Using transmission lines to lift trains

[chhitiz]

well, if we lay power lines along rails and use the magnetic field to lift trains, would it increase the transmission loss?

 

[Bob S]

Yes. To increase the magnetic field, the transmission line voltage has to be reduced, and the current increased. To minimize transmission loss (I²R losses in cable), the voltage has to be increased in order to reduce current.
Bob S

 

[Adjuster]

I would hazard a guess that there are several difficulties that make such an idea impracticable. Most likely, the previous poster could have made further comments, but the reply was limited to giving a direct answer to the question in the original post.

For my information, having recently joined this forum, is that what is usually done? Clearly anything in the way of sarcasm or mocking of someone's ideas would be wrong, but is it OK in general to suggest reasons why (in one's opinion) something is not a good idea?

 

[Borek]

is it OK in general to suggest reasons why (in one's opinion) something is not a good idea?

As long as you stick to the facts and not get personal, there is no problem with giving a little bit more info. Just don't let the thread to drift too far from the original topic (but don't feel very bad if it happens, sometimes it happens even to Mentors ).

 

[chhitiz]

I would hazard a guess that there are several difficulties that make such an idea impracticable. Most likely, the previous poster could have made further comments, but the reply was limited to giving a direct answer to the question in the original post.

For my information, having recently joined this forum, is that what is usually done? Clearly anything in the way of sarcasm or mocking of someone's ideas would be wrong, but is it OK in general to suggest reasons why (in one's opinion) something is not a good idea?

i don't mind sarcasm. i have a lot of doubts myself. whatever faults you show me would be of great help

 

[sophiecentaur]

To get a high magnetic field you need many turns of wire- a single loop of wire would need to carry far too much current.
If you want a magnetic field to generate lift, it would make more sense to generate it with on-board magnets, fed from the transmission line. The high currents needed for high fields could be produced using transformers on the vehicle and allow the transmission to be at a much lower current and higher voltage.
Maglev has been used successfully but not on a commercial scale, I think. I don't think the rolling losses of a modern railway are particularly high so I wonder whether this is the reason it hasn't been tried on a large scale. Every now and then 'they' seem to propose a project which then doesn't get implemented.

 

[Adjuster]

OK chhitiz, you did ask for this:-

Apart from needing huge currents, with consequently increased losses, and enormously heavier capacity cables, it may be as well to mention one or two other problems which would seem to prevent this idea from getting very far:

The overhead cables and their support towers would effectively be carrying the weight of each train as it passed, which would require them all to be extremely strong. As well as being heavy in themselves, the cables would also exert huge forces on each other, so they would require a great deal of extra support.

An attractive (as opposed to repulsive) levitation system would require precise control of the current in the lines, but as described the line current depends on whatever the load current happens to be. Along comes an overload; the train takes off, smashes into the cables, and presumably the train and everything in it gets incinerated.

Last but not least, consider the horrors likely to be produced by the unconfined AC magnetic fields from this system. For instance, what would happen to any poor passenger unfortunate enough to depend on an artificial heart pacemaker?

 

[chhitiz]

@adjuster. i thought so too
1)the cables would be on ground(insulated) with a minimum clearance area cordoned off. so no need to support overhead cables.
2)i have no idea how to deal with power surges, or grid failures. suggestion are welcome
3)if the wagons are hollow metal boxes, wouldn't they have a shielding effect on em waves?
@sophiecentaur
i don't understand

 

[sophiecentaur]

chhitiz
From what you say, you seem to be implying that you would be using a single pair of conductors to form a vast loop (the whole length of the track) which would act as an electromagnet and suspend the train.
I was making the points that
1. You need the field only at the train's location
2. The current is unthinkably huge, because you have only a single turn for your electromagnet. The field is proportional to the current and all electromagnets use a lot of turns.
3. Even if you were to use many turns in your transmission line 'coil' (field is also proportional to number of turns), the length of wire with such large currents flowing would involve enormous losses (resistive) in such a long length of wire.
4. Having the magnet mounted on the train (conventional maglev) means that the electromagnet would be much, much smaller, yet still produce the necessary field between train and track.
5. Distributing the power at high voltage would mean much less resistive loss in the supply cables (that's why all power distribution networks use high voltages and lower currents).

In addition, how could you control the current, back at the station, so as to keep the train at a given height? Much better to control it on board the train where you can measure the train's height.
There is another problem in that the whole length of track would accumulate every bit of ferrous material in the area.
Any maglev system has to be able to support the train when the field dies. A strong structure with a rail is necessary (not normally in contact) to deal with this possibility, so the train can come to a halt safely.
There is little point in using maglev unless it is going to be much more efficient than a wheeled system. As I already commented, maglev has not been used seriously for this reason. Your proposal would be hugely inefficient, as well as being impractical. Sorry! :-)

I might also make the point that, afaik, most maglev systems use AC and the resulting repulsive magnetic force, due to eddy currents. The train is pushed up rather than lifted.