Electro Magnetic Retarder – Working and Principles

Electro Magnetic retarders (EMR) works on the principle of eddy currents. Basically they have a fixed stator and a pair of rotors that are attached to the propeller shaft and allow it to rotate. Its working methodology is opposite to that of a generator or alternator. Alternators produce electricity by using a rotating magnetic field with a stationary armature. So when electricity is passed through armature, the magnetic field will be developed and will resist rotating shaft.

In case of retarders the stator –which carries the electromagnets, and the rotors are mounted coaxially, opposite to one another. These two are separated by a narrow air gap, thereby avoiding any friction. The stator plays the role of the inductor and is made of a pair of electromagnets, which generate electromagnetic fields when electricity continuously flows through the stator coils, thereby producing eddy currents in the mass of the rotor.

Electro Magnetic Retarder

Rotors act as induced current. Manufactured with a specially designed conducting material, rotors are only subjected to eddy currents when passed through by magnetic fields generated by the stator while being brought into rotation by the transmission shaft.

By definition, eddy currents originate from conducting metallic masses when the latter is placed in variable magnetic fields. In the case of Electro Magnetic retarders, the variability of the magnetic fields to which rotors are subjected is a result of the rotation of the rotors. Eddy currents, also known as swirling currents, wrap around the magnetic flow lines.

The generation of eddy currents in the mass of the rotor leads to the appearance of Laplace forces that counteract with the rotation of the rotor. This leads to generation of a braking torque and  this is applied to the transmission shaft slows the vehicle.

The eddy currents produce a gradual rise in rotor temperature and the heat is then released into the atmosphere through ventilation systems. With EMR, it is thus possible to effectively slow down a rotating shaft without friction and therefore without wear. Since there is no direct contact of any moving parts its called friction free braking as well.

Although induction braking systems might seem simple in principle, they involve complex physical laws such as materials resistance principles, electromagnetism, thermodynamics and fluid mechanics.

Primarily there are two types by which a EMR can be fitted. One is Focal retarders which designed to be adapted at the gearbox output flange or at the transmission axle input (suitable for RE chassis). Second one being Axial retarders which are designed to be interfaced with a propeller shaft. Though EMR is fitted in both RE and FE chassis its easier for FE chassis where we have substantial space to fit the EMR, though there are lot of RE models that do have EMR.

Telma from France and Brakes India from India are the two major manufacturers that are found in Indian buses. Telma is a global leader and is much superior in technology and reliability.

Indian buses that were and is fitted with EMR:

  • Volvo B7R, Mark 1 – Telma
  • Ashokleyland 12M – Telma / Brakes India (Option of both)
  • Ashokleyland 12M RE – Telma
  • Ashokleyland Luxura – Telma

Major merits and demerits of Electromagnetic retarders:

  • Super swift and quick in response once activated. As it works on the basis of electric principles, the response time is normally less the 10micro seconds.
  • Can be operated from as low as 20kmph speed as it just requires a moving field to generate electricity.
  • It’s a natural cooled system and doesn’t depend on engine cooling system, it means EMR doesn’t put any additional stress in the engine cooling system. And this makes the cooling and performance is uniform across the weather condition.
  • This also means less load on engine and relatively better fuel performance.
  • Electrical system usually requires higher capacity batteryAlong with the retarder and electrical system, this makes the whole system costlier.
  • In case of RE chassis, EMR packaging is little complex as it needs to be mounted in between gear box and differential and normally this space is very much less in case of RE chassis and hence many manufactures moved towards to HDR.


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