Q1. State the meaning of various terms of OHE labeled in fig no 1. Give their normal values.
Ans:
Q 19. With neat diagram explain trolley collector or pole collector for overhead system.
Ans:
a->Mid span height of
contact wire - 5.5 m
b->Span length — 72 m
c->Encumbrance — 1.4 m
d->Dropper spacing --9 m
Q 2. Describe the criteria for designing height of
contact wire for OHE.
i)
Ans:
Height of contact wire is decided by
considering the following:
1) Rolling stock gauge (Broad gauge, meter gauge
etc.)
2) System voltage.
3) Location of the track such as in tunnels, on
bridges, under bridges, level crossings, etc
4) Working at inspection pits, loco
sheds
5) Type of OHE: regulated or
un-regulated.
6) Surrounding temperature
Q 3. What is importance of contact wire
gradient in OHE?
Ans:
Importance of contact wire
gradient:
* When OHE passes under an over line
structure, the contact wire height is to be reduced.
* Height of OHE is increased at level
crossing.
* The change-over is achieved very
gradually depending on the speed of the pantograph.
* Else pantograph may lose contact or
excessive pressure may be exerted on contact wire.
* Higher the running speed smaller
should be the gradient.
* The gradient should not be too
much; else the system will be disturbed.
* Gradient of 4 mm per meter for speeds up-to
100 kmph.
* Gradient
of 3 mm per meter for speeds above 100 kmph.
* Gradient of 10 mm per meter for siding is
allowed.
Q 4. Compare between diamond and faiveley type
pantograph.
Ans:
|
Q 5. Define and give normal
values of the following terms used in OHE:
i) Encumbrance ii)Stagger
Ans: i) Encumbrance: It
is the axial distance between catenary and contact wire. It is 1.4 m normally.
ii) Stagger : It is the distance between contact wire and pantograph axis. Range is
675 mm to 800 mm in worst conditions.
Q 6. State any four points
which are considered while deciding the span length in overhead equipment.
Ans:
Points
considered while deciding the span length in overhead equipment:
1)
Maximum wind pressure on contact wire. (98 kg/sq.m
for coastal areas to 74 kg/sq.m for interiors,) Higher the pressure lower is
the span.
2)
Curvature of paths: low span for curves.
3)
Track layout and other local conditions such as
restricted head room etc.
4) Current collection requirements necessitate
lower spans.
5) Adjacent spans should not differ by more than 18
m.
6) Span lengths are in multiples of 4.5 m and vary from 27 m to 72 m.
Q 7. With neat sketch explain automatic weight tension and
temperature compensation.
Ans:
Automatic
weight tension and temperature compensation:
The tension in overhead lines in traction systems
depends on the temperature. Higher temperatures lead to expansion of conductors
and hence sag. It is therefore necessary to minimize the sag to least by giving
more tension to the conductors. This adjustment of tension and sag is done
automatically to facilitate sparkless collection of current from the overhead
lines by the current collectors mounted on the top of high speed loco. It works
as follows.
The
tensioning device consists of pulley block or a winch with suitable reduction
ratio. This arrangement is made on the two sides of the tensioned catenary and
contact wire. With variations in the temperature the tension in the contact
wire and catenary are maintained due to the pull of the counter weights of
around 400 kg shown in the figure. The tension in the contact wire and catenary
is around 1000 kg.
Q 8. State any four advantages of automatic weight
tensioning and temperature compensation.
Ans:
Advantages of automatic weight tensioning and
temperature compensation:
•
Sparkless current collection at higher speeds.
•
OHE becomes more dynamically stable under all
atmospheric conditions.
•
Reduction in wear of both contact wire and
pantograph collecting strips.
•
Creep of conductors with passage of time is
automatically taken up by the tensioning device.
•
Only one critical velocity of propogation of
waves in contact wire which is greater than that of unregulated OHE.
• Because of fixed tension, fluctuations in the height are minimized.
Ans:
Purpose and location of
insulated & un-insulated overlap:
Insulated overlap:
-
To isolate for operation and maintenance of OHE
if two lengths of OHE belong to two elementary sections.
-
Located at feeding posts, sub sectioning posts
and booster sections.
Un-insulated overlap:
-
Two contact wires belonging to two adjacent
sub-elementary sections are run parallel to each other for at least one span
and electrically connected to each other by means of jumpers.
-
To rectify faults in one contact wire of two
running in parallel without affecting power to OHE.
-
Located at each sub elementary section.
Q 10. State the function of (i) Neutral section, (ii) Section insulator,
(iii) Un-insulated overlap, (iv) Insulated overlap.
Ans:
(i) Neutral section: Passing of pantograph under
insulated overlap will cause short circuit between two phases of the supply
system, damaging OHE & pantograph. To avoid this bridging of two different sections
fed by two different phases, a neutral section is inserted between them.
Located between two
substations.
(ii) Section insulator: Section
insulators are provided to insulate OHE of one elementary section from the OHE
of another adjacent elementary section. Located at cross over from one track to
other, from main line to siding.
(iii) Un-insulated overlap: Two contact wires belonging to
two adjacent sub elementary sections running parallel to each other for one
span are electrically connected by jumpers. This is called as uninsulated
overlap. To rectify faults in one contact wire of two running in parallel wires
without affecting power to OHE. Located at two adjacent sub-elementary
sections.
(iv) Insulated overlap: To
provide isolation for operation and maintenance of OHE if two lengths of OHE
belong to two elementary sections.
Located at feeding posts,
sub sectioning posts and booster sections.
Ans:
Neutral
section:
Purpose - Passing of pantograph under insulated overlap
will cause short circuit between two phases, damaging OHE & pantograph. To
avoid this bridging a neutral section is inserted between them.
Location - Located between two substations.
Section
insulator:
Purpose - Section insulators are provided to insulate
OHE of one elementary section from the OHE of another adjacent elementary
section.
Location - Located at cross over from one track to
other, from main line to siding.
Q 12. Give purpose and location of
i) Uninsulated overlap. ii) Insulated overlap iii) Neutral section. iv) Section insulator.
Ans:
i) Insulated overlap: To
isolate for operation and maintenance of OHE if two lengths of OHE belong to
two elementary sections.
Located at feeding posts, sub
sectioning posts and booster sections.
ii) Un-insulated overlap: To
rectify faults in one contact wire of two running in parallel wires without
affecting power to OHE.
Located at two adjacent
sub-elementary sections.
iii) Neutral section: Passing
of pantograph under insulated overlap will
cause short circuit between two
phases, damaging OHE & pantograph. To avoid this bridging a neutral section
is inserted between them.
Located between two substations.
iv) Section insulator: Section
insulators are provided to insulate OHE of one elementary section from the OHE
of another adjacent elementary section. Located at cross over from one track to
other, from main line to siding.
Q 13. With neat sketches list the various types of construction of polygonal OHE and give
their scope of application.
Ans:
Various types of construction of
polygonal OHE and Scope of application (speed ranges),
•
Simple - for train speeds upto 120 kmph
•
Compound - for
train speeds between 190 to 225 kmph
•
Stitched or modified Y simple - for train speeds
upto 160 kmph
•
Modified Y compound - for
train speeds upto 220 kmph
Q 14. Draw a neat sketch of stitched
catenary. State the speed limit for i) 10 mY ii) 20 m Y of this type.
Ans:
i) 10
m Y : 140 - 160 kmph
ii) 20
m Y : beyond 160 kmph
Q 15. Explain effect of speed on OHE (Any four points).
Àns :
The type of the construction of OHE to be used is primarily determined by
the train speed. The effect of train speed on the current collection by the
pantograph will be understood when the behavior of OHE under the passage of pantograph is followed.
1) The pan of pantograph along the
overhead wire pushes it up by a certain amount of pushup.
2) The contact wire after the passage of
pantograph, suddenly sags and starts vibrating vertically due to elasticity of
the system.
3) When the collecting speed exceeds 120
kmph, a contact break phenomenon is likely to occur in the overhead wire system
having simple catenary system.
In order to have satisfactory current
collection at various speeds, various modifications in the construction of OHE are carried out.
i) For
speeds up to 100 kmph: Simple Catenary construction (for employing regulation
of mechanical tension)
ii) For
speeds up to 120 kmph: Simple Catenary construction
iii) For speeds up to 160 kmph: Modified Y
Simple Catenary construction
iv) For speeds beyond 160 kmph: Modified Y Compound Catenary construction
Q 16. List different types of
OHE supporting structures and describe any one of them.
Ans:
Name of
structure
|
Short Description
|
|
1
|
Normal mast
|
Have single
swiveling bracket and support one OHE.
|
2
|
Overlap
intermediate mast
|
Have two
swiveling brackets and support two OHE.
|
3
|
Anchor mast
|
In addition
to one swiveling bracket to support one OHE it provides anchoring facility
for other OHE as same happens to be at the end of tension length.
|
4
|
Anti-creep
anchor mast
|
Anchor mast
provided on both sides of anti-creep central mast.
|
5
|
Anti-creep
central mast
|
Located in
centre of tension length.
|
Q 17. Give purpose and location of following signals,
1) Warner: 2) Co-acting: 3) Shunt: 4)
Detonating:
Ans:
1) Warner signal - When in ’on' position, it indicates
that the signal ahead is at danger and driver must be prepared to stop at next
signal. When in ‘off position it indicates that the signal ahead is in proceed
position and driver can take the train speed. It is the first signal for train
entering the station.
2) Co-acting signals- Where due to obstruction such as
over bridge, arm or light of main signal does not remain in the view of driver,
during whole time co-acting signals are provided. They are fixed below the main
signal.
3) Shunt signal - For controlling the shunting
operations in the station yards shunt signals are used. They are mounted about
0.5m above ground or fixed on same post as departure signal.
4) Detonating signals- Are used in foggy weather to
indicate the locality of a signal to the driver of approaching train. These are
placed on the rails and explode when engine passes over them.
Q 18. For conductor rail system used for current
collection system:
i)
State its merit (any one) and demerit (any one).
ii) What
is done to reduce the voltage drop at joints?
iii) Suitable
system voltage.
Ans:
i) Merits:
a) Compact than OH system, b) can be used with small diameter tunnels.c) less
cost compare to OH.
Demerit: Current collection at higher speeds can lead to
accidental contact & unsatisfactory current quality; only DC workable.
ii) Copper
bonds (between rails) provided either by rivetting or welding to reduce voltage
drops at joints.
iii) (Direct
voltage) 750 V for top contact systems and 1200 V for side contact systems.
Q 19. With neat diagram explain trolley collector or pole collector for overhead system.
Ans:
Trolley collector for overhead system:
- This consists of a grooved gun metal wheel or grooved slider shoe with carbon insert carried attached to the end of a long pole provided on the top of the car.
- Other end of this pole is hinged to a swiveling base fixed to roof of vehicle.
- Necessary upward pressure for the pole and current collector is achieved by means of springs.
- As two trolley wires are required for a trolley bus a separate trolley collector is provided for each wire, the bases being mounted side by side. The pressure for wheel is approximately 10 kg and for a carbon insert slider is approximately 17 kg.
- The main drawback of trolley collector is that it has to be rotated through 180° for reversing the direction of motion of the vehicle.
- Another drawback is that there is poor contact between the wheel and trolley wire which gives rise to high current density.
- Suitable for comparative low speed (say 22 to 30 kmph).
Q
20. What is a bow collector ? Explain It with neat sketch
Ans. :
• A bow collector is a sliding current
collector, consisting of a bow-shaped strip mounted on a pivoted framework used
on locomotives, to collect current from an overhead wire.
• The main advantage of bow collector is
that it can be used for higher speed as compared to pole collector.
- It consists of two trolley collector poles at the end which is placed a light weight metal strip upto one meter long as shown in the Fig.
•
The bow collector should be mounted in
such a way that the top edge of the collector plate would rise several inches
over the wire when the bow and bow poles standing straight up.
•
Thus the collector usually inclines
opposite to the direction of the travel.
•
When it is to be travelled in opposite
direction, the collector must be swung over. It is normally achieved by ropes
and pulleys. The collector is folded down to a horizontal position when the
locomotive is not working or not in use.
•
They are having very less moving parts
than trolley collector but are heavier and sometimes difficult to construct.
•
In this, the overhead wires are
stretched more tightly as compared to trolley locomotive.
Q 21.
What is pantograph collector ?
Ans.:
• A
pantograph is a device mounted on the roof of an electric locomotive to collect
power through contact with an overhead catenary wire.
• It
is commonly used current collector equipment.
• In this typically a single wire is used, with
the return current running through the tracks.
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