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 loadcondition the starting torque is less and
motor rotates at dangerous speed and maybe damaged.52. What is
meant by transformer?The transformeris a static piece of apparatus
by means of which electricalenergy is transformed from one circuit
to another with desired change in voltageand current , without any
change in the frequency.It works on the principle ofmutual
induction.53. What are the different types of single phase
motor?i)Single phase induction motorii)Single phase synchronous
motor.iii)Single phase series motor54. What are the two types of
rotors of an induction motor?i) Squirrel cage rotorii)Slip ring or
wound rotor
6 MARKS1. A 3 4 pole 50 hz induction motor runs at 1460 r.p.m.
find its % of slip.SolutionN s = 120f/p= 120*50/4=
1500r.p.m.Running speed of motorn= 1460r.p.m.Slip S=( N sN)/ 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 toanother by electromagnetic
induction (transformer action). The electrical energy isalways
transferred without a change in frequency, but may involve changes
inmagnitudes of voltage and current. Because a transformer works on
the principle ofelectromagnetic induction, it must be used with an
input source voltage that varies inamplitude. There are many types
of power that fit this description; for ease of explanationand
understanding, transformer action will be explained using an ac
voltage as the inputsource.The amount of power used by the load of
an electrical circuit is equal to thecurrent in the load times the
voltage across the load, or P = EI. If, for example, the load inan
electrical circuit requires an input of 2 amperes at 10 volts (20
watts) and the source iscapable of delivering only 1 ampere at 20
volts, the circuit could not normally be usedwith this particular
source. However, if a transformer is connected between the
sourceand the load, the voltage can be decreased (stepped down) to
10 volts and the currentincreased (stepped up) to 2 amperes. Notice
in the above case that the power remains thesame. That is, 20 volts
times 1 ampere equals the same power as 10 volts times 2amperes.A
Transformer consists of the following partsA 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 magneticfield (flux) builds up (expands) and
collapses (contracts) about the primary winding. Theexpanding and
contracting magnetic field around the primary winding cuts the
secondarywinding and induces an alternating voltage into the
winding. This voltage causesalternating current to flow through the
load. The voltage may be stepped up or downdepending on the design
of the primary and secondary windings.3. Calculate the amount of
resistance (R) in a circuit, given values of voltage (E) andcurrent
(I):the amount of resistance (R) offered by the lamp4. calculate
the amount of voltage supplied by a battery, given values ofcurrent
(I) and resistance (R):the amount of voltage provided by the
battery5. Calculate the electric power in the given circuit and
discuss the effect ofincreasing the battery voltage.The formula for
determining the power in an electric circuit: by multiplying
thevoltage in "volts" by the current in "amps" we arrive at an
answer in "watts." Let's applythis to the given circuit.In the
above circuit, we know we have a battery voltage of 18 volts and
alamp 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 bythe 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 voltageand causes the moving system
of the instrument to move from its zero positionwhen the instrument
is connected in an electrical circuit to measure the
electricalquantity. The method of producing this torque depend upon
the type ofinstrument. In attracting the type of instrument, this
torque to equal toTd = 1/2 I2 dL/dWhereas in Pmmc instrumentsTd =
BilurWhere B - magnetic densityi - current flowingl - length of
coilu - number of turnr - radius of coil7. 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 ohms.Determine the value of
two resistorsSeries R1+R2=100 => R2 =100 - R1R1R2/R1+R2 =
24R1R2/100 = 24R1R2 =2400R1 (100-R1) = 2400100 R1-R1^2-2400 =
0R1^2-100 R1 + 2400 = 0(R1-60)(R1-40) = 0There Fore R1 = 60; R1 =
40When R1 = 60 ; R2 = 100 60 = 40When R1 = 40 ; R2 = 100 - 40 =
609. Find the Req between two points A & B.1/Req = +1/3+1/3 =
1.17 (Req = 1/1.17= 0.8547)1/Req = 2+.85+4Req = 7.210. Explain
about Krichoffs voltage and current laws.Kirchhoffs Current LawThe
sum of current flowing towards a function is equal to the
currentflowing away from it.Consider a function formed by 6
conductors. The current in theseconductors are i1, i2, .i6.Some of
these currents are flowing towards a 8others away from AAccording
to Kirchhoffs Law,i1+i4+i5+i6 = i2+i3(Flowing towards) (Flowing
away from A)Kirchhoffs Voltage Law (II Law)In a closed circuit, the
sum of the potential drops is equal to thesum of the potential
resistanceABCDA 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+IR3Potential rise from D -> A
=VIR1+IR2+IR3 = V11. 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 isdependent on the direction of the current flow. In terms
of conventionalcurrent flow (positive to negative) then, using your
right hand point yourthumb in the direction of the current flow and
your fingers will wrap aroundthe conductor in the same direction of
the flux lines.On the side of the conductor where the lines of flux
oppose each other, themagnetic field will be made weaker. On the
side of the conductor where the lines of fluxare not opposing each
other, the magnetic field will be made stronger. Because of
thestrong field on one side of the conductor and a weak field or,
the other side, the conductorwill be pushed into the weaker
field.The armature is connected to the commutator which rides along
the brushes whichare connected to a DC power source. The current
from the DC power source flows fromthe positive lead, through the
brush labeled A1 through one commutator section, throughthe
armature coil, through the other commutator section, through the
brush labeled A2and back to the negative lead.This current will
generate lines of flux around the armatureand affect the lines of
flux in the air gap. On the side of thecoil where the lines of
fluxoppose each other, the magnetic field will be made weaker. On
the side of the coil wherethe 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 torotate, it will rotate.The torque available
at the motor shaft (turning effort) is determined by themagnetic
force (flux) acting on the armature coil and the distance from the
renter ofrotation that force is. The flux is determined by the
current flowing through the armaturecoil 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 rotatesin the ac case. Three electrical phases are
introduced through terminals, each phaseenergizing an individual
field pole. When each phase reaches its maximum current,
themagnetic field at that pole reaches a maximum value. As the
current decreases, so doesthe magnetic field. Since each phase
reaches its maximum at a different time within acycle of the
current, that field pole whose magnetic field is largest is
constantly changingbetween the three poles, with the effect that
the magnetic field seen by the rotor isrotating. 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 thestator winding. For a standard 60 Hz
supply, as used in the United States, the maximumsynchronous speed
is 3,600 rpm.In the three phase induction motor, the windings on
the rotor are not connected toa power supply, but are essentially
short circuits. The most common type of rotorwinding, the squirrel
cage winding, bears a strong resemblance to the running wheel
usedin cages for pet gerbils. When the motor is initially switched
on and the rotor isstationary, the rotor conductors experience a
changing magnetic field sweeping by at thesynchronous speed.From
Faraday's law, this situation results in the induction of currents
round therotor windings; the magnitude of this current depends on
the impedance of the rotorwindings. Since the conditions for motor
action are now fulfilled, that is, current carryingconductors 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 relativemotion between the magnetic field
and the rotor windings and no current could beinduced. 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) andoperates with a
single-phase power supply. In all single-phase induction motors,
the rotoris the squirrel cage type. The single-phase induction
motor is not self-starting. When themotor is connected to a
single-phase power supply, the main winding carries analternating
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 forthe 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 canprovide the starting kick for the
motor to rotate.The starting mechanism of the single-phase
induction motor is mainly anadditional stator winding (start/
auxiliary winding) as shown in Figure. The start windingcan have a
series capacitor and/or a centrifugal switch. When the supply
voltage isapplied, current in the main winding lags the supply
voltage due to the main windingimpedance. At the same time, current
in the start winding leads/lags the supply voltagedepending on the
starting mechanism impedance. Interaction between magnetic
fieldsgenerated by the main winding and the starting mechanism
generates a resultant magneticfield rotating in one direction. The
motor starts rotating in the direction of the resultantmagnetic
field. Once the motor reaches about 75% of its rated speed, a
centrifugal switchdisconnects the start winding. From this point
on, the single-phase motor can maintainsufficient 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 ofelectrical energy supplied to or produced by a
residence, business or machine.The most common type is a kilowatt
hour meter. When used in electricityretailing, the utilities record
the values measured by these meters to generate an invoicefor the
electricity. They may also record other variables including the
time when theelectricity was used.Modern electricity meters operate
by continuously measuring the instantaneousvoltage (volts) and
current (amperes) and finding the product of these to
giveinstantaneous electrical power (watts) which is then integrated
against time to giveenergy 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 discwhich is made to rotate at a speed proportional to
the power. The number of revolutionsis thus proportional to the
energy usage. It consumes a small amount of power, typicallyaround
2 watts.The metallic disc is acted upon by two coils. One coil is
connected in such a waythat it produces a magnetic flux in
proportion to the voltage and the other produces amagnetic flux in
proportion to the current. The field of the voltage coil is delayed
by 90degrees using a lag coil. This produces eddy currents in the
disc and the effect is such thata force is exerted on the disc in
proportion to the product of the instantaneous current andvoltage.
A permanent magnet exerts an opposing force proportional to the
speed ofrotation of the disc - this act as a brake which causes the
disc to stop spinning whenpower stops being drawn rather than
allowing it to spin faster and faster. This causes thedisc 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.
Differentphase configurations use additional voltage and current
coils.The aluminium disc is supported by a spindle which has a worm
gear whichdrives the register. The register is a series of dials
which record the amount of energyused. The dials may be of the
cyclometer type, an odometer-like display that is easy toread where
for each dial a single digit is shown through a window in the face
of themeter, or of the pointer type where a pointer indicates each
digit. It should be noted thatwith the dial pointer type, adjacent
pointers generally rotate in opposite directions due tothe gearing
mechanism.
10 MARKS1. DETERMINE THE EQUIVALENT RESISTANCE BETWEENTERMINALS
A & BSOLUTION50 & 12.5 ARE PARALLEL50*12.5 / 50+12.5 =
10STEP ISTEP IIB20*30/20+30 = 12STEP IIIB35 860A60*20/60+20 =
15STEP I VSTEP VRAB = 502. Find the mesh currents in the following
networkSolution:The solution of -1 amp for I2 means that our
initially assumed direction of current wasincorrect. 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 atcommercial frequencies because these are cheaper in
cost, robust in construction andcan 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 ofsoft iron into a
magnetic field and repulsion type instrument operate on the
principle ofrepulsion of two adjacent iron pieces magnified by the
same magnetic field. Repulsiontype instrument are more sensitive
than attraction type instrument as in repulsion typeinstrument
large separating torque is developed by having two iron element
positionalclass 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 coilof wire. This
current carrying coil set up the necessary field depending on the
magnitudeof the current to be measured. The coil may be of a few
turns of very heavy conductor orof many turns of fine wire. The
instrument to be used as an ammeter is provided with acoil of few
turns of thick wire in order to have low resistance and carry large
current andthat to be used as a voltameter is provided with a coil
of large number of turns of wire inorder 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 ofq radians
and change in inductance be dL correponding to small change
indeflection angel dq due to small change in current.The change in
energy of magnetic field,dw = Td dSince change in energy dE =
workdone, dwTd d = I2dLTd = I2dL/dwhere I is in amperes, L is in
Henry and is in Radians.Thus toruqe is proportional to the square
of the instrument current and to the rateof change of inductance
with deflection.5. An energy meter revolves 10 revolutions of disc
for unit of energy. Findthe number of revolutions made by it during
an hour when connectedacross when connected 20A at 210V and 0.8
power factor leading. Ifenergy meter revolves 350 revolutions, find
the % error.Answer.Energy consumed in one hour = VI cos / 1000= 210
x 20 x 0.8 / 1000= 3.360 kwh.The number of revolution the meter
should make it is correct = 3.360 xregistration const in revolution
per kwh= 3.360 x 100= 336Number of revolution actually made = 350%
error = (350-336) x 100 / 350% error = 0.1466 %6. Explain how
following torque are produced in pmmc instrument andattracted type
moving iron instruments1. Deflecting torque2. Control torque3.
Damping torque1. DEFLECTING TORQUE:- The deflecting torque is
produced by makinguse of one of the magnetic, chemical,
electrostatic and electromagneticinduction effects of current or
voltage and causes the moving system of theinstrument to move from
its zero position when the instrument is connected inan electrical
circuit to measure the electrical quantity. The method ofproducing
this torque depend upon the type of instrument. In attracting
thetype of instrument, this torque to equal toTd = 1/2 I2
dL/dWhereas in Pmmc instrumentsTd = BilurWhere B - magnetic
densityi - current flowingl - length of coilu - number of turnr -
radius of coil2. CONTROLLING TORQUE:- The magnitude of the movement
to themoving system would be somewhat indefinite under the
influence ofdeflecting torque unless some controlling torque exist.
This torque opposesthe deflecting torque and increases with
increase in deflection of the movingsystem without controlling
system the irrespective magnitude of current andmoreover, once
deflected it would not return to its zero position on removingthe
current.In attraction type instrument it is produced by spring
control and in PMMCtoo it would be produced by spring control.3.
DAMPING TORQUE:- This torque is also necessary to avoid
oscillationof the moving system about it's final deflected position
owing to the inertia ofthe moving parts and to bring the moving
system to rest in it's final deflectedposition 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, acylindrical iron core is mounted between the two
poles of the magnet givingvery 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 usingcore is to make
the field uniform and to reduce the reluctance of the
magneticcircuit. A low reluctance helps to retain permeance of
magnet for a longerperiod.WORKING OF TORQUE EQUATION:- When the
current to be measuredis passed through the coil, say in the
direction as shown in fig. (a), deflectingtorque is produced. On
account of relation between permanent magnetic fieldand coil
magnetic field, the direction of deflecting torque can be
determinedby applying Fleming's left hand rule.It is the current in
amperes flowing through the coil of turns N and length lmeters 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
NmFrom the above expression it is obvious that if flux density B in
the air gap isconstant, thendeflecting torque Td = i , Tc= TdSo, =
iAnd since q is directly proportional to current, the scale of the
basic dcPMMC instrument are usually linearly spaced. Hence scale is
linear.8. Discuss the construction and working of an electrodynamic
wattmeterwith the help of diagram?Answer.This type of instrument is
similar in design and principle to the dynanometertype ammeter and
voltameter.WORKING AND CONSTRUCTION:- When the instrument of this
type isused as wattmeter, the fixed coil which is divided into two
equal portions inorder to provide them uniform field , is employed
as current coil and movingcoil is used as pressure coil. The fixed
coil which is divided into two equalportion in order to provide
them uniform field, is employed as current coil andthe moving coil
is used as pressure coil, i.e the fixed coil carries the
currentproportional to the voltage across the circuit. A high non
inductive resistanceis connected in series with the moving coil in
order to limit current. Themagnetic field of the fixed and moving
coil react on one another causing themoving coil to turn about it's
axis. The movement is controlled by hair springswhich also leads
the current into and out of the moving element. Damping isprovided
by light aluminium moving in an air dash pot. The pointer is fixed
tothe moving coil spindle and moves over a suitable caliberated
scale.THEORY:- Let us be the supply voltage, i the load current and
R the loadresistance of the moving coil circuit.Current through
fixed coil, if = iCurrent through moving coil, im = V/Rdeflecting
torque, Td if im V/RFor a DC circuit the deflecting torque is thus
proportional to the power and forany circuit with fluctuating
torque. The instantaneous to the instantaneouspower.9. Compare
merits and demerits of moving iron type instruments anddynamometer
type instruments. Which one is superior why?Answer.1. TORQUE HEIGHT
RATIO:- Dynamometer type instruments have equal smalltorque height
ratio.2. FRICTION ERROR:- Dynamometer type instruments have
considerablefriction error.3. FRICTION LOSS:- Owing to heavy moving
system, dynamometer typeinstruments have more friction losses.4.
COST AND SENSITIVITY TO OVERLOAD:- As a result of measures toreduce
the frictional error, the dynamometer type instruments are more
sensitiveto overloads and mechanical impactsis in comparison to
moving iron typeinstruments.5. SENSTIVITY:- The senstivity of
dynamometer instrument is typically verypoor due to poor deflecting
torque.6. POWER CONSUMPTION:- Dynamometer type instrument
havecomparatively higher power consumption.7. EFFECT OF STRAY
MAGNETIC FIELD:- There is no effect of straymagnetic field on
moving iron type while dynamometer type are most sensitivetowards
it.8. HYSTERISIS AND EDDY CURRENT ERRORS:- Dynamometer
typeinstruments are free from these erors while moving iron have
these errors.9. EFFECT OF WAVE FORM:- Dynamometer type instruments
are very usefulfor accurate measurement of runs voltage while
frequency change serious e rrorsin AC measurement in moving iron
type instruments.10. CALIBRATION:- Dynamometer type instruments
have same calibration forAC and DC measurements while moving iron
type have a difference between ACand DC calibration.10. Why shunt
is usually used voltmeter and ammeter? A moving coilinstrument has
a resistance of 5 _ and gives full deflection of 100mv.Show how the
instrument may be used to measure:-1. voltage upto 50V2. current
upto 10AAnswer.Shunt is usually used in voltmeter and ammeter to
extend the range ofvoltmeter and ammeters.Rm = 5_Vm = 100mvIm =
Vm/Rm = 100mv/5_ = 20mA1. For measuring voltage upto 50V.Series
resistance is used with the instrument whose resistance isR = V/Im
- Rm = 50/(20 x 10-3) - 5R = 2.5 x 10-3 - 5 = R = 2495 _2. Such
resistance of resistance Rf is used to be connectedRf = Rm/[I/Im -
1]= 5/[10/20 x 10-3 -1] = 5 x 2/998Rf = 0.01002004 _11. Explain the
principle of operation of attraction type moving ironinstruments
and explain how the controlling and damping forces
areobtained?Answer. The earliest and simplest form of attraction
moving iron instrumentsuses a solenoid and moving oval shaped soft
iron pinoted eccentrically. Tothis iron a pointer is attached so
that it may deflect along with the moving ironover a graduate
scale. The iron is made of sheet metal specially shaped to givea
scale as nearby uniform as possible. The moving iron is drawn into
field ofsolenoid when current flows through it. The movement of the
iron alwaysfrom weaker magnetic field outside the coil into the
stronger field inside thecoil regardless the direction of flow of
current. When the current to bemeasured is passed through the
solenoid, a magnetic field is set up inside thesolenoid, which in
turn magnetises the iron. Thus the iron is attached into thecoil
causing the spindle and the pointer to rotate.So much instruments
normally have spring control and pneumatic dampingforces.12. 3- 4
pole induction motor is supplied from 3 50Hz ac supply. Find(1)
synchronous speed(2) rotor speed when slip is 4%(3) the rotor
frequency when runs at 600r.p.m.Sulution1) Ns =120f/p=120*50/4=
1500 r.p.m.2) speed when slip is 4% or .04N=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= 0.613. Explain the method of temperature
control in open loop and closed loopsystems.Temperature controllers
are needed in any situation requiring a giventemperature be kept
stable. This can be in a situation where an object is requiredto be
heated, cooled or both and to remain at the target temperature (set
point),regardless of the changing environment around it. There are
two fundamentaltypes of temperature control; open loop and closed
loop control. Open loop is themost basic form and applies
continuous heating/cooling with no regard for theactual 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 thecar much warmer than the desired 75.Open
loop control block diagramClosed loop control is far more
sophisticated than open loop. In a closedloop application, the
output temperature is constantly measured and adjusted tomaintain a
constant output at the desired temperature. Closed loop control
isalways conscious of the output signal and will feed this back
into the controlprocess. Closed loop control is analogous to a car
with internal climate control. Ifyou set the car temperature to 75,
the climate control will automatically adjustthe heating (during
cold days) or cooling (during warm days) as required tomaintain the
target temperature of 75.A temperature controller is a device used
to hold a desired temperature at aspecified value. The simplest
example of a temperature controller is a commonthermostat found in
homes. For instance, a hot water heater uses a thermostat to
controlthe 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, acontroller monitors
the actual temperature inside of the oven. If it falls below the
settemperature, it sends a signal to activate the heater to raise
the temperature back to the setpoint. Thermostats are also used in
refrigerators. So if the temperature gets too high, acontroller
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 systemwhich is
expected to respond based on the specifications. If the system
fails to respondcorrectly (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-backsystem. The response C(s) is measured using the sensor
H(s) and the resultant iscompared with the input R(s). The
resultant difference (error) is acted upon by thecontroller which
works on the actuator. The actuator then applies the required force
onthe system.The closed loop thus contains the sensor dynamics, the
controller dynamics, theactuator dynamics in addition to the system
we are interested in. It should be noted thatall measurements have
to be done or converted if necessary into one unit so
thatcomparison with the target signal is possible. Usually,
measurements result in currentsand voltages. Hence, this conversion
from a mechanical input to an electrical output isalso included in
the sensor, controller and actuator dynamics.In designing the full
control system the dynamics of all the components need to
beaccounted for. If the controller is very slow compared to the
system, it will not send theright input at the right time. In this
class, we will assume perfect sensor and actuatordynamics, i.e.,
what goes into the sensor (it is commonly denoted by H(s)) and
theactuator comes out unmodified instantaneously. So we replace
them with unity transferfunctions.15. Explain the relation between
voltage, current and resistanceAn electric circuit is formed when a
conductive path is created to allow freeelectrons to continuously
move. This continuous movement of free electrons throughthe
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. Voltageis 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
arereferring to the measurement of how much potential energy exists
to move electronsfrom one particular point in that circuit to
another particular point. Without referenceto two particular
points, the term "voltage" has no meaning.Free electrons tend to
move through conductors with some degree offriction, or opposition
to motion. This opposition to motion is more properlycalled
resistance. The amount of current in a circuit depends on the
amount ofvoltage available to motivate the electrons, and also the
amount of resistance inthe circuit to oppose electron flow. Just
like voltage, resistance is a quantityrelative between two points.
For this reason, the quantities of voltage andresistance are often
stated as being "between" or "across" two points in a circuit.To be
able to make meaningful statements about these quantities
incircuits, we need to be able to describe their quantities in the
same way that wemight quantify mass, temperature, volume, length,
or any other kind of physicalquantity. For mass we might use the
units of "kilogram" or "gram." Fortemperature we might use degrees
Fahrenheit or degrees Celsius. Here are thestandard 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 andthe rotating part called stator.The stator consists of
main poles used to produce magnetic flux ,commutatingpoles or
interpoles in between the main poles to avoid sparking at the
commutator but inthe case of small machines sometimes the
interpoles are avoided and finally the frame oryoke which forms the
supporting structure of the machine.The rotor consist of an
armature a cylindrical metallic body or core with slots in itto
place armature windings or bars, a commutator and brush gearsThe
magnetic flux path in a motor or generator is show below and it is
called themagnetic structure of generator or motor.Cross sectional
view of a DC MachineFrameFrame is the stationary part of a machine
on which the main poles andcommutator poles are bolted and it forms
the supporting structure by connecting theframe to the bed plate.
The ring shaped body portion of the frame which makes themagnetic
path for the magnetic fluxes from the main poles and interpoles is
called Yoke.YokeIn 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 saturationwith cast iron
steel is about 1.5 Wb/sq.m.So for the same magnetic flux density
the crosssection area needed for cast steel is less than cast iron
hence the weight of the machinetoo. If we use cast iron there may
be chances of blow holes in it while casting. so nowrolled steels
are developed and these have consistent magnetic and mechanical
properties.End Shields or BearingsIf the armature diameter does not
exceed 35 to 45 cm then in addition to poles endshields or frame
head with bearing are attached to the frame. If the armature
diameter isgreater than 1m pedestral type bearings are mounted on
the machine bed plate outsidethe frame. These bearings could be
ball or roller type but generally plain pedestralbearings are
employed. If the diameter of the armature is large a brush holder
yoke isgenerally fixed to the frame.17. Explain the Working of
dynamometer type wattmeterThe principle of operation of the
electrodynamometer-type wattmeter is the sameas that for
dynamo-electric machines. The deflection torque is produced by
theinteraction of two magnetic fluxes. One of the fluxes is
produced by a fixed coil whichcarries a current proportional to the
load current and therefore called the current coil. Theother flux
is created by a movable coil which carries a current proportional
to the loadvoltage and thus called the voltage or potential coil. A
high non-inductive resistance isconnected 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 inseries
with the load.This coil is divided in to two parts and they are
kept parallel to eachother. 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
hundredsof turns. It has a non - inductive high resistance in
series with itThe wattmeter is an electrodynamic instrument for
measuring the electric poweror the supply rate of electrical energy
of any given circuit. The device consists of a pair offixed 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 isconnected in parallel. Also, on analog
wattmeters, the potential coil carries a needle thatmoves over a
scale to indicate the measurement. A current flowing through the
currentcoil generates an electromagnetic field around the coil. The
strength of this field isproportional to the line current and in
phase with it. The potential coil has, as a generalrule, a
high-value resistor connected in series with it to reduce the
current that flowsthrough it. The result of this arrangement is
that on a dc circuit, the deflection of theneedle is proportional
to both the current and the voltage, thus conforming to the
equationW=VA or P=EI. On an ac circuit the deflection is
proportional to the averageinstantaneous product of voltage and
current, thus measuring true power, and possibly(depending on load
characteristics) showing a different reading to that obtained by
simplymultiplying the readings showing on a stand-alone voltmeter
and a stand-alone ammeterin the same circuit. The two circuits of a
wattmeter are likely to be damaged by excessivecurrent. The ammeter
and voltmeter are both vulnerable to overheating - in case of
anoverload, their pointers will be driven off scale - but in the
wattmeter, either or even boththe current and potential circuits
can overheat without the pointer approaching the end ofthe scale!
This is because the position of the pointer depends on the power
factor, voltageand current. Thus, a circuit with a low power factor
will give a low reading on thewattmeter, 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 certainvoltage level to AC power at a different voltage,
but at the same frequency.The construction of a transformer
includes a ferromagnetic core around whichmultiple 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 theprimary coil induces an alternating
magnetic flux that 'flows' around the ferromagneticcore, changing
direction during each electrical cycle. The alternating flux in the
core inturn induces an alternating current in each of the secondary
coils. The voltage at each ofthe secondary coils is directly
related to the primary voltage by the turns ratio, or thenumber 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 asecondary coil consists of 25
turns, the secondary voltage is then:secondary voltage = (480
volts) * (25/100) = 120 voltsTwo coils of wire (called windings)
are wound on some type of core material. Insome cases the coils of
wire are wound on a cylindrical or rectangular cardboard form.
Ineffect, the core material is air and the transformer is called an
AIR-CORETRANSFORMER. 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 transformeris called an
IRON-CORE TRANSFORMER. Most power transformers are of the
ironcoretype. 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
primarywinding 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 bephysically small, yet efficient. The iron-core
transformer provides better power transferthan does the air-core
transformer. A transformer whose core is constructed of
laminatedsheets 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 laminatedsteelcores.
These steel laminations are insulated with a non conducting
material, such asvarnish, and then formed into a core. It takes
about 50 such laminations to make a core aninch thick. The purpose
of the laminations is to reduce certain losses.