The Birth of Eddy Current Retarder

In 1855, Leon Foucauit, a French physicist, discovered the eddy current phenomenon. In 1903, Mr. STECKEL, a French engineer, declared the world's first patent for eddy current retarder. From the 1930s, some European manufacturers paid more attention to the necessity of using retarders for commercial vehicles in mountainous areas and accident-prone areas. Because of World War II, the technical development of retarders was forced to stop. It was not until after the war that it was developed and applied. The retarder not only guaranteed the safety of vehicles, but also reduced the economic cost caused by the wear of brake pads.

According to the latest "Classification and Grade Evaluation of Operating Passenger Cars" issued by the Ministry of Communications on July 1, 2002, the brake retarder must be installed on passenger cars above the second-grade level. Almost all high-end buses have installed eddy current retarders, such as Shanghai Shenwo Bus, Zhengzhou Yutong Bus, Dongfeng Nissan Chassis, Xiamen Jinlong and other imported products. Most of them are French Telma and Spanish Frenasa eddy current retarders. Domestic manufacturers with advanced eddy current retarders technology are Keimer, Terga and so on.

Brief Introduction of Eddy Current Retarder

First of all, a clear concept is eddy current, that is, eddy current, which refers to the current formed in the conductor under electromagnetic induction. Eddy current brake is usually used in combination with traditional brake. It plays a role in controlling the speed of most commercial vehicles (large and medium-sized passenger cars and trucks), so it is also commonly called eddy current retarder. Eddy current retarder is a device which is installed on the gearbox, transmission shaft or drive axle of automobile and reduces speed by the force of eddy current. In essence, he is equivalent to a DC high-power moment motor, based on the principle of "electromagnetic induction". Because the electromagnetic conductor of the rotor in the device is very large and the induced current on the rotor exists in the form of eddy current revolving around the magnetic force line, the retarder is called the eddy current retarder.

When a coil is wound outside a conductor and the coil is fed into an alternating current, the coil generates an alternating magnetic field. Because the conductor in the middle of the coil can be equivalent to a closed circuit of a circle in the circumferential direction, and the magnetic flux in the closed circuit is constantly changing, the induced electromotive force and the induced current will be generated in the circumferential direction of the conductor. The direction of the current will rotate along the circumferential direction of the conductor, just like the whirlpool of a circle, so this kind of electromagnetic induction occurs in the whole conductor and produces the induction. The phenomenon of current response is called eddy current phenomenon.

The longer the circumference of the conductor, the higher the frequency of the alternating magnetic field and the larger the eddy current.

The eddy current inside the conductor also generates heat. If the resistivity of the conductor is small, the eddy current is strong and the heat generated is large.

Detailed Working Principle of Eddy Current Retarder

When the driver pushes the hand-stop switch of the retarder or pushes down the brake pedal to direct current to the stator coil of the retarder, the rotating disc fixed on the drive shaft of the eddy current retarder rotates driven by the shaft. Because there is a certain gap between the working face of the rotating disc and the magnetic pole, it can be seen from the principle of electromagnetic induction that the rotating disc is formed between the adjacent iron core, magnetic pole plate, air gap and rotor. In the electromagnetic circuit, when there is relative motion between the rotor and the stator, the motion is equivalent to the conductor cutting the magnetic force line, then the eddy induced current will be generated inside the body, at the same time, the induced current will excite the electromagnetic force. The direction of the force is just opposite to the direction of the rotor rotation. Because the direction of the force is always hindering the direction of the conductor's movement, it leads to the deceleration of the rotor, which is the braking force source of the reducer.

Heat Dissipation of Eddy Current Retarder

From the point of view of the law of conservation of energy, when the retarder brakes, it converts the kinetic energy of the car into eddy current electric energy and releases it in the form of short circuit. The eddy current flows in the rotor disc with a certain resistance value and is short-circuited. The thermal effect of the current will cause the rotor to heat up. In this way, the kinetic energy of the vehicle is converted into the heat energy of the rotor, and the heat is emitted through the strong wind generated by the blades on the rotor disc.

Therefore, the eddy current retarder will generate a lot of heat when it works. Improving the heat dissipation ability of the rotor and controlling the thermal deformation direction of the rotor is the key to the design of the rotor structure. It is also one of the core technologies of the eddy current retarder, and ensuring the surface cleanliness of the cooling components such as the rotor blade has become a key item in the daily maintenance of the retarder.