‘Maglev’ is a portmanteau of ‘magnetic levitation’. Instead of running on wheels, maglev technology uses strong magnetic fields to lift the train and move it forward. Since the train floats above the track, there is no rolling friction during normal operations, nor are operations affected by inclement weather, like rain or snow. The result: the train can move at extremely high speed, topping 400 km/hr, while also giving passengers a smooth ride.
How is a train lifted?
Nature has four fundamental forces: the two nuclear forces, the electromagnetic force, and gravity. Of them, gravity is the weakest by far — around 1039 times weaker than the electromagnetic force. This is why a small magnet can stay stuck to your fridge despite having the entire gravitational force of the earth pulling it down. The same idea applies to lifting a train weighing somewhere from 150 to 500 tonnes.
Engineers get the train to float above the tracks using electromagnetic suspension (EMS). The train has two ‘arms’ that wrap around the guideway over which it runs. Strong electromagnets on these arms point magnetic fields up towards the protruding parts of the guideway. Say you are lifting a bag with your hand. Your fingers wrap around the bag’s strap. The train’s arms do something like this — they wrap only just halfway, and from the bottom part, i.e., where the tip of your finger would be, exert a magnetic field into the underside of the strap. When these electromagnets are turned on, they exert a magnetic field that attracts them towards the guideway, i.e., your fingers would be attracted towards the strap. As a result, the train is pushed upwards. The train is fit with sensors that help it float around 10 mm above the guideway at all times.
In a closely related technology called electrodynamic suspension (EDS), there are superconducting magnets on the train that are cooled to a very low temperature. When the train moves, the magnets that move with it induce an electric current in coils built into the sides of the tracks. (The phenomenon is called electromagnetic induction.) The result is a repulsive force that pushes the train up. Since the magnets have to move to induce the current, the train often needs to be travelling at around 100 km/hr before the effect can kick in, and until then it uses wheels. The train also hovers higher above the track, at around 100 mm.
But in exchange for requiring wheels, EDS can support higher speeds. EMS systems operate at around or under 500 km/hr whereas the SCMAGLEV EDS system in Japan achieved 603 km/hr during testing in 2015.
How does the train move forward?
The walls of the guideway are lined with coils that carry an alternating current (AC). This current creates a magnetic field that constantly shifts its poles (north to south) along the track. The magnets on the train are attracted to the ‘opposite’ pole ahead and repelled by the ‘like’ pole behind. As a result, the train is simultaneously pulled forward from the front and pushed forward from the back. And to make the train go faster, engineers just need to increase the frequency of the AC.
Similarly, the current is used to switch the direction of the magnetic field to slow the train or altogether stop it. This process may also use regenerative braking to divert the energy ‘saved’ by braking to the electric grid.
If a fast-moving maglev train starts to drift, there are magnets on the sides that repel/attract accordingly to ensure it stays centered. For emergencies or when the magnetic braking fails, the system is also fit with friction brakes. They come in two forms, although there are other technologies as well. One: the maglev train is also fit with ‘regular’ auxiliary wheels, and these wheels have a braking system akin to those in other trains and cars. Two: braking pads between the guideway and the train apply friction to further slow it down.
What are the downsides of maglev?
The maglev system including the guideway cannot reuse existing guideways and stations vis-à-vis non-maglev trains. It needs its own. And this infrastructure is very expensive as the guideway needs to have magnets and coils with precision engineering.
The Shanghai Maglev, which was completed in 2004, cost ₹580 crore to ₹720 crore per kilometre (adjusted for inflation). This is excluding the cost of preparing the local terrain first — and operational costs like maintenance. Ergo, there needs to be very high and consistent passenger demand. Otherwise, the economics won’t work out.
On the engineering front, while there is no rolling friction, a train moving very fast — especially over 300 km/hr — experiences aerodynamic resistance due to the flow of air around the train’s body. This is why maglev trains have aerodynamic designs (which can add to the cost), including a front portion that resembles a bird’s beak.
Is maglev technology used elsewhere?
Maglev is essential about using magnetic fields to support or move objects — a use-case that arises in many enterprises. High-speed turbines and compressors suspend their rotating shafts in a magnetic field instead of conventional bearings, minimising mechanical contact and allowing the blades to turn faster. Similarly, modern aircraft carriers use the linear motor technology that pushes maglev trains forward to accelerate aircraft on the vessels’ runways.
Factory automation systems also use it to achieve the rapid, precise motion crucial in semiconductor manufacturing, food and medicine packaging, CNC machining, inspection cameras involving lasers or X-rays, and DNA sequencing, among others.
Scientists have also used magnetic fields to levitate biological samples, liquid droplets, molten metals, and even small animals for experiments. Following a suggestion by his wife, the British physicists Andre Geim and Michael Berry used a magnetic field to levitate a frog in 1997. This was possible because a form of magnetism called diamagnetism gives all materials a very weak magnetic field. The duo had to work out the strength of the field required to ‘cancel out’ the gravitational force exerted on the frog by the earth. Thus, they were able to get the frog to float using a 10-tesla magnet.
For this work the duo won an Ig Nobel Prize in 2000. Today, Geim remains the only scientist to have won both an Ig Nobel Prize and a Nobel Prize. Berry is also a noted physicist, known for his work on semi-classical physics.
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