under the hoodEMS vs EDS

How Maglev Levitates

float it, then shove it with the same field

A maglev has to solve two problems at once: hold the vehicle a hair above a track it never touches, and still drive it forward without anything to push against. Both jobs are done with magnetic fields — but the lift comes in two completely different flavours, and they float in opposite directions.

Two ways to float

Every practical maglev picks one of two levitation families. One pulls the train up by attraction; the other pushes it up by repulsion. They are not two styles of the same trick — they are opposite physics, with opposite trade-offs in stability, air gap, and whether the thing can float while parked.

EMS — pull up, and never stop correcting

Electromagnetic Suspension (EMS) works by attraction. Electromagnets bolted to the vehicle wrap under the guideway rail and are drawn up toward the steel above them — the train hugs the track from below and hangs off it. The catch is that magnetic attraction gets stronger the closer you are: pull a little too close and the force spikes, snapping the magnet shut against the rail; drift a little too far and lift collapses.

The arrangement is inherently unstable, so it can never be left alone. A feedback controller measures the gap and adjusts the current thousands of times a second to hold a tight, roughly 10 mm spacing.
Because the lift comes from powered electromagnets, EMS works at a standstill — the train floats before it has moved an inch.
This is the Transrapid approach, the technology under the Shanghai Maglev.

EDS — push up, but only once you're moving

Electrodynamic Suspension (EDS) works by repulsion. Superconducting magnets on the moving vehicle sweep past coils in the guideway and induce currents in them; by Lenz's law those induced currents create a field that pushes back, shoving the train away from the track. The lift is generated by motion itself.

Repulsion is naturally stable — drift closer and the pushback grows, so the gap self-corrects with no fast controller babysitting it. That also allows a far larger air gap, on the order of 100 mm.
But induced currents need speed. Below some threshold there simply isn't enough motion to levitate, so an EDS train rides on wheels at low speed and only lifts off once it's fast enough.
This is how Japan's SCMaglev runs — stable and big-gapped at speed, on rubber until it gets there.

Rule of thumb: EMS is a tiny gap held by clever electronics and works parked; EDS is a big gap held by physics but needs a running start.

The track is the motor

Levitation only holds the train up — something else has to move it. Both families use the same answer: a linear motor. Take an ordinary rotary electric motor, slice it open, and unroll its stator flat along the guideway. The track becomes a stationary electric motor laid out end to end, and its windings drive a travelling magnetic wave that the train's onboard magnets chase down the line.

Because the pushing happens through fields rather than teeth or friction, propulsion is contactless: no wheels to grip, no rolling friction to fight, nothing to wear out. The only real resistance left at speed is the air. That single move — putting the engine in the track instead of the train — is what lets a maglev keep accelerating where steel wheels would slip.

For the vehicle-level view of where all this lives — guideway, records, why it hasn't spread — see the maglev vehicle page.