ELECTROMAGNETIC & EDDY CURRENT BRAKING SYSTEMS

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Andrew Sponsler Sean Kurtz Analysis of the stopping force provided by induced eddy currents proves to lend credibility to the system as a practical brake solution. Thompson presents an equation for the force generated by eddy currents [6]. $ % = 2' %,)*+ , --./ - ! " - ! 0 (6)[6] ./ Describing this equation, Thompson says, “By Faraday’s law, there is an induced eddy current in the conducting fin, and these eddy currents generate a velocitydependent braking force fb, given by [equation] where v is relative velocity between the conductor and the permanent magnets, Fb,max is the maximum braking force; and vpk is a characteristic velocity at which the braking force peaks” [6]. A major drawback and maintenance cost of frictional systems is the effects of the sizeable amount of thermal heat generated from the braking process. For instance, when a brake pad makes contact with a rotating brake disc, thermal energy is transferred directly to the metal disc. The thermal energy transferred causes the metal disc to expand. Once the energy has been sufficiently transferred to the surroundings, the metal disc contracts to its original size. Over time, this constant expanding and contracting can crack the metal brake disc. Once this occurs, the brakes’ integrity is compromised. The disc cannot properly work and can fail at any time given the specific conditions of operation. In contrast, eddy current brakes diminish the occurrence of this safety issue. One alternate situation that can arise occurs under constant braking. In time, thermal heat can be generated, though, at a slow rate. This case is not a main concern, but should still be kept in consideration. As a result, with the absence of friction, safety and accountability of the braking system is improved. Power Supply Critically Analyzed A natural question posed of the limitations of ECB is that of the source of power supplication. In the majority of automobiles, power is supplied by the ignition of gasoline or diesel fuel. Onboard electronics are supplied by means of an alternator, a device that converts mechanical energy into alternating electrical energy. A brake system that is actuated by an electric current would naturally tax the alternator heavily for how often brakes are used. A greater need for electrical power may necessitate more powerful alternators or additional battery cells to fuel brake function. The greater electric consumption may also mean that the system can only be implemented on electric cars that already generate enough current, or can be modified to generate enough current, to additionally supply power to the ECB. As insufficient experimental research has been performed on the subject of implementing ECB into automobiles, it is difficult to establish quantitatively the power demand versus stopping power of ECB systems or the feasibility of powering ECB through traditional automobile alternators. COMPARISON TO FRICTIONAL <strong>BRAKING</strong> <strong>SYSTEMS</strong> Friction braking systems work by forcing direct contact between a moving surface and a fixed surface. The physical processes involved in this interaction are mechanical in nature. Since constant grinding occurs between the two surfaces, friction based brakes are not comparatively efficient – in terms of energy and heat application – to eddy current brake systems. ECB systems work through the use of applied electromagnetic fields without ever requiring physical contact between the disc wheel and the coils. The elimination of friction allows an increased timespan of utility and greater dependability. Advantages The primary advantages of applying EBS are reliability and efficiency. EBS will have to be replaced less frequently than friction based brakes, thus saving time and money in the long run. The applications of EBS are unlimited since they can be altered to meet a desired shape or size. Since EBS is a relatively young field, much advancement can easily be made through testing to find ways to make the product less costly. An additional factor to consider is the safety of EBS. The increased reliability and lack of surface contact leads to less chance of brake failure, overheating or slipping. The primary advantages of applying eddy current brakes to passenger vehicles include efficiency, safety, low pollution, and a low cost of maintenance. Since there is no direct contact between the moving body and the braking system, there is no deterioration of the braking system. ECB use electromagnetic forces to slow a moving body without creating much wasted energy. Even though thermal energy arises from the workings of the system, the waste is not close to traditional frictional systems that are in place today. Since ECB does not use friction, airflow or liquid coolants can easily cool the system without disturbing the braking process. The airflow and/or liquid coolants can absorb some of the thermal energy given off by the braking system, keeping the system stable. While in the frictional system, there is no room between the contact between the brake and the moving body causing constant wear on the system every time it is used. This direct contact friction also causes increased temperatures and is limited in functionality by the time of dissipation of the thermal energy. Friction-based braking systems function by transforming kinetic energy entirely into thermal energy [2]. One solution to the heat dissipation problem could be to use advanced engineering materials to construct lightweight, internally ventilated brake discs. Another solution would be to eliminate the direct University of Pittsburgh Swanson School of Engineering April 13, 2013 4
Andrew Sponsler Sean Kurtz contact of brake pads entirely, using ECB. The disc brake system has an unstable nature when the concentration of heat becomes extreme. This leads to safety concerns in frictional systems that will fail above certain temperatures, while ECB can overcome these concerns since the temperature of the system can be controlled so that it does not rise appreciably. Installations of ECB can be one of two types: permanent magnet systems or charged systems. Permanent magnet systems have an inherent safety mechanism. These systems involve a magnet, commonly a neodymium-iron-boron magnet [6] that permanently generates a magnetic field. In this case the system works the same ways as the charged system with electric coils; however in this system the coils apply a magnetic field opposite of the permanent magnet so that when fully powered, the coils cancel out the magnetic field, creating a net magnetic field of zero. This way, if the system loses power, the magnet will automatically engage. Whether the system has power or the power fails, the brake will still work since the magnetism is constant. Charged systems, on the other hand, work like friction systems in the sense that they require power to slow a moving body. This charged system requires an emergency brake that can be applied in the chance that the power source fails. This emergency brake is most commonly a traditional friction brake that can be initiated through the use of a mechanical lever to apply braking force to slow the system. While, in a frictional case, normal friction applied brakes and emergency brakes can slip. Since eddy current brakes do not use direct contact, there is minimal chance that the system can fail or slip. The electromagnetic force is constantly applied during braking. A slip in a frictional system occurs when a horizontal force overcomes the force of friction. Since there is no contact in an ECB and since a constant electromagnetic field cannot slip, the system eliminates the chance of a situation where the braking can slip unintentionally. It must be noted that this reduction of slipping probability in the brake system does not preclude intentional engineering to provide for safety. Anti-locking safety features may be hardwired into the circuitry of the system to prevent tire skid or brake lock at high speeds or low-traction roadways. Another advantage of eddy current brakes is the minimization of pollution. The various forms of pollution related to traditional frictional braking systems include noise, smell, and physical waste. Debris from the brake in the contact area, thermal energy, and sound energy all result from the rubbing between the braking system and the moving body at the point of contact. As the brake pads are worn down, the material has to go somewhere, so the refuse usually turns into soil, air, and water pollution [7]. The thermal energy usually translates to create a burning smell not found in nature, creating air or odor pollution [7]. Lastly, the rubbing also causes various sounds of grinding, which is noise pollution [7]. Eddy current braking resolves the noise, odor and debris issues and is a green alternative. Since it lacks a frictional component, little energy is given off in the form of thermal pollution. The process is quiet and clean. No excess material is produced from the braking process and the only byproduct is a small quantity of thermal energy. In addition to being a green alternative, eddy current brakes require little maintenance. Foreseeable maintenance required for upkeep of the system would not be significantly more than coolant replacement and wiring replacement, and only infrequently through the life of the product. The life of the ECB system could foreseeably exceed that of the vehicle on which it is installed. Both wiring and coolant are inexpensive products and, if only purchased intermittently through the life of the product, would amount to low upkeep costs. Since the maintenance costs are so low, the cost of the installation and production of eddy-current brakes can be recouped over an estimated seven years, according to one study [8]. Life-cycle costs over a nominal 25 years are expected to be little more than half of those for conventional disc brakes [8]. While traditional friction brakes need consistent seasonal replacement of brake pads, braking fluid, and rotors, the long-run economic value of non-friction brakes overcomes the initial cost that the brakes require in a few years when compared to frictional systems. Disadvantages The problems facing the application of eddy current brakes are simply the time and cost associated with research and development of a system compatible with traditional automobiles. Since this is largely an experimental subject on systems as small and as ubiquitous as the common automobile, developers need to find the balance point between cost of production and marketability while still maintaining the quality of the product. A further aspect that disadvantages EBS is the need for the development of safety features analogous to anti-lock brakes in friction systems. The primary disadvantages of applying eddy current brakes to modern systems are: limited research, reliability at low speeds, and interference with other signaling processes. A search for information and statistics regarding the brake system yields few results because little field research has been done. This is a massive field where only general systems have been designed but minimal testing has been done beyond applications to trains, propellers, and roller coasters. This leads to unknowns in the everyday use of ECB for the common person, specifically in cars. Almost no studies have been done for real designs of ECB for consumer cars. One more concern of using eddy current brakes is their effectiveness at low speeds. Once a body’s motion has decreased, ECB almost become useless at slow speeds because nothing is there to hold the body at rest. Once the brakes have slowed the body down, there still needs to be a contact brake to hold the body in place. Then, when you turn the power off to the braking system, the body will be put back into motion by any force willing. Even the University of Pittsburgh Swanson School of Engineering April 13, 2013 5
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ELECTROMAGNETIC & EDDY CURRENT BRAKING SYSTEMS

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