Magnetic levitation · EDS vs EMS · lift, drag & lift-off speed
A maglev train floats on a magnetic field instead of wheels. There are two main ways to do it. EDS (electrodynamic suspension) uses superconducting or permanent magnets on the train that induce currents in the guideway as they move. By Lenz's law those induced currents repel the magnets, so the lift grows with speed: below a lift-off speed the train still rides on wheels, and only above it does it rise. EDS is passively stable but pays a price in magnetic drag, which peaks near lift-off and then falls off, so the lift/drag ratio keeps improving as the train accelerates.
EMS (electromagnetic suspension) uses electromagnets that are attracted upward to a steel rail. Attraction grows stronger as the gap shrinks, which is inherently unstable — pull too close and the magnet slams into the rail. EMS therefore needs fast active control: a sensor measures the gap and a controller adjusts the coil current thousands of times per second to hold a small, constant gap (typically ~10 mm) even at standstill. Here the control gain governs that loop; too little gain and the train sags, too much and the gap oscillates and the suspension goes unstable.
In both cases the lift must balance weight, F_lift = m·g. A simplified model is used: EDS lift rises with speed as F ∝ v²/(v²+v₀²) and decays with gap, while drag follows F_drag = F_lift·(v₀/v). Propulsion comes from a separate linear motor: the guideway acts as the stator and a travelling magnetic wave pushes the train forward, so there is no contact and no traction limit from wheels.