A magnetic brake is a device that leverages strong magnetic forces to slow a vehicle down. There are various different types of magnetic brake systems, including ones that use electromagnets to actuate traditional friction pads, and those that leverage magnetic repulsion itself to provide resistance. These can be found on a variety of vehicles, from trains to roller coasters.
The most common type of magnetic brake is the electromagnetic brake, in which an electric current is run through magnets attached to a pair of brake pads. As they are attracted to each other, the magnets clamp the brakes over a brake disk, which generates friction to provide stopping power. There are several variations on this design, including so-called failsafe brakes, which employ very strong permanent magnets that are attracted by default. Only by an opposing electromagnet actively keeping them apart do they remain open, so in a power outage or other emergency, they automatically engage.
Another kind of magnetic brake is known as the Eddy current brake — an eddy current being the form of resistance the electromagnets create to slow a vehicle. Instead of simply being the force that brings friction pads together, in an Eddy current brake, when an electric current is run through two round magnets situated on either side of a magnetic disk, the eddy currents resist the motion of disk. This type of force is commonly known as electromagnetic induction.
By increasing or decreasing the amount of electric current, the stopping power of an Eddy current brake can be correspondingly attenuated up or down. Rather than pads pressing harder on a rotor, the resistive magnetic force is amplified. Though there is no physical contact, the process still generates increased slowing, along with heat, as a result of the resistance. Eddy current brake systems are used mostly in larger vehicles, like trains.
A sub-type of the Eddy current brake is known as the linear Eddy current brake. Instead of the normal circular design, magnetic coils are wound around a straight rail. The coils alternate between a positive and negative magnetic charge, so, when activated, generate resistance and slowing action. This design is less widely used than traditional electromagnetic brakes on train systems, but, in places like Europe, is becoming more common on high-speed rail systems.
Unpowered versions of the linear design — which instead use permanent, rare Earth magnets — are the brake of choice on most roller coasters. As anyone who has ridden a roller coaster will be aware, these non-electromagnetic types work on an on-off basis, and cannot be easily modulated. This results in very abrupt periods of deceleration, and, for this reason, they are not a popular choice on more comfort-oriented vehicles, like trains.