In OHE, i.e Over Head Electrification systems, electricity is provided through an overhead system of suspended cables.In such a system you can see two wires as shown in the image below.
In the first set of images you can see pantograph .In the second set you can see contact and messenger wire and in the third set you can see Dropper or spacers.
Getting the basics:
Let us understand each component one by one.
- Pantograph: A pantograph is used to make contact with overhead contact cable.It can be raised or lowered mechanically to make contact with the contact wire.This pentograph supplies power to motor through transformer .
- Contact wire: It carries the current and remains in contact with the pantograph.A contact wire is suspended from other wire above it which ensures that the wire is at a constant distance from pantograph.If this is not done then sagging of wire can lead to variation in the distance between pantograph and contact wire which can lead to breaking of contact wire.Hence this Catenary system is used.
- Messenger wire: Contact wire is suspended from another wire called as messenger wire.This wire has a shape of Catenary i.e hyperbolic cosine curve shape. Hence this system has a name of Catenary system.
- Dropper: Also called spacers these wires are present at regular intervals which maintains the shape of contact wire.
Let us now get into some detail of catenary system.
This image is from Mumbai (India). As you can see that there is a contact wire and Messenger wire. Here you can see that Messenger wire has a sag or its forming a catenary shape ( In easy words a stretched ‘U” shape).
Now just imagine a pantograph in contact with messenger wire! Due to the variation in height two possible situations can occur:
- Wire will break due to excess tension.
- Contact between pantograph and Wire can break.
We don’t want either situation to occur. To prevent this we hold the contact wire from messenger wire with Dropper of different length ( You can observe different length of Dropper wire in the above image).
Why Not just stretch the wire???
You might be thinking why we cannot just stretch the wire till it get straight !.Well theoretically its possible to straighten the wire but cant be done practically.Here are a few reasons behind it:
- Wind and effect of temperature requires some flexibility.Wind can cause wire to move which can break it if it is fully stretched. Moreover Wire expands due to heat which again requires flexibility!! Have you ever seen a structure as shown in the image below?? Well that structure is used as a counter weight to compensate for change in length due to thermal expansion. If this structure is not present it can lead to breaking of wire due to expansion or contraction.
- Also a train never moves straight.It takes turns, experiences jerks and when the train is crowded then the jerks are even severe.In India, especially in mumbai where local train is the lifeline and experiences heavy crowd it is important that the wire doesn’t break due to jerks.Pantograph here plays an important role.It adjust itself with jerks within required limits.Here if wire is fully stretched these jerks will break it!! Catenary system provides required flexibility due to droppers and hence prevents breaking of contact wire.
In OHE various types of Pantographs are used.This will be explained in a separate blog.Link will be provided here.
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