Help, please, boys and girls - I’m not clear on how maglev trains are supposed to work. An electromagnet in the track repels a magnet under the train to levitate it? OK. This is the basic explanation I can find on the net. But doesn’t the train’s magnet have to be a north or south pole to be repelled? Wouldn’t an electromagnet simply attract a metal train otherwise? Is the electromagnet in the suspension system only north or only south? And how do these magnetic fields keep from interfering with the magnetic fields of the propulsion system? I thought I understood all this until a kid asked me a series of questions.
Well, I’m no expert, and I wish I could help more, but I’ve read:
Overall, the track does serve to repel the train thus creating magnetic leviation (mag-lev). And, whether it is N-N or S-S repulsion is just a trivial detail.
However, the fact you might be missing is that the track just ahead of the lead car has a pole opposite to that of the train…which pulls the train forward up until some point at which that piece of track flips poles to repel.
(Otherwise, the’d need jet propulsion to move the thing, I’d WAG!)
Maybe someone can add to this, but I believe this is known as a dipole, and the flipping of poles is a concept used in radio broadcasting - where the poles are flipped at regular cycles.
- Jinx
Check this out. Maybe it’ll help.
Or try this, http://www.howstuffworks.com/maglev-train.htm
After spending millions of D-Marks the Germans have abandoned the proposed Maglev line between Berlin and Hamburg. A pity realy because the potential is there for a breakthrough in transport technology. I believe that work is still going on in Japan on building a line.
Thank you, posters, but I’ve seen both of those sites and they don’t explain it well enough for me. The train “floats on a magnetic field”? How? I know that electric current can create an electric field, and a hunk of iron placed inside it will become a magnet - i.e. an attractive source with poles. So, how does a train float on this unless it is being repelled, which is what my question is about. How is the repulsive force created? And, since that’s a different bit of action, how does the levitation keep from interfering with the propulsion? anyone else?
Isn’t the basics behind it all the same as a railgun?
so - can I get some help on this one?
The train is suspended by opposing magnetic force and the forward motion comes from selectively reversing the field on a few sections of the track to attract the train, as the train approaches the point of attraction the magnetic field is matched to the train again with another section reversing to continue drawing the train forward.
Electrical current creates a magnetic field. An iron core placed in a coil just enforces the magnetic field, but a simple coil of wire with current running through it becomes an electromagnet.
But that’s not important. All that matters is that such an electromagnet acts just like a permanent magnet. It has a N pole on one side and S pole on the other. If you have two magnets side by side, with the same pole facing each other, they repel. Put one magnet on the train with the N pole pointing down, and another on the ground with the N pole pointing up, and they repel each other. This repulsive force acts to lift the train. It doesn’t matter if one or both of the magnets are electromagnets.
The magnets used for propulsion are either in the different part of the train, or carefully controlled to prevent it from interfering with the lifting force.
Does that help? I could elaborate but I’m still not sure which part of it is the main question.
I thought it had something to do with that right hand rule…?
Current goes left, field goes up the screen, force goes into the screen??
Can be applied to the track as current travels over the track perpendicular to the direction of travel, the electric field holds the train in the air and the force is forward along the track.
I might be thinking of something else or misheard someone.
PerfectDark
There’s lots of ways to do it.
For normal magnet repulsion, you need a North-North or South-South configuration. You could achieve this by having DC electromagnets on the track and DC electromagnets on the train, but this involves having a source of electrical power on the train and really you want the train to be as light as possible.
However, if you have AC feeding into your electromagnet, then the magnetic field is constantly fluctuating in strength and swapping North-South. Bring a conductor into a fluctuating magnetic field and you induce electric currents in it, and these currents produce another magnetic field in the conductor. This magnetic field “opposes” the first one.
So you can levitate a sheet of aluminium over your AC electromagnet, because the AC electromagnet induces a North pole in the sheet when it has its own North pole, and vice versa when it has a South pole. If you have a row of AC electromagnets and do something clever with the relative phases of the AC, you can make the repulsion stronger at the back of the sheet than the front and it will travel “downhill”, accelerating along the row. This is the principle behind Laithwaite’s original demonstration model and appeared briefly in the “Q” scene in the Bond film “The Spy Who Loved Me”. So in this scheme, the levitating magnets are also the driving magnets.
(Incidentally, you have to use a nonmagnetic conductor such as copper or aluminium for this induced repulsion to work. If you use ferrous metals, they attract as you would expect. This principle can be used to separate steel scrap from aluminium scrap, the steel being pulled one way by the AC magnet and the aluminium being pushed the other.)
Another way to do it uses the same principle, but instead of using AC magnets you use DC, and the field fluctuation is provided by the movement of the train itself. The Japanese developed a version on this principle - it had wheels which were retracted when the train got above a certain speed. It was cancelled due to a prohibitively expensive track. With this scheme, the levitating magnets were below the train and the driving magnets somewhat to the side to prevent them interfering. If memory serves, the induced-field magnets were actually coils rather than sinple plates of aluminium.
Another way to do it is suspended maglev - you use magnetic attraction to hang a train from a rail, with a positional feedback mechanism to make the field stronger when the train moves further away and weaker when it gets too close. I’ve seen similar widgets used in some shop displays. I think this approach was developed by Germany. Not sure about the driving arrangement.
Yet another way to do it involves superconductors - we’ve all seen pictures of floating superconductor pellets. This works due to a particular property of superconductors -they expel magnetic fields from within them (they are actually perfect dia-magnets, if you care.) You can also make a kind of super-efficient electromagnet using superconductors. The question is, do the power savings you get from superconductors make up for all the power you have to use keeping them cold?
Then there’s a whole other set of levitating trains using air-cushion technology, but that’s enough for now!
Great post, matt. Just curious where you heard that the Japanese program has been cancelled? I just checked their web page and they are still continuing test runs. Their method uses superconducting magnets mounted on the train, whose movement induces current in coils mounted on the track. I hear the extreme magnetic field can be a problem though - definitely not recommended for people wearing pacemakers.
Just curious where you heard that the Japanese program has been cancelled?
Oops! I probably didn’t. All I know about the Japanese program is from a brief feature on the UK popular science programme “Tommorrow’s World” in the late '80’s. My memory is more reliable about the technology than the status!