Transformers in electric trains?

[cabellos]

I have an exam on monday and looking through the past papers I came across a question I couldn't find a suitable answer to:

List the economical advantages of using a transformer in a train rather than run the overhead lines at 400V?

any help would be very much appreciated...

Phil N

 

[Astronuc]

Well very simply, overhead lines carry electrical current long distances and since Power = Current * Voltage (for AC it's an integral of I(t)*V(t), but essentially the same principle), the current is reduced as voltage increases.

So to minimize power resistive (I²R) and reactive losses, high voltage is used. In the transformer the high voltage is reduced (dropped down) to a voltage suitable for a motor.