It's not just the extra weight of the batteries themselves which would need to be considered, there would also be extra traction motors, extra control systems, extra fire suppression equipment, upgraded processing capabilities for the unit's brains, etc - all of which becomes deadweight the moment that the boost runs out.
You're more likely to see flywheel recovery systems on mainline trains than lithium ion batteries. A three tonne flywheel going to 8,000 rpm can store up to 33MJ of energy - enough to provide 33 seconds worth of 1,000kW boost, or 82.5 seconds worth of full power to a pair of 200kW traction motors - all without requiring any exotic minerals like batteries.
Bombardier are currently working on the technology to create a battery electric MU and will be running a prototype at some unspecified date in the future using a Cl379. The precise specifications are still to be decided, but I believe that it will be used to test different technologies including lithium ion batteries. I don't believe they are intending to trial a flywheel storage system because the idea is to allow an electric unit to operate "off the juice" rather than merely giving it a boost.
As for the additional weight requirements, not all of the things listed will be required. There certainly won't be any additional fire suppression, as most electric units have no fire suppression at all due to the lack of combustible material. Most fires are electrical in nature and can be dealt with either by the train's own fault protection devices or by physical disconnection from the power supply.
It doesn't need to go "back up" to 25kV AC because it's not a linear process of "conversions" - the basics of how a transformer works are covered in every half-decent secondary school physics curriculum, but evidently not in the training to become a train driver.
Thanks, but if you'd read a little further you would have seen that I got there in the end.
Forgive me, but my secondary school education was 25 years ago but I do remember the lessons I had about transformers. I am aware of how they work and what they do. However, I will just say that the education I received was about how they work rather than how to wire one up.
What I was attempting to do was tally this knowledge with the traction training I had received when I drove EMUs and was not searching simply for generic information on the subject but something more specific to explain how a transformer is connected up inside an electric train. This is what took the time.
As for my driver training being deficient, again we were simply taught what we needed to know in order to carry out our duties. We're train drivers, not electrical engineers.
To put it simply, the the 25kV circuit passes through the train, in via the active lead and the pantograph and out via the wheels and the neutral lead going to the return circuit, with it doing work along the way - in the same way that a 240V circuit passes through a lightbulb from the active wire to the neutral wire in your house and does work along the way.
The work done by the 25kV circuit on the train is to induce a magnetic flux within the transformer's core. The transformer's core then induces a current within the secondary windings which have no electrical connection to the high voltage AC circuit.
The secondary windings for traction power are low voltage (therefore high current) circuits rectified to DC, which may be used to run DC motors directly or connected into an inverter to run AC motors. There will also be other secondary windings running off the same transformer - the number of loops in the wire dictate the voltage - for providing power to other onboard systems such as HVAC, lighting, control systems and so on.
That is all understood already, but it is a very good précis for those who may wish to know a little more. Thank you.
As for it being dangerous to have low current 25kV running through the rails between the train's current location and the nearest connection from the rails to the return wires, others have explained all about bonding already, and it's a feature of AC electrification that it shuts itself down within a few milliseconds of the circuit getting broken. Your concern about inducing magnetic fields is irrelevant - we use much stronger permanent magnets to hold papers to our fridge doors!
I have no concerns about induced electrical currents. Never did. Everything around my workplace is adequately bonded.
However - it's probably useful to maintain a bit of mystique about the tracks being dangerous due to electricity because people still respect electricity thanks to seeing the effects but not the electricity itself - unlike trains which they may reason they can see coming. Maybe that's why your instructors kept you in the dark.
Again, I don't believe that my instructors
"kept me in the dark" about the physics of the return path. It is simply not required to understand all the whys and wherefores, only that you are safe from the electrification system at track level as long as you don't touch any detached "red bonds".
But now I know a bit more than I did. So thanks for that.
O L Leigh
