Investigation on Eddy Current Braking Systems A Review

Transcription

1 Applied Mechanics and Materials Submitted: ISSN: , Vols , pp Revised: doi: / Accepted: Trans Tech Publications, Switzerland Online: Investigation on Eddy Current Braking Systems A Review G.L.Anantha Krishna 1,a, K.M.Sathish Kumar 2,b 1 Assistant Professor, BMS Institute of Technology, Bangalore , India 2 Associate Professor, BMS Institute of Technology, Bangalore , India a glamech@bmsit.in, b sathishmech@bmsit.in Keywords: Eddy current, Brakes, Magnetic field, Magnets, Neodymium Iron Boron (NdFeB) Abstract. The changing magnetic field will induce eddy currents in the conductor. These currents will dissipate energy in the conductor and generate drag force. It is found that Aluminium is the best material as conductor compared to Copper and Zinc. Also, it is found that the larger thickness of disc, more number of turns of electromagnet and higher electrical conductivity of conductor influences the generation of greater braking torque. Permanent magnet eddy current brake uses Neodymium Iron Boron (NdFeB) magnets. The analysis of permanent magnet eddy current shows that the parallel magnetised eddy current topology has the superior braking torque capability. In electrically controlled eddy current braking system subjected to time varying fields in different wave forms, the triangular wave field application resulted in highest braking torque. Electromagnetic brakes were found to interfere with the signalling and train control system. Permanent magnet eddy current brakes are a simple and reliable alternative to mechanical or electromagnetic brakes in transportation applications. Greater the speed greater is the eddy current braking efficiency. Hence, author intends to work on the development and investigation of permanent magnet eddy current braking system. Introduction The movement of a metal plate in a magnetic field induces a voltage which in turn creates eddy currents and metal plate decelerates. The better the conductivity and permeability of the plate, the stronger the braking force. There is no mechanical contact between the brake and track and hence is wear free and silent and requires minimal maintenance. The braking force is independent of the coefficient of friction ensuring high efficiency regardless of wheel rail adhesion for example damp conditions. This means that relatively high braking forces can be applied which remain almost constant even in high speed applications. The braking force can be accurately controlled by regulation of the magnetic field. Intensity of magnetic field allows the brake to be finely controlled as the magnetic field is created using electromagnets fed from an external power supply. The obstacle to commercial application would be electromagnetic compatibility with signalling and train control systems. This could result in interference or potentially even irreparable damage to the signalling systems by the electromagnetic forces generated when the eddy current brake was activated. A thorough investigation with the use of eddy current brake for service applications resulted in a substantial reduction in the wear of the train s conventional brake discs. So, there are All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/02/16,08:42:45)

2 1090 Dynamics of Machines and Mechanisms, Industrial Research obviously significant potential savings if the use of eddy current brakes can be extended. Eddy current brakes provide non-contact means to produce braking forces required to decelerate the motion of a moving object. With the rapid expansion of high speed lines and highways around the world, there is clearly considerable potential for a wider application of frictionless braking. Literature Review M.Z.Baharom et.al [1] found that Aluminium is the best material to be used as brake disc compared to Copper and Zinc as Aluminium has higher electrical conductivity. M.Z.Baharom et.al [2-3] has focussed on two series of Aluminium as the brake disc which are Al6061 and Al7075. The authors compare both the series for various Eddy current parameters such as air gap, number of turns and brake disc thickness. The findings shows that smaller the air gap, the larger the electromagnetic turns and higher the disc thickness, higher braking torque is generated and hence a great performance for Eddy current braking. Also, it is found that the higher electrical conductivity influenced the generation of greater braking torque. Der Ming Ma, Jaw Kuen Shiau [4 6] have presented four systematic engineering design scenarios to design a braking system. They are - A constant magnetic field - An optimal magnetic field distribution - Piecewise constant magnetic fields - Section wise guide rail with a constant magnetic field Simulation results of the above four designs show that the optimal magnetic field has a deceleration peak of 9g which is not suitable for most people. Piecewise constant magnetic field has the advantage of a pre-set terminal speed and predictable wire current but it produces a higher speed. Piecewise constant magnetic fields and section wise guide rail with a constant magnetic field have tolerable deceleration and easy manufacturing. An experimental braking system using constant magnetic field was built to demonstrate the design procedure. For high speed trains (speeds up to 350Km/hr.) with heavier loads on the axle and more complex functions, the conventional braking systems relying on the adhesion force between the rail and the wheel are no longer adequate. Firstly the increase in braking distance is unusable. Secondly, weather dependence of braking system is unsuited. Thirdly, in the event of failure of brake based on adhesion force between wheel and rail, it requires alternate braking system [7] that shall perform the profitable braking. Sergey Kitanov and Anatoly Podol ski [8] describe the investigation of eddy current and magnetic rail brake structures. A brake that contains permanent magnet pieces [9] and combining both magnetic rail brake and eddy current brake is built. Comparisons on experimental [10] and computed operating characteristics of eddy current and magnetic rail brakes for use on a tram car, on a rail road vehicle and on high speed train are

3 Applied Mechanics and Materials Vols presented. It is found that a brake built up from permanent magnet pieces that combines both magnetic rail brake and eddy current brake permits the most profitable braking action through the whole range of acceptable speeds from zero (a parking brake) to 350Km/hr. It is found that braking effect becomes more pronounced at the speed value 50Km/hr. than at speed 14Km/hr. Measured braking distance value is 520m. Hyun Rok Cha et al [11-12] examines permanent magnet eddy current couplings and brakes. Using 2 dimensional finite element analysis, the authors deals with the influence of the magnetisation patterns and one of the design parameters on the performance of the permanent magnet eddy current devices. The basic structure of eddy current device comprises the moving (or stationary) set of magnets that are separated from the stationary (or moving) conducting cylinder by an air gap. The eddy current couplings and brakes employed high energy product Neodymium iron Boron (NdFeB) permanent magnets that act on iron backed Copper drums to provide torque transfer from motor to load without mechanical contact. A two dimensional finite element analysis is performed to predict the electromagnetic behaviour and the torque speed characteristics of permanent magnet type eddy current couplings and brakes under constant speed operation. It is found that the parallel magnetised eddy current topology has the superior braking torque capability. Also, it is found that by increasing magnet thickness, the air gap flux density increases there by increase in braking torque. Eddy current brakes (ECB) are electrically controlled and non contact actuators used as assistive brakes in vehicles. ECBs exhibit insufficient generated braking torque at low speeds. In order to overcome the problem of insufficient braking torque generation at low speeds., Kerem Karakoc et al [13] worked on the braking performance improvement eg. Contact less braking, silent operation, no friction can be achieved with the replacement of the existing conventional hydraulic brakes with eddy current brakes (ECBs). They used alternating current magnetic fields with fixed and variable frequencies in different wave forms at both low and high speeds. Finite element analysis validated by an existing analytical model is performed for direct current and alternating current magnetic fields. It is shown that improved braking performance can be obtained when alternating current magnetic fields are used both at low and high velocities. Time varying fields in different wave forms (e.g. Sinusoidal, square, saw tooth and triangular waves) were applied. The triangular wave field application resulted in the highest braking torque. In addition, the frequency of the applied field is optimised using generic algorithms on a generic ECB configuration. Induced currents appear when electrical conductors undergo conditions of variable magnetic flux. Two procedures to achieve such conditions are - Exerting a time varying magnetic field on a static piece - Exerting a steady magnetic field on a moving one The latter case is 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 [14-15]. Following several kinds of measurements that can be carried out with the set up are

4 1092 Dynamics of Machines and Mechanisms, Industrial Research - Braking time of the disc which is measured as function of excitation intensity - Eddy current losses versus angular velocity - Eddy current losses versus excitation intensity The results show a reasonable agreement with theoretical predictions. Experiments deal with concepts, instruments and measurement techniques with high didactic value [7]. Marc T.Thomson [16] used a scale model fixture to test several different linear passive brakes based on Neodymium Iron Boron (NdFeB) permanent magnets. Test fixture composed of a rotating disc driven by an adjustable speed direct current motor. The 3.125mm thick aluminium disc having 370mm diameter is used and is designed to operate at speeds up to 2000 rpm which corresponds to a linear peripheral speed of up to 40m/s. Results from this high speed rotating test fixture were used to develop a set of magnetic scaling laws for the sizing and cost analysis of brakes. Using these results, a full scale brake was designed for a roller coaster application. Good agreement is found between the experimental results from the test fixture and analytical result based on electrodynamics theory. It is shown that in permanent magnet braking, the dominant material cost is for the permanent magnet material. Conclusions Electromagnetic brakes may interfere with signalling and train control system because of electromagnetic forces generated when the eddy current is activated. Permanent magnet eddy current brakes are a simple and reliable alternative to mechanical or electromagnetic brakes in transportation applications. Since, high powered vehicles and high speed expressways are being constructed all over the world, permanent magnet eddy current brakes are expected to have great success in future as greater the speed greater is the eddy current braking efficiency. Hence, author intends to work on the development and investigation of permanent magnet eddy current braking system. References [1] M.Z.Baharom, M.Z.Nuawi, G.Priyandoko, SM Harris, L.M.Siow: Eddy current braking study for brake disc of Aluminium, Copper and Zinc. Regional engineering postgraduate conference, (2011). [2] M.Z.Baharom, M.Z.Nuawi, G.Priyandoko, SM Harris: Electromagnetic braking system using Eddy current for brake disc of Al6061 and Al7075, International review of Mechanical Engineering, Vol.6, Issue 3, (March 2012), pp [3] M.I.Gonzalez: Experiments with Eddy currents: The Eddy Current Brake, European Journal of Physics,25,(2004), [4] Der Ming Ma, Jaw Kuen Shiau: The design of Eddy current magnet brakes. Transactions of the Canadian Society for Mechanical Engineering, Vol.35, No.1,(2011).

5 Applied Mechanics and Materials Vols [5] M.Jou, J.K.Shiau and C.C.Sun, Design of Magnetic Braking System, Journal of Magnetism and Magnetic Materials, 304, c234 c236, (2006) [6] Gagarin G., Kroger U., and Saunweber E, Eddy current magnetic track brakes for high speed trains, Joint ASME/IEEE/AAR. Railroad Conference, pp 95 99,(1987) [7] Herald M.A: Magnetic braking: Improved theory,american Journal of Physics, Vol.56, No.6, (1988), pp [8] Sergey Kitanov, Anatoly Podolskii: Analysis of Eddy current and magnetic rail brakes for high speed trains,the Open Transportation Journal, 2, (2008), pp [9] D.Z.Karminsky, M.D.Fokin, O.V.Bescennaya and N.S.Shlyakhov: Rail brakes with permanent magnets, Vestnik VNIIGT, No.8, pp 43 45, (1972) [10] P.J.Wang and S.J.Chiueh: Analysis of Eddy current brakes for high speed railway, IEEE Trans.Magn. Vol.34, (July 1998), pp [11] Hyun Rok Cha, Han Wook Cho and Sung Ho Lee: The Influence of Magnetisation pattern on the performance of permanent magnet Eddy current couplings and Brakes. Journal of Electrical Engineering & Technology, Vol.3, No.3, (2008), pp [12] B.Lequesne, B.Liu and T.W.Nehl, Eddy current machines with permanent magnet and solid rotors, IEEE Trans.Ind.Applicat, Vol. 35, pp (1997) [13] Kerem Karakoc, Edward J.Park, Afzal Suleman: Improved braking torque generation capacity of an eddy current brake with time varying magnetic fields: A numerical study. Finite elements in Analysis and Design. Vol.59, (2012), pp [14] Marshall S.V and Skitek GG: Electromagnetic concepts and Applications. 3 rd edition, Englewood Cliffs, NJ, Prentice Hall, (1990), pp [15] Tippler P: Physics for Scientists and Engineers. 4 th edition, NewYork, Freeman, (1999), pp [16] Marc T.Thompson: Permanent Magnet Electrodynamic Brakes Design principles and Scaling laws. Online Symposium for Electronics Engineers.

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