[Q:-01]
What are the various traction systems in practice In our country ?
Ans
:
The various traction systems prevailing in
our country are Steam Engine drive, Internal Combustion Engine drive, Diesel
Electric drive, Battery Electric drive & Electric drive in which Steam
Engine drive and Internal Combustion Engine drive fall in the category of non-electric
traction system and Diesel Electric drive, Battery Electric drive and Electric
drive fall in the category of electric traction system.
[Q:-02]
What do you mean by electric traction system and what are its advantages?
Ans
:
The system of traction involving the use
of electricity is called the electric traction system.
Advantages —
(1) It is the
cleanest of all other traction systems.
(2) The
acceleration and retardation in electric traction system is comparatively
higher.
(3) Electrical
braking is superior than mechanical braking.
(4) It
requires less maintenance.
(5) The
wastage of energy is comparatively less in electrical traction system
[Q:-03]
What are the requirements of an ideal traction system ?
Ans
:
The requirements of an ideal traction
system are as follows:—
(1) High
starting tractive effort in order to have rapid acceleration.
(2) Capability
of withstanding large temporary overloads.
(3) Easy speed
control.
(4) Braking
arrangements with minimum wear caused on the brake shoes.
(5) Ability to
run on any route by its self-contained compactness.
(6) Minimum
wear on the track.
(7) Minimum
equipment of high efficiency and low initial and maintenance cost.
(8) No
interference to the communication lines running along the track.
[Q:-04]
Why Steam Engine drive is not suitable for urban or sub-urban services ?
Ans
:
Acceleration and retardation with steam
engine drive is not possible as high as with electric drive due to low power
weight ratio for its low co-efficient of adhesion. Therefore steam engine drive
is not suitable for urban or sub-urban services where distance between stations
is small.
[Q:-05]
Why Steam Engine drive is not suitable for underground railway?
Ans
:
Owing to smoke steam engine drive is not
suitable for underground railway.
[Q:-06]
Name different systems of electric traction?
Ans
:
Systems of electric traction may be
divided into two main groups :
(1) In the
first group, the vehicles depend on the external source of electric power for
their running. They receive electric power from a distributing network fed at
suitable points from either a central power station or substations suitably
spaced.
This group is
sub divided into —
(a) Systems operating with d.c. such as
trolley buses. train ways and railways.
(b) Systems operating with a.c. such as
railways.
(2) In the
second group, the locomotives are self-contained in which the sources of
electric power are incorporated within them. This group is sub divided into —
(a) Diesel-electric trains and ships.
(b) Petrol-electric trucks and lodes.
(c) Battery driven road vehicles.
[Q:-07]
What are the various systems of track electrification?
Ans
:
The systems of track electrification are
as follows :—
(1) D. C.
System,
(2) Single
phase A. C. System.
(3) Three
phase A.C. System and
(4) The
composite system of
(a) Single phase to three phase system or
kondo system &
(b) Single phase to direct current system.
[Q:-08]
In D.C. system what types of motors are usually used ?
Ans
:
Usually d.c. series motors are used but in
case of the application of regenerative braking compound motors are preferred.
[Q:-09]
What ranges of operating voltage are used in D.C. system?
Ans
:
The operating voltage within the distance
of 2.5 kms from the substation is from 600 volts to 750 volts for tramways and
for many sub urban railways whereas the operating voltage in sub urban services
within the distance of 20 kms from the substation is from 1500 volts to 3000
volts for main line railways.
[Q:-10]
Where is D.C. system preferred?
Ans
:
The D.C. System is preferred for sub urban
services and road transport where stops are frequent and distribution distances
are small.
[Q:-11]
network in single phase A.C. system?
Ans
:
The operating voltage for distribution
network in single phase A.C. system is 15 kV to 25 kV at 16 2/3
or 25 Hz which is stepped down on the locomotive to a low voltage of 300 to 400
volts suitable for supplying to single phase a.c. series motor by mean of a
step down transformer carried in the locomotive.
[Q:-12]
What is the necessity of changing supply frequency in single phase a.c. system?
Ans
:
The change of supply frequency becomes
necessary because of better performance of the a.c. series motors at low
frequency.
[Q:-13]
Where is single phase A.C. system is adopted ?
Ans
:
Single phase A.C. system is adopted for
main line service where the cost of overhead structure is of more importance
and rapid acceleration and retardation is not so important as for sub-urban
railways.
[Q:-14]
In three phase A.C. system how the three phase supply is given?
Ans
:
The three phase supply is given in
distribution system by using two overhead wires for two phases and track rail
for the third phase receiving power either directly from the generating station
or through the transformer substation at the operating voltage of 3300 to 3600
volts and normal frequency or 16 2/3 Hz.
[Q:-15]
Why three phase traction system employing induction motors is now obsolete ?
Ans
:
The three phase A.C. traction system
employing induction motors is now obsolete on account of disadvantages of two
overhead contact wires and constant speed characteristics of three phase
induction motor and also for a chance of getting electrocuted from the live
third phase track rail .
[Q:-16]
What is the distribution voltage and how the driving action is adopted in
single phase to three phase composite system?
Ans
:
The voltage used for distribution network
is A.C. single phase 15 kV at 50 Hz. The single phase a.c. supply is converted
into three phase supply by using a phase converter carried in the locomotive.
In addition an inverter circuit using silicon controlled rectifier is used in
it to get variable low frequency supply (1/2 to 9 Hz) at which three phase
induction motor develops high starting torque without excessive current and
speed control is also conveniently obtained by varying the supply frequency.
[Q:-17]
Which system of track electrification is being commonly adopted in our country
and why?
Ans
:
Single phase to Direct current system of
traction employing 25 kV, 50 Hz A.C. supply is adopted for track
electrification in our country and this is being installed on sections which
are being electrified. This system is adopted in all the areas of major traffic
density on main lines because in this system the advantages of the two
electrical systems are combined together by employing high voltage a.c. single
phase system for overhead distribution with high efficiency and desirable characteristics
of d.c. series motors for the a disadvantages of D.C. system can be partly
eliminated by this system.
[Q:-18]
Why single phase a.c. system is preferred for main line railway service?
Ans
:
Single phase a.c. system is preferred for
main line railway service on account of the following advantages —
(1) The
important requirement of higher maximum speed can be fulfilled.
(2) Frequent
acceleration and retardation are not required as in sub urban services.
(3) By
employing high voltage with smaller section of conductors to carry the low
current required the cost of distribution system is reduced.
(4) By
employing high voltage for distribution purpose the spacing between substations
can be higher thus reducing the number of substations.
(5) The
installation, operation and maintenance cost of a.c. substations are less due
to low cost small quantity of substation equipments.
[Q:-19]
What system of traction is preferred for sub-urban and urban services?
Ans
:
The d.c. system is preferred for sub-urban
and urban services because this system fulfils the following requirements
necessary for sub-urban railway service :—
(1) Frequent
starting & stopping with rapid acceleration and retardation.
(2) Negligible
effect of voltage fluctuation on the working of motors.
(3) No
interference to the communication line running along the track on account of power
system adopted.
[Q:-20]
What are the advantages of d.c. system?
Ans
:
(1) The energy
consumption is less in d.c. system as compared with that in a. c. system for
similar nature of service.
(2) For
exerting same torque the current requirement is comparatively less in d.c.
system.
(3) The d.c.
locomotive along with its allied equipments is lighter in weight, cheaper in
initial as well as in maintenance cost and more efficient.
[Q:-21]
What are the disadvantages of d.c. system ?
Ans
:
(1) Due to low
voltage the current required becomes high for which large section of conductors
are required resulting higher expenditure.
(2) For low
voltage distribution the spacing of substations is less requiring more number
of substations.
(3) For
providing extra converting machineries the cost involvement for substations
becomes comparatively higher.
(4) Efficiency
of d.c. substation is less.
(5) Additional
equipment such as negative booster is required for keeping the voltage of
return rail within limits.
[Q:-22]
What do you understand by speed time curve ?
Ans
:
The curve drawn between speed and time
taking speed in km/hour on the Y-axis and time is seconds or minutes on x-axis
is known as speed time curve.
[Q:-23]
What is the use of speed time curve in practice ?
Ans
:
The speed time curve gives the complete
information about the motion of the train i.e. speed at various times after
start of run directly. The slope of the curve at any point gives the
acceleration / retardation at the corresponding instant or speed. The area
covered by the curve gives the distance covered in the corresponding time.
[Q:-24]
What are the constituents of speed time curve ?
Ans
:
Speed time curve consists of
(i) initial
acceleration,
(ii) constant
speed run or free run,
(iii) coasting
&
(iv)
retardation or braking.
[Q:-25]
What are the parts of acceleration period ?
Ans
:
Acceleration period has two parts, namely
(1) Constant
acceleration or acceleration while notching up and
(2) Speed
curve running or acceleration on the speed curve.
During notching up period in constant
acceleration the current is maintained approximately constant and the voltage
across the motor is gradually increased by cutting out the starting resistance.
Acceleration remains constant during this period due to constant tractive
effort.
In the second phase during speed curve
running the voltage acting across the motor remains constant and the current
starts decreasing with the increase in speed according to the characteristics
of the motor across the motor remains constant and the current starts
decreasing with the increase in speed according to the characteristics of the
motor and finally the current becomes constant. During this period though the
train accelerates but acceleration decreases and becomes to zero at the speed
at which the tractive effort developed by the motor becomes exactly equal to
the resistance to motion of the train.
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