c) Electrical Retarder
The Electrical Retarder, also known as the Eddy-Current Brake, shown in the figure, functions on the electromagnetic principle. Excitation coils are mounted on a disk which forms the stator. Two braking rotors are arranged, one in front and the other behind the stator, through which the drive shaft passes. Cooling fins are present on the braking rotors for better heat dissipation. To create a braking torque, the exciting coils are fed with electric current (from the battery or alternator) in order to produce a magnetic field. If the brake rotors rotate in this field, eddy currents are induced which generate a braking torque whose magnitude depends on the exciting current of the stator coils and the speed of the drive shaft.
When it comes to the design of this retarder, a comparatively low construction expense is confronted against high weight. A trouble-free operation requires a sufficiently dimensioned battery and alternator. The achievable braking effect drops with increasing temperatures. Hence when the braking rotors are in a red-hot state for example, only half of the nominal torque is available.