Permanent-Magnet Eddy-Current Brakes

How PM eddy-current brakes work

Fixed permanent magnets and a conductive reaction plate move relative to one another. That relative motion sets up eddy currents in the plate, and those currents produce a field that opposes the motion — so the brake resists whatever is trying to move past it. Nothing touches, nothing rubs, and no drive current is supplied: the braking force is drawn straight from the kinetic energy of the moving body and dissipated as heat in the reaction plate.

The force-speed curve

The retarding force is not flat across speed. From a standstill it rises with increasing relative speed, reaches a broad peak, and can then taper off at higher speeds as the induced currents begin to shield the plate. The shape and position of that curve are set by the magnet geometry, pole pitch, air gap and the reaction-plate material and thickness, so the brake can be tuned to put its strongest retardation where the duty needs it. Predicting where the peak falls and how the force behaves either side of it is a design exercise, typically bounded by the achievable gap and the plate's thermal limit rather than by any single figure.

Track-mounted vs vehicle-mounted magnets

Which part carries the magnets is a system decision, and it trades cost against vehicle mass much as it does for a linear motor. Mounting magnets along the track keeps each vehicle light and passive but means paying for magnet material over the whole braked length; carrying the magnets on the vehicle needs only a plain conductive rail on the route but adds mass and magnet cost to every unit.

Double-sided caliper arrangements

Placing magnets on both sides of the reaction plate, in a caliper arrangement, works the plate from each face and lets the two normal attractions largely cancel in the supporting frame. Under comparable gap, plate and thermal limits this can approach roughly twice the retarding force of a single active face for a given plate width, while keeping the structure that holds the magnets in a manageable state of balance. It is the natural layout where high, compact braking force is wanted without a heavy back-structure to react a one-sided magnetic pull.

No-power dynamic braking

Because the force comes from motion through a fixed magnetic field, the brake needs no electrical supply while it is working — an appealing property for fail-safe and emergency duty, since there is nothing to power up before it acts. There are no friction pads to wear, glaze or fade, so repeated or sustained braking does not degrade the retarding surfaces the way it does on a mechanical brake. The energy still has to go somewhere, though: it becomes heat in the reaction plate, which is why the thermal path is part of the design rather than an afterthought.

Zero-speed holding caveat

The one behaviour to design around is that the force is zero at a standstill — with no relative motion there are no eddy currents, and therefore no braking. A PM eddy-current brake is a dynamic brake: it is excellent at shedding speed but cannot hold a load stationary. Where zero-speed restraint is required — parking on a grade, holding at a station, securing a launch sled — it should be paired with a mechanical holding brake that takes over once the vehicle is slow or stopped.

Thermal and demagnetisation checks

All the braked energy ends up as heat in the reaction plate, so we run the full duty cycle in simulation and check the plate against its thermal limits before committing to a design — peak temperature under a worst-case stop, and steady temperature under repeated duty. Because permanent magnets can lose strength if driven too hot, we also check that the magnets stay comfortably within their safe temperature and operating range across the envelope. We size the brake with equivalent-circuit models, cross-check the force and the fields with FEA, and confirm the thermal and demagnetisation margins in simulation before cutting metal.

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