3. The pantograph is employed in railways for
collection of current where the operating speed
is as high as 100 or 130 Kmph and the current
to be collected are as 2000 or 3000 amperes.
Pantograph are mounted on the roof of the
vehicles and usually carry a sliding shoe for
contact with the overhead trolley wire.
These consist of a jointed frame usually of steel
tubing.
The contact shoes are usually about 1.2 meters
long.
4. Pantograph collector maintain link between
overhead contact wire and power circuit of the
electric locomotive at different speed under all
wind conditions and stiffness of overhead
equipment.
This necessities that positive pressure must be
maintained at all times to avoid lass of contact
ad sparking but the pressure must be as low as
possible so that wear of overhead contact wire
is minimum.
5.
6. 1) It can be operate in either direction of motion.
2) There is no risk of leaving wire junction etc.
3) The erection of the overhead network is very
simple due to absence of points and grooved
crossings required for bows.
4) Its height can be varied from the driver’s cabin
by carrying out simple operations.
7. Since in railways a sliding current collector is
employed, therefore, the contact wire is required
to be suspended with very small sag so that the
contact between the wire and collector may be
maintained at high speed.
For such short spans an indirect method of
suspension is adopted, in which the trolley wire
is supported by another wire known as the
“Catenary” or “Messenger” wire and is fixed with
considerable sag between supports at suitable
distance.
8. The wire is usually made of more than seven
strands of steel.
To avoid the use of insulated droppers the
whole of the messenger wire is usually
insulated, and it may therefore be used if
required for carrying current.
The position of contact wire remains practically
constant at all temperatures which may cause
large amount of sag in the catenary wire.
Now a days the catenary construction employed
is of either the single catenary or compound
catenary.
9. In single catenary construction, a steel wire with
7 or more strands supports the trolley wire by
means of droppers clipped to catenary and
trolley wire at equidistant horizontal intervals.
The span of catenary wire may be from 45 to 90
meters with a sag of from 1 to 2 meters
respectively for straight track.
The distance of droppers is between 3 and 5
meters.
Such construction provides sufficient flexibility
for speeds upto 120 kmph without any problem
of current collection.
10. Besides this it is relatively cheaper, easy to
erect and does not require heavy maintenance.
Single catenary construction is considered
suitable for most light weight ac systems and for
branch and loop lines and secondary routes
where traffic density is low and operating
speeds are low.
11.
12. The compound catenary construction is
employed for heavy current i.e. where traffic
density is high.
It consist of three wires, all in the same vertical
plane.
The upper wire is catenary wire, which is
insulated from the supporting structures.
From this wire another wire, known as
“intermediate catenary”, is supported by
droppers clipped to both wires.
13. The third wire i.e. trolley wire is maintained in
definite and constant tension by means of
automatic tightening gear.
Both contact and catenary wires are divided in
sections of 150 meters and each section is
anchored at the centre.
The purpose of intermediate catenary besides
increasing the current carrying capacity is to
provide more uniformity in elasticity.
14. The smoothing reactor is used to make a.c. ripple
free d.c. available from rectifire and pure d.c. is
given to d.c. series motor better performance.
When the thyrister is used to rectify from AC to DC,
the sinusoidal wave display on the voltage and time
graph is confined to above the positive axis.
15. In order to create a steady supply of voltage and
smoothen the delivery, a smoothening circuit is
connected.
The smoothening circuit may include inductors
and capacitors .
