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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, whichi^ubjected 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 arc some practical applications. Such an application is the eddy
current brake.
EDDY CURRENT BRAKING:
Eddy current brakes arc^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 tne gap of a toroidal electromagnet, with diameter D. When the conductive disc rotates, eddy currents arc 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.l 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 technicai^problems and in a variety of everyday life situations. Sometimes th oyOe undesirable because of their dissipative nature (e.g. transformer c cores, metallic parts of generators and motors etc). In many othercSrses, however, eddy currents are valuable (metal detectors, cofaVfEognition 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 cours^^they are often dealt with only from a phenomenological point qf^feew, 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.
PRINCIPLE OF OPERATIONS:
.er
Eddy current brake works accor*
A .
According to this law, whenever a OQupictc the conductor, the magnitude ch is proportional to the strength of magnetic field and the speed of the conductor. If^jV-conductor is a disc, there will be circulatory currents i.c. 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.
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 simplified sketches
of this pattern. It is easy, however, to obtain an approximate expre&ion for the power dissipated by eddy currents. Since the magnetic field B is steady, the indhecd electric field in each point of the disc is given by £=vx B. where v is the velocity point [4]. Instead of measuring B
directly, we will relate it to the excitation curreut the coil of the electromagnet, which is easily measurable. For the moment we will assuine that B is proportional to lex (the validity of this hypothesis, which is not true for nrtagnetic media, will be discussed later). Then the following proportionality law holds: • T'
E « colex
where a> is the angular spc^Aofthc disc. This means that for any loop of eddy current the induced electromotive forceToeing the line integral of the induced field, is also proportional to colex . 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 = Kco2I2exp(\)
Air Cooled Eddv-Current Brakes:
The Dynamatic® Air-Cooled. Adjustable Torque Eddy-Current Brakes consist of a rotating member (rotor), keyed to a straight-through double extension shaft, and a stationary brake coil. The brake imposes controlled deceleration at variable speeds. There is no physical contact
between rotating and stationary members. This results in smooth response, thereby eliminating shock loading and extending equipment life. These brakes may be equipped with an optional tachometer generator mounted externally to provide a feedback signal to the brakes controller. The controller provides the DC excitation for the brake coil. The feedback signal from the tachometer generator is compared against a reference signal within the controller to provide accurate, smooth, controlled braking or constant speed throughout the period of excitation. Constant torque can be obtained with highly accurate torque adjustments. The controllers for the brakes are described in the Product Catalog.
Liquid Cooled Eddv-Current Brakes:
Liquid cooled eddy-current brakes consist of a rotating member (rotor), keyed to a straight- through double extension shaft, a stationary brake coil and an automatic water piping flow¬through cooling system. The Eddy-Current brake imposes controlled deceleration at variable speeds. With the Eddy-Current principle of torque transmission, there is not physical contact between input and stationary members. This results in smooth response, thereby eliminating shock loading and extending equipment life. Eddy-Current brakes may be equipped with an optional tachometer generator mounted internally to provide a feedback signal to the brake’s controller. The controller provides the DC excitation for the brake coil. The feedback signal from the tachometer generator is compared against a reference signal within the controller to provide accurate, smooth, controlled braking or constant speed throughout the period of excitation. Constant torque can be obtained with highly accurate torque adjustments.
CONCLUSION:
As we have seen, our experimental setup allows students to investigate one of the most outstanding aspects of eddy currents, namely their dissipated power. Simple tests to measure Pe and to study its functional dependence on the velocity, the sources (jijthc. magnetic field and the sample resistivities have been carried out. The results show a j^J^nable but not indisputable agreement with theoretical predictions. Nevertheless the author believes that the experiments deal with concepts, instruments and measurement tetljhiqiifes with high didactic value for the students, regardless of whether the agreement betwew?Hjeory and experiments is good or poor.