Submitted By
U.SUNIL KUMAR

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EDDY CURRENT BRAKES
ABSTRACT:
Many of the ordinary brakes, which are being used now days stop the vehicle by means of mechanical blocking. This causes skidding and wear and tear of the vehicle. And if the speed of the vehicle is very high, the brake cannot provide that much high braking force and it will cause problems. These drawbacks of ordinary brakes can be overcome by a simple and effective mechanism of braking system 'The eddy current brake'. It is an abrasion-free method for braking of vehicles including trains. It makes use of the opposing tendency of eddy current
Eddy current is the swirling current produced in a conductor, which is subjected to a change in magnetic field. Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy such as kinetic energy into heat, which is much less useful. In many applications, the loss of useful energy is not particularly desirable. But there are some practical applications. Such an application is the eddy current brake.
EDDY CURRENT BRAKING:
Eddy current brakes are simple magnetic devices that consist of a non-ferromagnetic conductor that moves through a magnetic field. An example is shown in Figure 1 where a magnetic field is created in the gap of a toroidal electromagnet, with diameter D. When the conductive disc rotates, eddy currents are induced at an average distance R from the axis of rotation where the pole’s magnetic field moves as a function of the angular velocity of the disk.1 Power is dissipated in the conductive disk by the Joule Effect, which creates a viscous-like torque applied to the disk.
INTRODUCTION:
Eddy currents are one of the most outstanding of electromagnetic induction phenomena. They appear in many technical problems and in a variety of everyday life situations. Sometimes they are undesirable because of their dissipative nature (e.g. transformer cores, metallic parts of generators and motors etc). In many other cases, however, eddy currents are valuable (metal detectors, coin recognition systems in vending machines, electricity meters, induction ovens, etc). However, little attention is paid to eddy currents in many of the textbooks commonly used in introductory physics courses they are often dealt with only from a phenomenological point of view, and they are considered in some cases only as a topic for optional reading Furthermore, most of the commercially available experimental setups concerning eddy currents treat only their qualitative aspects. This paper presents a set of laboratory experiments intended to help students better understand the phenomenon from a quantitative point of view.
Above figure is the sketch of eddy currents in a rotating disc. The crosses represent a steady magnetic field perpendicular to the plane of the disc. According to Faraday’s law, eddy currents appear in those points of the disc where the magnetic field increases or decreases.
PRINCIPLE OF OPERATIONS;
Eddy current brake works according to Faraday's law of electromagnetic induction. According to this law, whenever a conductor cuts magnetic lines of forces, an emf is induced in the conductor, the magnitude of which is proportional to the strength of magnetic field and the speed of the conductor. If the conductor is a disc, there will be circulatory currents i.e. eddy currents in the disc. According to Lenz's law, the direction of the current is in such a way as to oppose the cause, i.e. movement of the disc.
Essentially the eddy current brake consists of two parts, a stationary magnetic field system and a solid rotating part, which include a metal disc. During braking, the metal disc is exposed to a magnetic field from an electromagnet, generating eddy currents in the disc. The magnetic interaction between the applied field and the eddy currents slow down the rotating disc. Thus the wheels of the vehicle also slow down since the wheels are directly coupled to the disc of the eddy current brake, thus producing smooth stopping motion.
Theoretical foundation
Induced currents appear when electrical conductors undergo conditions of variable magnetic
flux. In particular, we talk about eddy currents when bulk conductor pieces instead of wires
are involved. There are two basic procedures to achieve such conditions:
• exerting a time-varying magnetic field on a static piece;
• exerting a steady magnetic field on a moving one.
An example of the latter class will be investigated. It consists of a rotating metallic disc, which is subjected to the magnetic field present at the gap of an electromagnet. Eddy currents appear inside the disc and brake its rotation. This is the foundation of the electromagnetic braking systems used by heavy vehicles such as trains, buses or lorries. Even in such a geometrically simple case, the pattern of eddy currents is complex. Figure 1 and [3] show simplified sketches of this pattern. It is easy, however, to obtain an approximate expression for the power dissipated by eddy currents. Since the magnetic field B is steady, the induced electric field in each point of the disc is given by E = v × B, where v is the velocity of that point [4]. Instead of measuring B directly, we will relate it to the excitation current Iex in the coil of the electromagnet, which is easily measurable. For the moment we will assume that B is proportional to Iex (the validity of this hypothesis, which is not true for magnetic media, will be discussed later). Then the following proportionality law holds:
E ∝ ωIex
where ω is the angular speed of the disc. This means that for any loop of eddy current the induced electromotive force, being the line integral of the induced field, is also proportional to ωIex . Finally, the basic laws of electric current state that the power dissipated in that particular loop is proportional to the square of the electromotive force and to the inverse of the electrical resistivity of the disc. The same holds for the power dissipated in the whole disc:
Pe = K ω2I 2 exρ(1)