Introduction to eddy current retarder

The birth of eddy current retarders

In 1855, the French physicist Mr. Leon Foucauit discovered the phenomenon of eddy current, and in 1903, the French engineer Mr. STECKEL declared the world's first eddy current retarder patent. Due to World War II, the technical research and development of the retarder was forced to stop. It was not until after the war that it was developed and applied, and the retarder not only ensured the safety of the vehicle, but also reduced the economic cost of brake pad wear.

The latest "Classification and Rating of Operating Bus Types" standard issued by the Ministry of Communications, which was implemented our country July 1, 2002, stipulates that buses above the second level must be equipped with brake retardation devices. Almost all high-end buses have been installed with eddy current retarders, such as Shanghai Shenwo bus, Zhengzhou Yutong bus, Dongfeng Nissan diesel bus chassis, Xiamen Jinlong and other imported products, most of which are French Taylor and Spain's Frenassa eddy current retarder, and domestic eddy current retarder technology is more advanced manufacturers such as Kaimai and Terjia.

Introduction to eddy current retarder

The first concept that needs to be clarified is eddy current, that is, eddy current, which refers to the current formed inside the conductor under electromagnetic induction. Eddy current braking is usually used in conjunction with conventional braking and is used to control the speed of most commercial vehicles (large and medium-sized buses and trucks), so it is also commonly referred to as eddy current retarder. An eddy current retarder is a device installed on an automobile transmission, drive shaft or drive axle that relies on the force of eddy current to slow down. In essence, it is equivalent to a DC high-power torque motor, which works based on the principle of "electromagnetic induction". Because the electromagnetic conductor of the rotor in the device is large, and the induced current generated on the rotor exists in the form of a vortex rotating around the magnetic field line, this kind of retardor is called an eddy current retarder.

Wrap a coil around the outside of a conductor and let the coil pass through the alternating current, then the coil will generate an alternating magnetic field. Because the conductor in the middle of the coil is a closed circuit that can be equivalent to a circle in the circumferential direction, and the magnetic flux in the closed circuit is constantly changing, the induced electromotive force and induced current will be generated in the circumferential direction of the conductor, and the direction of the current will rotate along the circumferential direction of the conductor, just like a circle of vortexes, so this phenomenon of electromagnetic induction in the whole conductor and the induced current is called eddy current phenomenon.

The longer the outer circumference of the conductor and the higher the frequency of the alternating magnetic field, the greater 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 presses the brake pedal to pass the stator coil of the retarder into direct current, the turntable fixed on the transmission shaft of the eddy current retarder is driven by the shaft and rotates, because there is a certain gap between the working surface of the turntable and the magnetic pole, it can be known from the principle of electromagnetic induction, in the electromagnetic circuit formed between the adjacent iron core, magnetic pole plate, air gap, and rotor, when there is relative movement between the rotor and the stator, this movement is equivalent to the conductor cutting the magnetic field line. At this time, a vortex-like induced current will be generated inside the body, and this induced current will excite the electromagnetic force. The direction of this force is exactly opposite to the direction of rotation of the rotor, because the direction of the force is always to hinder the direction of movement of the conductor, so it causes the rotor to slow down, which is the source of the braking force of the reducer.

Heat dissipation of eddy current retarder

From the perspective of the law of conservation of energy, when the retarder brakes, it converts the kinetic energy of the car into the electrical energy of the eddy current and releases it in the form of a short circuit. (The eddy current flows inside the rotor disc with a certain resistance value and shorts the circuit, and the thermal effect of the current will heat up the rotor.) In this way, the kinetic energy in the form of the vehicle is converted into heat energy from the rotor, which is dissipated by the strong wind generated by the blades on the rotor disc)

Therefore, the eddy current retarder will generate a lot of heat when working, and improving the heat dissipation capacity of the rotor and controlling the thermal deformation direction of the rotor is called the key to the design of the rotor structure, and it is also one of the core technologies of the eddy current retarder, and ensuring the cleanliness of the surface of heat dissipation parts such as rotor wind blades has also become a key item in the daily maintenance of the retarder.