UNIT 1
Electrical Energy generation utilization and conservation
GENERATION
1) What are the sources of energy?
Electrical energy is produced from energy available in various forms in
nature. The sources of energy are
• The Sun
• The wind
• Water
• Fuels
• Nuclear energy
2) Name the different types of power generation.
Conventional methods (With prime movers)
a) Hydro power generation
b) Thermal power generation
c) Nuclear power generation
Non-Conventional methods (Without prime movers)
d) MHD (magneto hydro dynamic) power generation
e) Solar power generation
f) Fuel cells generation
g) Thermo electric generation
h) Thermionic converters
i) Solar cells
j) Wind power generation
k) Geo-thermal energy generation
l) Tidal power generation
3) How will you classify hydro-electric plants according to nature of load?
Classification according to nature of load
Base load plants
Pear load plants
Pumped storage plants for peak load.
4) What is penstock?
From the reservoir the water is carried to valve house through pressure
tunnel and from valve house to the water turbine through pipes of large
diameter made of steel or reinforced concrete, called the penstock.
5) What is the principle of pumped storage scheme?
The basic principle of pumped storage scheme is to convert the surplus
electrical energy generated by a power plant or available in a system in offpeak
periods, to hydraulic potential energy, in order to generate power in
periods where the peak demand on the system exceeds the total available
capacity of the generating stations.
6) What are the factors to be considered while selecting a site for steam
power plants?
The factors to be considered for selecting the steam power plant are as
follows:
Nearness to the load centre
Availability and supply of cooling water
Availability of coal
Land requirement
Transport facilities and Ash disposal facilities.
7) What is the function of deaerator in steam power plant?
The function of deaerator is to reduce dissolved oxygen content in the
condensate i.e. in the feed water. The feed water is then pumped into boiler
through economizer in which it is further heated by the heat of the flue gas
passing through it on the way to chimney.
8) What are the disadvantages of steam power plants?
The disadvantages are
High maintenance and operating cost.
Pollution of atmosphere due to fumes and residues from
pulverized fuels.
Requirement of water in huge quantity.
Handling of coal and disposal of ash is quite difficult.
Troubles from smoke and heat from the plant.
Requires long time for erection and put into action.
9) What is nuclear fission?
In nuclear station, heavy elements such as Uranium (U235) or Thorium
(Th232) are subjected to nuclear fission in a special apparatus known as
reactor.
Fission:
The breaking up of nuclei of heavy atoms into two nearly equal parts with
release of huge amount of energy is known as nuclear fission.
10) What are the components of nuclear reactor?
The nuclear reactor consists of the following basic components
• Reactor core
• Moderator
• Control rods
• Coolant
• Reflector
• Thermal shielding
• Reactor vessel
• Biological shield.
11) What are the merits of nuclear power plants?
The advantages are
The amount of fuel required is small; therefore, there is no
problem of transportation, storage etc.
The demand for coal, oil and gas is reduced which are
tending to rise in cost as the stocks are becoming depleted.
These plants need less area as compared to any other plant.
A 2000MW nuclear plant needs 80 acres whereas thermal
stations need about 250 acres of land.
Most economical in large capacity.
The operating cost is quite low and once the installation is
completed, the loading of the plant is always operated as a
base load plant.
12) What are the advantages of MHD generation?
The advantages are
The conversion efficiency is around 50% to 60%.
No moving part, so more reliable.
Capital cost is less compared with conventional steam
plants.
Overall generation cost is less.
Economic and reduced fuel consumption.
The closed cycle system produces power free of pollution. 9 Elimination of energy losses.
13) What is solar cell?
The solar cells operate on the principle of photo voltaic effect, which
is a process of generating an emf as a result of the absorption of ionizing
radiation.
It is possible to convert solar energy directly into electrical energy by
means of silicon wafer photo-voltaic cells, also called the solar cells, without
any intermediate thermodynamic cycle.
Thus a solar cell is a transducer, which converts the sun’s radiant
energy directly into electrical energy and is basically a semi-conductor diode
capable of developing a voltage of 0.5-1 volts and a current density of 20-40
mA/sq.cm depending on the materials used and the conditions of sunlight.
14) What are the types of collectors used in solar power generation?
Types of collectors æ Flat plate collectors(60°C) æ Focusing or concentrating collectors
• Cylindrical parabolic concentrator (100-200°C)
• Paraboloids, Mirror Arrays( <200°C)
15) What is the basic principle of wind power generation?
Winds are essentially caused by the solar heating of the atmosphere.
They carry enormous quantity of energy. Wind as a source of power is very
attractive because it is plentiful, inexhaustible, renewable and non-polluting.
There is no depletion of scarce resources. In large portion of the world, wind
blows for 320 days in a year and this gives them an advantage over sunlight
in direct conversion programmes, operating cost of a wind mill is negligible.
Further, it does not impose extra burden on the environment.
The ideal maximum efficiency using Froud momentum theory is
equal to 59% but an overall efficiency of 30% could be had due to
aerodynamic and other mechanical losses. This gives a power of about
0.3KW/sq.m for a wind velocity of 10m/s.
16) What is tide?
TIDE is a periodical rise and fall of the water level of sea which are
carried by the action of the sun and moon on the water of the earth.
The main feature of the tidal cycle is the difference in water surface
elevations at the high tide end, the tidal energy can be converted into
electrical energy by means of a generator.
17) What is the function of moderator in nuclear power plant?
A nuclear reactor is a cylindrical stout pressure vessel and
houses fuel rods of Uranium, moderator and control rods. The
fuel rods constitute the fission material and release huge
amount of energy when bombarded with slow moving
neutrons.
The moderator consists of graphite rods which enclose the fuel
rods. The moderator slows down the neutrons before they
bombard the fuel rods.
18) What are the merits and demerits of tidal power generation?
The advantages of tidal power are
It is free from the problems of uprooting the people and
disturbing the ecology balance.
It is everlasting and is not influenced by the charging mood
of the nature such as failure of the monsoon.
No extra submerging of land is involved.
The major drawback of tidal power plants is their uneven operation.
Variations in the tidal energy available through the lunar day and lunar
month different from their solar counterparts prevent the tidal power from
being regularly used in power systems during the periods of peak demand.
19) What is thermal efficiency?
The ratio of heat equivalent of mechanical energy transmitted to the
turbine shaft to the heat of combustion of coal is known as Thermal
efficiency of Steam power station.
20) What are the types of wind mills?
Wind energy conversion system are classified into two types,
i) Horizontal axis wind mills
The axis of rotation is horizontal and in the aero turbine, plane is
vertical facing the wind.
ii) Vertical axis wind mills
The axis of rotation is vertical, the blades also be vertical.
UNIT-2
CONSERVATION
21) What do you mean by Economics of power generation?
The art of determining the per unit i.e. one KWh cost of production of
electrical energy is known as Economics of power generation.
22) Explain the term depreciation.
The decrease in the value of the power plant equipment and building
due to constant use is known as depreciation.
In practice, every power station has a useful life ranging from fifteen
to thirty years. From the time the power station is installed, its equipment
steadily deteriorates due to wear and tear so that there is a gradual reduction
in the value of the plant. This reduction in the value of plant every year is
known as annual depreciation.
23) Define load factor.
Load factor is the ratio of average demand to the maximum demand
during a certain period of time and is applicable to both generating
equipment and receiving equipment.
Load factor = Average demand
Maximum demand
24) What is load curve?
The curve showing the variation of load on the power station with
reference to time is known as a load curve.
The load curves supply the following information
The variation of the load during different hours of the day.
The area under the curve represents the total number of units
generated in a day.
The peak of the curve represents the maximum demand on the
station on the particular day.
The area under the load curve divided by the number of hours
represents the average load on the power station.
The ratio of the area under the load curve to the total area of
the rectangle in which it is contained gives the load factor.
25) Define diversity factor.
Diversity factor is defined as the state of being dissimilar to one
another. It is defined as the ratio of sum of the maximum demands of
individual consuming units in a group during a specified period to the
maximum demand of the whole group during the same period.
Diversity factor = Sum of individual demands of different units in a group
Maximum demand of the entire group
The value of diversity factor is always greater than one. If the diversity
factor is higher, the cost per unit of generation will be lesser.
26) What do you mean by utilisation factor?
It is a measure of the utility of the power plant capacity and is the
ratio of maximum demand to the rated capacity of the power plant. It is
always less than unity.
Utilisation factor = Maximum Demand on the power station
Rated capacity of the power station
A low value of utilisation factor indicates that the plant has been installed
much in advance of need. A high value indicates that the plant is probably
most efficient in the system. If its value exceeds unity, it means that the load
has been carried in excess of rated capacity of the plant
.
27) Write short note on load duration curve?
When the load elements of a load curve are arranged in the order of
descending magnitudes, the curve thus obtained is called load duration
curve. The load curve is obtained from the same data as the load curve but the
ordinates are arranged in the order of descending magnitudes. In other
words, the maximum load is represented to the left and decreasing loads are
represented to the right in the descending order. Hence the area under the
load duration curve and the load curve are equal.
or and diversity factor. Load factor and diversity factor play a vital role in the cost of the
supply of electrical energy. Higher the values of load factor and diversity
factor, lower will be the overall cost per unit generated.
28) Write the significance of load factor?
Higher load factor means greater average load, resulting in greater
number of units generated for a given maximum demand. Thus, the standing
charges, which are proportional to maximum demand and independent of
number of units generated, can be distributed over a large number of units
supplied and therefore overall cost per unit of electrical energy generated
will be reduced.
29) What is mean by base load?
The unvarying load which occurs almost the whole day on the station
is known as base load.
30) What are the methods for determining depreciation charges?
The decrease in the value of the power plant equipment and building
due to constant use is known as depreciation. The cost of depreciation will
depend on the size and type of equipment and on its estimated life.
The reduction in the value of the plant every year is known as annual
depreciation. Due to depreciation the plant has to be replaced by new one
after its useful life. Therefore a suitable amount must be set aside every year,
so that by the time the plant retires the collected amount by way of
depreciation equals the cost of equipment.
The methods commonly used for determination of annual depreciation
charges are
• Straight line method
• Diminishing value method and
• Sinking fund method.
31) What are the objectives of tariff?
Electrical energy is sold at such a rate so that it not only returns the
cost but also earns reasonable profit. Therefore, a tariff must cover the
following items:
Recovery of cost of capital investment in generating, transmitting and distributing equipment.
Recovery of cost of operation, supplies and maintenance of
equipment.
Recovery of cost of metering equipment, billing, collection costs etc.
A satisfactory return on the total capital investment.
32) Define energy audit.
Energy Audit means studying the energy consumption pattern in the
utilities or equipments by obtaining necessary data analyse the same to
identify the areas where wastages or loses occur and suggest methods to
avoid wastages or loss and also other consumption measures to ensure
efficient use of energy.
33) What are the causes of low power factor?
The following are the causes of low power factor 9 Most of the a.c motors are of induction type which have low
lagging power factor. These motors work at a power factor
which is extremely small on light load 0.2 to 0.3 and rises to 0.8
to 0.9 at full load. 9 Arc lamps, electric discharges lamps and industrial heating
furnaces operate at low lagging power factor. 9 The load on the power system is varying, being high during
morning and evening and low at other times. During low load
period, supply voltage is increased which increases the
magnetization current. This results in the decreased power
factor.
34) Define the term connected load factor.
It is the sum of the continuous rating in KW of all electrical devices
installed at the consumer’s premises and connected to the supply system.
35) What are the important points to be taken into consideration while
selecting the size and number of units? 9 The load on a power station is never constant due to variable demands from time to time. The nature of these demands can be
seen from the load curve. The load variation is greater with a
poorer load factor.
The selection of the number and sizes of the units is decided
from the annual load curve of the station. The number and size
of the units are selected in such a way that they correctly fit the
station load curve.
The capacity of the plant should be made 15% to 20% more than
the maximum demand to meet the future load requirements.
There should be a spare generating unit so that repairs and
overhauling of the working units can be carried out.
UNIT-3
ILLUMINATION, HEATING AND WELDING
36) Define luminous flux.
It is defined as the total quantity of light energy emitted per second
from a luminous body. It is represented by symbol F and is measured in
lumens. The conception of luminous flux helps us to specify the output and
efficiency of a given light source.
37) What is meant by candle power?
It is defined as the number of lumens given out by the source in a unit
solid angle in a given direction. It is denoted by CP.
CP=lumens
38) Define MHCP.
The mean of candle power in all directions in the horizontal plane
containing the source of light is termed as Mean Horizontal Candle Power.
39) Define utilisation factor.
It is defined as the ratio of total lumens reaching the working plane to
total lumens given out by the lamp.
Utilisation factor= Total lumens reaching the working plane
Total lumens given out by the lamp
40) What are the laws of illumination?
Law of Inverse Squares:
Illumination at appoint is inversely proportional to square of its distance
from the point source and directly proportional to the luminous intensity
(CP) of the source of light in that direction.
If a source of light emits light equally in all directions be placed at the
centre of a hollow sphere, the light will fall uniformly on the inner surface of
the sphere. If the sphere be replaced by one of the larger radius, the same
total amount of light is spread over a larger area proportional to the square of
the radius.
Lambert’s cosine law:
The illumination at a point on a surface is proportional to cosine of the angle
which ray makes with the normal to the surface at that point.
41) What is meant by luminance?
It is defined as the luminous intensity per unit projected area of either
a surface source of light or a reflecting surface and is denoted by L.
42) Define space-height ratio.
It is defined as the ratio of horizontal distance between adjacent lamps
and height of their mountings.
Space-height ratio= Horizontal distance between two adjacent lamps
Mounting height of lamps above working plane
43) What is polar curve?
In most lamps or sources of light the luminous intensity is not the same
in all directions. If the luminous intensity, i.e. the candle power is measured
in a horizontal plane about a vertical axis and a curve is plotted between
candle power and the angular position, a curve is obtained is called as
horizontal polar curve. The luminous intensity in all the directions can be represented by polar curves. If the luminous intensity in a vertical plane is plotted against the
angular position, a curve known as vertical polar curve is obtained.
44) Name the various photometer heads.
1. Bunsen Head (or) Grease spot photometer
2. Lummer-Brodhun photometer head
There are two types of Lummer Brodhun heads
a) Equality of Brightness type photometer head
b) Contrast type photometer head
45) What are all the sources of light?
According to principle of operation the light sources may be grouped
as follows.
Arc lamps
High temperature lamps
Gaseous discharge lamps
Fluorescent type lamps
46) What is stroboscopic effect of fluorescent tubes?
With a.c. supply frequency of 50 cycles per second, discharge through
the lamp becomes zero, 100 times in a second. Due to the persistence of
vision, our eyes do not notice this. If this light falls on moving parts, they
may appear to be either running slow or in the reverse direction or even may
appear stationary. This effect is called stroboscopic effect.
47) Define beam factor.
The ratio of lumens in the beam of a projector to the lumens given out
by lamps is called the beam factor. This factor takes into account the
absorption of light by reflector and front glass of the projector lamp. Its
values vary from 0.3 to 0.6.
48) Mention the types of lighting schemes.
The distribution of the light emitted by lamps is usually controlled to
some extent by means of reflectors and translucent diffusing screens or even
lenses. The interior lighting schemes may be classified as
Direct lighting
Semi-direct lighting
Indirect lighting
Semi-indirect lighting
General lighting
49) What are the drawbacks of discharge lamps?
Drawbacks of discharge lamps:
• Take time to attain full brightness.
• High initial cost and poor power factor.
• Starting requires trigger-starter.
• Light output fluctuates at twice the supply frequency. The
flicker causes stroboscopic effect.
• These lamps can be used only in particular position.
50) What are the requirements of lighting system/
The following factors are required to be considered while designing
the lighting scheme.
Illumination level
Uniformity of illumination
Colour of light
Shadows
Glare
Mounting height
Spacing of luminaries
Colour of surrounding walls.
51) What are the advantages of electric heating?
The main advantages of electric heating over other systems of heating
such as coal, oil or gas heating are given below.
o Economical
o Cleanliness
o Absence of flue gases
o Ease of control or adaptation
o Automatic protection
o Upper limit of temperature
o Special heating features
o High efficiency of utilisation
o Better working conditions
o Safety
o Heating of non-conducting materials
52) Classify the methods of electric heating.
Kinds of electric heating
A. Power frequency heating
a. Resistance heating
i) Direct resistance heating
ii) Indirect resistance heating
iii) Infrared or Radiant heating
b. Arc heating
i) Direct arc heating
ii) Indirect arc heating
B. High frequency heating
a. Induction heating
i) Direct induction heating
ii) Indirect induction heating
b. Dielectric heating
53) What is meant by indirect resistance heating? 9 In this method, the current is passed through a high resistance
wire known as heating element. The heat produced due to I2 R
loss in the element is transmitted by radiation or convection to
the body to be heated.
Applications are room heaters, in bimetallic strip used in
starters, immersion water heaters and in domestic and
commercial cooking and salt bath furnace.
54) What are the properties of heating element material?
The material of the heating elements should posses the following
desirable properties for efficient operation and long life.
High resistivity: It should have high specific resistance so that the overall
length to produce a certain amount of heat may be smaller.
High melting point: It should have high melting point so that high
temperatures can be produced without jeopardizing the life of the element.
Free from oxidation: It should be able to resist oxidation at high
temperatures, otherwise its life will be shortened.
Low temperature coefficient: It should have a low temperature coefficient so
that resistance remains appreciably constant even with increases of
temperature. This helps in accurate control of temperature.
55) What are the causes of failure of heating elements?
Principle causes are
Formation of hot spots
General oxidation of the element and intermittency of
operation
Embrittlement caused by grain growth
Contamination of element or corrosion
56) Write short note on infrared heating.
In radiant heating, the elements are of tungsten operating about 2300°C
as at this temperature a greater proportion of infra-red radiation is given off.
Heating effect on the charge is greater since the temperature of the
heating element is greater than in the case of resistance heating. Heat
emission intensities up to 7500 watts/sq.m can be obtained leading to heat
absorption up to 4300 watts/sq.m. This reduces the time taken by various
drying process.
57) What is the basic principle of induction heating?
It works on the principle of electromagnetic induction as same as a
transformer. It has a metal disc surrounded by a copper coil in which a.c
supply is flowing. The disc has a finite value of diameter and thickness and
is spaced a given distance from the coil and concentric to it. We find that a
secondary current is caused to circulate around the outer surface of the disc.
58) What are the different types of resistance welding?
The different types are as follows
Butt welding
Spot welding
Projection welding
Seam welding
Percussion welding
60) What is LASER welding?
LASER (Light Amplification by Stimulated Emission of Radiation)
welding is a welding process that uses the heat from a laser beam impinging
on the joint. The process is without a shielding gas and pressure.
UNIT-4
ELECTRIC TRACTION
61) What are the requirements of an ideal traction system?
The requirements of an ideal traction system are as follows
The starting tractive effort should be high so as to have
rapid acceleration.
The wear on the track should be minimum.
Pollution free
Speed control should be easy.
The equipment should be capable of withstanding large
temporary loads.
Low initial and maintenance cost.
There should be no interference to the communication lines
running along the lines.
Braking should be such that minimum wear is caused on the
brake shoes.
62) Name the various systems of traction.
1. Direct steam engine drive
2. Direct Internal Combustion Engine Drive
3. Steam Electric Drive
4. Internal Combustion Engine Electric Drive
5. Petrol Electric traction
6. Battery Electric Drive
7. Electric Drive
63) Classify the supply system for electric traction.
A. D.C system
B. A.C system
i) Single phase
ii) Three phase
C. Composite system
i) Single phase AC-DC
ii) single phase-Three phase
64) What are the advantages of electric traction?
High starting torque
Less maintenance cost
Cheapest method of traction
Rapid acceleration and braking
Less vibration ô Coefficient of adhesion is better
It has great passenger carrying capacity at higher speed.
65) What are the disadvantages of electric traction?
• High capital cost
• Problem of supply failure
• Additional equipment is required for achieving electric
braking and control
• The leakage of current from the distribution mains and
drop of volts in the track are to be kept within the
prescribed limits.
• The electrically operated vehicles have to move on
guided track only.
66) What is meant by speed-time curve? Why it is used?
The movement of the train and their energy consumption can be
studied by means of speed-time and speed-distance curves, which shows the
speed at different time instants after start of run and speed at different
distances from the starting point respectively. Of the two, the speed-time
curve is generally used. The curve drawn between speed in Kw/hr along Y-axis and time in seconds along X-axis is called speed=time curve. The speed-time curve
gives the complete information about the motion of the train.
This curve gives the speed at various time instants after the start of run
directly. Slope of the curve at any point gives the speed at that instant. The
area under the curve gives the total distance traveled by the train.
67) What do you mean by average speed in electric traction?
The mean of the speeds from the start to stop i.e the distance between
two stops divided by the actual time of run is known as average speed.
Average speed= Distance between stops in km
Actual time of run in hours
68) What do you mean by schedule speed in electric traction?
The ratio of distance covered between two stops and total time of run
including time of stop is known as schedule speed.
Schedule speed= Distance between stops in km
Actual time of run in hours+ Stop time in hours
The schedule speed is always smaller than the average speed. The difference
is large in case of urban and suburban services and is negligibly small in
case of main line service.
69) What is tractive effort?
The effective force necessary to propel the train at the wheels of the
locomotive to which the motor is geared is called the geared effort. It is
measured in Newtons and is tangential to the driving wheels.
Total tractive effort required to run a train on track
= Tractive effort to produce acceleration +
Tractive effort to overcome effect of gravity +
Tractive effort to overcome train resistance.
70) What are the factors affecting energy consumption?
The various factors affecting energy consumption are
(i) Distance between the stops
The greater the distance between the stops, the lesser will be the
specific energy consumption for suburban service is 50 to 75 watts-hour/
ton-km and for main line service it is between 18 to 32 watt-hour/ton-km.
(ii) Train resistance
The train resistance depends upon the nature of track, speed of the
train and shape of the rolling stock, particularly the front and rear portions of
the train. If the train resistance is greater, the specific energy consumption is
more.
(iii) Acceleration and retardation
If the acceleration and retardation increases, the specific energy
consumption is increased.
(iv) Gradient
The steep gradients will involve more energy consumption though
regenerative braking is applied.
(v) Train equipment
More efficient train equipment will reduce the specific energy
consumption.
71) Define dead weight, adhesive weight.
(i) Dead weight
The total weight of locomotive and train to be pulled by the
locomotive is known as dead weight.
(ii) Adhesive weight
The total weight to be carried on the driving wheels is known as
the adhesive weight.
72) Name the various methods of traction motor control.
There are various methods for controlling the speed of d.c series motors.
They are
Rheostatic control
Series parallel control
Field control
Buck and Boost method
Metadyne control
Thyristor control
73) What are the basic requirements of braking system?
The basic requirements of a braking system are given below
It should be simple, robust, quick and reliable in action.
Easy to use for driver to operate.
Maintenance should be minimum.
The braking system should be inexhaustible.
In case of emergency braking, safety consideration is taken
into account.
Kinetic energy of the train must be storable during braking
which could be used subsequently during acceleration of the
train.
74) What are the various methods of applying electric braking?
There are three methods of applying electric braking are
• Plugging or Reverse current braking
• Rheostatic braking
• Regenerative braking.
75) Name the advanced methods of speed control of traction motors.
The latest methods of speed control of traction motors are
Tap changer
Thyristor control
Chopper control ô Microprocessor control
76) What are the advantages of microprocessor based control of traction
motors?
The advantages of microprocessor based drives are 9 High speed of response 9 High accuracy 9 Over voltage and over speed protection. 9 Electronic interlocking 9 Less sensitive to temperature variations and drift. 9 Numbers of components used are less.
UNIT-5
DRIVES AND THEIR INDUSTRIAL APPLICATION
77) What is meant by electrical drives?
Systems employed for motion control are called “DRIVES” and drives
employ any of the prime movers such as, diesel or petrol engines, gas or
steam turbines, hydraulic motors and electric motors for supplying
mechanical energy for motion control. Drives employing electric motion
known as “Electric Drives”.
78) Mention the parts of electrical drives?
• Electric motors and load
• Power modulator
• Sources
• Control unit
• Sensing unit
79) Mention the application of electric drives?
Paper mills
Electric traction
Cement mills
Steel mills.
80) Mention the different types of classes of duty?
• Continuous duty
• Discontinuous duty
• Short time duty
• Intermittent duty
81) Define equivalent current method.
The motor selected should have a current rating more than or equal to
the current. It is also necessary to check the overload capacity of the motor.
This method of determining the power rating of motor is known as the
equivalent current method.
82) What are the three methods of operation of electric drive?
Steady state
Acceleration including starting
Decceleration including stopping.
83) Define four-quadrant operation?
A motor operate in two modes, motoring and braking. In motoring, it
converts electrical energy into mechanical energy, which supports its
motion. In braking it works as a generator converting mechanical energy
into electrical energy and thus, opposes the motion. Motor can provide
motoring and braking operations for both forward and reverse directions.
84) Mention the types of braking?
• Regenerative braking
• Dynamic braking
• Plugging
85) Define and mention different types of braking in a dc motor?
In braking, the motor works as a generator developing a negative torque
which opposes the motion. Types of regenerative braking are Dynamic (or)
Rheostat braking; and plugging (or) reverse voltage braking.
86) List the drawbacks of armature resistance control?
In armature resistance control, speed is varied by wasting power in
external resistors that are connected in series with armature. Since it is an
inefficient method of speed control, it was used in intermittent load
application where the duration of low speed operation forms only a small
proportion of total running time.
87) Mention the methods of armature voltage control dc motor?
When the supply voltage is ac
i) Ward-Leonard schemes
ii) Transformer with taps and uncontrolled rectifier bridge
iii) Static Ward-Leonard scheme (or) controlled rectifiers
When the supply is dc
i) Chopper control
88) What are the disadvantages of conventional ward-Leonard schemes?
• Higher initial cost due to use of two additional machines.
• Large weight and size.
• Needs more floor space and proper foundation.
• Required frequent maintenance.
• Higher noise and higher loss.
89) Mention the drawbacks of rectifier fed dc drives? É Distortion of supply É Low power factor É Ripple in motor current
90) What are the different methods of speed control of induction motors?
• Stator voltage control
• Supply frequency control
• Rotor resistance control
• Slip power recovery control
91) What is meant by stator voltage control?
The speed of the induction motor can be changed by changing the
stator voltage. Because the torque is proportional to square of the voltage.
92) Mention the applications of stator voltage control?
The stator voltage control method is suitable for applications where
torque demand reduced with speed, which points towards its suitability for
i) Fan ii) Pump drives
93) Mention the applications of AC drives?
AC drives are used in a number of applications such as fans, blowers,
mill run-out tables, cranes, conveyors, traction etc.
94) What are the advantages of stator voltage control method?
• The control circuitry is simple
• Compact size
• Quick response time
• There is considerable savings in energy and thus it is economical
method as compared to other methods of speed control.
95) What are the features of variable frequency control?
Speed control and braking operation are available from
zero speed to above base speed.
Drop in speed from no load to full load is small.
Copper losses are low. Hence efficiency and power factor are high as the operation is restricted between synchronous speed and maximum torque point at all frequencies. During transient (starting, braking and speed reversal) operation can be carried out at the maximum torque with reduced current giving good dynamic response.
96) What is meant by frequency control of induction motor?
The speed of the induction motor can be controlled by changing the
supply frequency, because the speed is directly proportional to supply
frequency. This method of speed control is called frequency control.
97) What is meant by V/F control?
When the frequency is reduced, the input voltage must be reduced
proportionally so as to maintain constant flux. Otherwise the core will get
saturated resulting in excessive iron loss and magnetizing current. This type
of induction motor behaviour is similar to the working of dc series motor.
98) What is meant by regenerative braking?
Regenerative braking occurs when the motor speed exceeds the
synchronous speed. In this case, the induction motor would runs as the
induction machine is converting the mechanical power into electrical power,
which is delivered back to the electrical system. This method of braking is
known as regenerative braking.
99) What is meant by dynamic braking?
Dynamic braking of electric motor occurs when the energy stored in
the rotating mass is dissipated in an electrical resistance. This requires the
motor to operate as a generator to convert this stored energy into electrical.
100) What is meant by plugging?
It is one method of braking of induction motor. When phase
sequence of supply of the motor running at a speed is reversed, by
interchanging connections of any two phases of stator with respect to supply
terminals, operation shifts from motoring to plugging region.
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