Why do maglev trains use attraction vs repulsion

[Paul Aitken]

I'm trying to find out why EMS maglev trains use magnetic attraction instead of repulsion. Given that the attractive force diminishes with distance it seems the forces would have to match exactly or else one of the two forces would win out and the train would come to a grinding halt.

But with like polarity between the track and train, any variation in the distance between the two would be self-correcting. As the train got closer to the track, magnetic repulsion would increase.

What am I missing? Excuse my ignorance if the answer is obvious. I'm a curious layman.

Reference https://www.physicsforums.com/threads/why-do-ems-maglev-trains-use-attraction-instead-of-repulsion.912614/

 

[davenn]

I'm trying to find out why EMS maglev trains use magnetic attraction instead of repulsion.

they don't, at least not to levitate the train
to move the train they use both

you need to do some googling and reading about maglev trains
then ask specific questions relating to anything you read that you don't understand
in that case , supply links to what you were reading and the part you were unsure about

regards
Dave

 

[Paul Aitken]

Thanks Dave. Here's what I'm basing this on.

http://emt18.blogspot.ca/2008/10/maglev-suspension-systems.html

And specifically the following excerpt:

The electromagnetic suspension EMS uses attractive force system to levitate. The train’s levitation magnet will be attracted to the conductors on the underside of the guideway. The attractive force between them will overcome the gravitational force. This will in turn levitates the train on the track.

My question is: Why attraction as opposed to repulsion?

 

[NascentOxygen]

In EMS it sounds like they are using electromagnets mounted on the carriage's underside to interact with the magnetic field surrounding the high-current-carrying 3^rd and 4^th rails (so to speak). The current in these copper bus bars will include the current used by the propulsion motors, so it will be a very high current.

In EDS, it sounds like they are relying on induced eddy currents to form the opposing magnetic field, and eddy currents are small at low relative velocities, but increase as speed picks up.

 

[Paul Aitken]

It's EMS that concerns me. And for the purposes of this discussion I'm excluding the forward propulsion provided by linear induction. I'm trying to determine whether it's possible to levitate a train using only the repulsive force of like polarity. In other words could a train with permanent magnets on the bottom and sides, levitate in track hemmed in by corresponding electromagnets of like polarity. It seems like it should be possible but all existing EMS trains use attraction as described in the above post. I'm writing a television story about a character who builds such a train and I'm trying to determine what would happen.

 

[Nidum]

It's an underslung system . The levitation magnets are below the guide rail and with poles facing upwards .

 

[Nidum]

Technically excellent but expensive compared to other systems .

 

[Paul Aitken]

According to wikipedia:

Magnetic attraction varies inversely with the cube of distance, so minor changes in distance between the magnets and the rail produce greatly varying forces. These changes in force are dynamically unstable – a slight divergence from the optimum position tends to grow, requiring sophisticated feedback systems to maintain a constant distance from the track.

So why not use magnetic repulsion instead where the distance between track and train is self-reinforcing?

 

[Nidum]

The guide rail construction is much simpler for attraction type systems and there is minimal magnetic field leakage into the passenger compartment .

System tends to be costly overall because of the complicated construction of the underworks of vehicles and because of the need for very sophisticated designs of levitation magnets .

Both attraction and repulsion systems are dynamically unstable and complex control systems are required to enable trains using them to run safely and smoothly .

These control systems have features in common with the systems used in unstable aircraft designs where the instability is turned to advantage so as to get very fast control responses .

 

[Paul Aitken]

Thank you for answering my question Nidum. I understand that both forces are dynamically unstable, Earshaw's theorum and all that. I'm not looking for modern world applications so magnetic leakage isn't a concern. I'm writing a television story set in the early 20th century where a Elon Musk type character conceives of a maglev train using magnetic repulsion on both sides and the bottom of the guide track. Ignoring for the moment whether the magnetic field leakage would affect the linear induction system of propulsion, what would happen? I'm trying to avoid building my own scaled down test model.

 

[PhysicoRaj]

Repulsion seems to work as well. But I don't think that could eliminate the need for feedback control. The gap between the rails and undercarriage still has to be controlled and cannot be left at the mercy of inertia and weight of the train. That's what I think.