What is an eddy current brake?

We use eddy current brakes in our dynos, so what is an eddy current brake? 

Eddy current brake: also known as an electromagnetic brake or electric retarder. 

Eddy current brake
Eddy current brake used in dynamometers

It is commonly used in various applications, including trains and trucks, roller coasters, and industrial machinery like dynamometers. 

In eddy brakes that are used in dynos, there is a fixed stator in the center and air-cooled rotors on both sides that are rotated when the dyno axle is rotating.  

The brake is controlled by varying the applied current and voltage. 

The stator is made of coils, multiple pairs of electromagnets and when electricity flows through them, they generate electromagnetic fields. As the rotors move through the magnetic field, electromagnetic induction occurs.  

Eddy current brake in Dynolyze hub dyno
Eddy current brake in Dynolyze hub dyno

The resistance generated by the eddy currents acts as a braking force, without friction or wear.  

The eddy currents produce heat. The rotors have air ducts which help transferring the heat into the atmosphere. After driving on the dyno you can feel the warm air rising from the brake units. But as it is air-cooled, you do not need to fill your dyno room with water hoses and tanks. 

The eddy current brakes do not require physical contact between components to work, which is an advantage for the dyno user as they don’t need regular maintenance. It is not uncommon to find a 20-year-old dyno with eddy current brakes that have never been repaired.