Application of Multiple Unipolar Axial Eddy Current Brakes for Lightweight Electric Vehicle Braking

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Article Application of Multiple Unipolar Axial Eddy Current Brakes for Lightweight Electric Vehicle Braking

Mufti Reza Aulia Putra 1 , Muhammad Nizam 2,3,4,*, Dominicus Danardono Dwi Prija Tjahjana 1,3, Muhammad Aziz 5 and Aditya Rio Prabowo 1

1 Mechanical Engineering Department, Faculty of Engineering, Universitas Sebelas Maret, Jl. Ir. Sutami 36A, Surakarta 57126, Indonesia; [email protected] (M.R.A.P.); ddanardono@staﬀ.uns.ac.id (D.D.D.P.T.); [email protected] (A.R.P.) 2 Electrical Engineering Department, Faculty of Engineering, Universitas Sebelas Maret, Jl. Ir. Sutami 36A, Surakarta 57126, Indonesia 3 National Center for Sustainable Transportation Technology (NCSTT) ITB, Bandung 40132, Indonesia 4 Lithium Battery Research and Technology Centre, Universitas Sebelas Maret, Jl. Slamet Riyadi 435, Surakarta 57146, Indonesia 5 Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan; [email protected] * Correspondence: muhammad.nizam@staﬀ.uns.ac.id

Received: 6 June 2020; Accepted: 2 July 2020; Published: 6 July 2020

Abstract: The braking system in a vehicle has the main role of slowing down the speed or stopping the moving vehicle. Compared to mechanical braking, which utilizes friction, non-contact braking has several advantages, including longer lifetime and less maintenance. One form of non-contact braking systems is the eddy current brake (ECB), an electric braking system that employs eddy currents to operate. This research focuses on the impact of magnetic ﬁeld sources used in the ECB. In addition, the number of magnetic ﬁeld sources is also observed. In order to achieve an ECB design that can be easily applied in any types of vehicles, including motorcycles and compact cars, a compact ECB design with an excellent braking torque is required. In this study, a compact design of the ECB is obtained by distributing the required braking torque from the disc brake into multiple electromagnets. Finite element method-based modeling has been performed to study ECB parameters, including the number of coil winding, the number of electromagnets, and the electric current. The results of this study show that the developed compact ECB could produce 93.66% of the torque required for braking.

Keywords: contactless braking system; eddy current brake; multi polar eddy current brake

1. Introduction The main role of the braking system is to decrease the speed of a moving vehicle or stop the vehicle. Braking can also be used to maintain the vehicle’s position in its state. During its usage, the brake absorbs the kinetic energy, and the drag force to stop the objects is provided by friction. The absorbed energy is then transformed into heat [1]. Good braking performance is crucial for maintaining vehicle speed and vehicle safety. The use of the mechanical braking system produces heat that must be released through cooling using its surrounding free air. Brakes that are used frequently can generate an amount of heat that is larger than the ability of the system to release it. The increase in heat decreases the braking performance, and a high temperature can eliminate braking forces, resulting in braking failure. The use of friction in this braking system has disadvantages. Brake components require maintenance and replacement at certain times. The service life of brake components can be increased by combining braking systems. Mechanical braking supported by engine brakes produces an excellent braking performance and leads

Appl. Sci. 2020, 10, 4659; doi:10.3390/app10134659 www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci. 20202020, 10, 10, x, 4659FOR PEER REVIEW 2 2of of 16 15 braking system generally employs a hydraulic system to generate sufficient drag force. This kind of to the extended service life of the braking components [2,3]. A conventional braking system generally braking system faces several problems, including a delay in braking time, wear of the brake pad, a employs a hydraulic system to generate sufficient drag force. This kind of braking system faces several low braking performance in the high-speed region, and bulkiness in size. problems, including a delay in braking time, wear of the brake pad, a low braking performance in the Electric and hybrid vehicles have been massively produced and adopted [4,5]. Electric vehicles high-speed region, and bulkiness in size. show better energy efficiency and are more environmentally friendly. Different to conventional Electric and hybrid vehicles have been massively produced and adopted [4,5]. Electric vehicles vehicles, electric vehicles employ no engine brake, and regenerative braking is adopted in order to show better energy efficiency and are more environmentally friendly. Different to conventional vehicles, substitute the engine brake. It is used as a support for braking and, additionally, to produce electricity electric vehicles employ no engine brake, and regenerative braking is adopted in order to substitute [6,7]. The generated electricity is then utilized to charge the battery. the engine brake. It is used as a support for braking and, additionally, to produce electricity [6,7]. In order to solve the problems of a friction-based braking system and to provide an excellent The generated electricity is then utilized to charge the battery. braking performance even in the high-speed region, a contactless braking system has been In order to solve the problems of a friction-based braking system and to provide an excellent developed. This kind of braking system is believed to have a longer lifetime and require less braking performance even in the high-speed region, a contactless braking system has been developed. maintenance. One form of contactless braking system is the eddy current brake (ECB). The ECB is an This kind of braking system is believed to have a longer lifetime and require less maintenance. One form electric braking system that employs the eddy currents principle. The main components of the ECB of contactless braking system is the eddy current brake (ECB). The ECB is an electric braking system that consist of a magnetic field in the exitation winding and brake caliper and conductor sources, as shown employs the eddy currents principle. The main components of the ECB consist of a magnetic field in in Figure 1. An eddy currents is an induction current that is formed in the conductor section to the exitation winding and brake caliper and conductor sources, as shown in Figure1. An eddy currents produce braking force. The ECB can be combined with regenerative braking to improve energy is an induction current that is formed in the conductor section to produce braking force. The ECB efficiency [8]. In addition, the ECB can also be adopted as a braking support system. Furthermore, can be combined with regenerative braking to improve energy efficiency [8]. In addition, the ECB compared to using resistive loads on regenerative braking, the ECB has a superior braking can also be adopted as a braking support system. Furthermore, compared to using resistive loads on performance because the torque components produced by the ECB are more effective in terms of regenerative braking, the ECB has a superior braking performance because the torque components braking performance [9]. produced by the ECB are more effective in terms of braking performance [9].

(a) (b)

FigureFigure 1. 1. AxialAxial unipolar unipolar eddy eddy current current brake brake (ECB): (ECB): (a ()a design) design unipolar unipolar ECB; ECB; (b (b) eddy) eddy currents. currents.

TheThe ECB ECB has has been been widely widely used used in in various various scientific scientific and and application application fields. fields. It It is is generally generally used used to to improveimprove braking braking performance performance and and reduce reduce the the risk risk of of mechanical mechanical failure. failure. It Itcan can be be applied applied to to wind wind turbinesturbines as as a a turbine turbine speed controlcontrol system, system, so so the the rotation rotation speed speed can can be suppressedbe suppressed such such that that it is lowerit is lowerthan itsthan critical its critical speed speed [10]. In[10]. addition, In addition, the ECB the can EC beB can used be in used high-speed in high-sp vehicleseed vehicles that have that a lowhave risk a lowof failure,risk of failure, such as such planes as andplanes trains. and Theretrains. areThere also are several also several studies studies evaluating evaluating its application its application in light invehicles light vehicles such as such cars as and cars motorbikes and motorbikes [11,12]. [11,12]. TheThe improvement improvement inin ECBECB performance performance can can be carriedbe carried out byout increasing by increasing the density the density of the magnetic of the magneticfield in thefield air in gap. the Inair general,gap. In inductancegeneral, inductance receives thereceives magnetic the magnetic field from field one from side. one However, side. However,recent research recent showsresearch that shows the use that of magneticthe use of fields magnetic from bothfields sides from can both increase sides thecan magneticincrease fieldthe magneticdensity [field13]. Thedensity changes [13]. The in the changes air gap in have the air been gap evaluated have been in evaluated [14]. It was in [14]. shown It was that shown a change that in a thechange gap einffect the generated gap effect the generated braking performance.the braking performance. This study focused This study mainly focused on the mainly magnetic on fieldthe magneticdensity producedfield density by airproduced gaps, but by the air researchersgaps, but the did researchers not pay attention did not topay the attention influence to of the the influence magnetic offield. the magnetic Another field. study Another stated thatstudy the stated change that in the the ch positionange in the of theposition magnetic of the field magnetic affects field the brakingaffects theperformance braking performance [15]. These [15]. changes These indicate changes that indicate changes that in changes the location in the of location the shoe of pole the result shoe inpole the resultchange in the of braking change performanceof braking performance by the ECB. by the ECB. Appl. Sci. 2020, 10, 4659 3 of 15

The limitation of using the ECB at low speeds is due to the induced currents, in which the magnitude is affected by the speed. To improve this performance, the ECB can be integrated with other systems that can provide braking at low speeds. Obara et al. [16] investigated hybrid braking, combining the ECB and the magnetic track brake. The ECB can provide good braking at high speeds and reduce mechanical contact, therefore reducing the possibility of braking system failure. At low speeds, the braking system uses magnetic track braking, which uses braking pads coated with braking material. A similar study was also carried out by Li et al. [15]. They combined the ECB with mechanical braking using a hydraulic system. The distance between the magnetic field source and the rail uses an electrically controlled hydraulic system. Furthermore, Ma et al. [17] developed the system by adding the design optimization using non-magnetic barriers to reduce the flux leak age. Providing a barrier to leakage increases the density of the magnetic field in the air gap and increases the braking force. Ryoo et al. [18] examined the current settings to obtain the optimal braking torque. To activate the ECB, an external source of electricity and/or a permanent magnet are necessary, leading to a high total cost. To overcome these issues, an ECB system in which the source of the magnetic field is generated not by a permanent magnet but by an electromagnet from the electricity generated from a pack battery has been made. This system can save energy and reduce costs, as has been studied by Bae et al. [1]. A self-generation system to improve energy efficiency was also evaluated by Cho [2] by adopting capacitors. The capacity of the capacitor seemed to influence the energy efficiency. Previous studies have examined how to improve performance without consideration of the size of the magnets used. The results of the existing studies show improvements in performance without considering the limitations of the size of the vehicle used. Unipolar research is still limited to the use of one magnet. There has been no research on the use of more than one magnetic field source with a unipolar design. This research further analyzes the effect of certain parameters. The magnetic field is generated by using an electromagnet, allowing for the possibility of controlling the current. This research focuses more on the study of the ECB braking system by making changes to the source of the magnetic field that is adopted. In addition, the use of more than one magnetic field sources is also evaluated further in this study. Both experimental and calculation analyses are conducted to validate the result. The purpose of this research is to make a good ECB for hybrid applications with conventional brakes for a lightweight vehicle. The ECB does not replace the conventional brake but supports it, so that increases the lifetime of conventional brakes.

2. Literature Review An eddy currents is an induced current generated due to changes in the magnetic field in the conductor. Eddy currents can appear in a stationary conductor that is affected by the changing magnetic field or the moving object across the fixed magnetic field. Initially, the magnetic field is induced in the direction of its main magnetic field; therefore, there is a repulsive force between the two. Furthermore, another magnetic field that is opposite to the source of the main magnetic field is obtained, resulting in tensile force. Both generated forces produce another force in the opposite direction to the direction of the motion of the magnetic field [19]. The main components of the ECB consist of magnetic field sources and inductance. Based on its structure, the ECB can be categorized into axial, radial, linear, and retarder types. In this research, a unipolar design of the ECB is developed and evaluated. The unipolar design has advantages over other ECB designs [13]. By adopting the electromagnet, it is possible to achieve a braking performance that is easier to control compared to a permanent magnet. The magnetic field is produced by an electrically wound coil. The amount of magnetic flux produced by a coil depends on the amount of electric current flowing to the system. The amount of torque is regulated by changing the amount of current. Greater electric power leads to a greater magnetic field. In addition, the frequency and shape of the current signal also affect the braking performance. It was also found that the braking torque can be increased by adopting AC power [14]. However, one of the challenges faced by the ECB is that it is difficult to control in the low speed region. Appl. Sci. 2020, 10, 4659 4 of 15

Increasing the braking capacity can be achieved by increasing the strength of the magnetic field source. Permanent magnets have a fixed capacity depending on the material used to make permanent magnets, such that the strength that can be obtained can no longer be increased, whereas the conductive winding has a limited capacity of strong electric current that can be produced. On the other hand, the limitations on the conductor material due to surface forces cause the limited volume of the area to be affected by the main magnetic field producing the eddy currents. In addition to determining the design parameters, increasing the ECB braking torque can be achieved by making modifications. The amount of braking torque is directly proportional to the density of the magnetic flux in the air gap, so the braking torque can be increased by increasing the addition of permanent magnets as an additional magnetic field source. Yazdanpanah (2015) [9] uses two magnetic field sources simultaneously. Permanent magnets are at once used as a winding core. The resulting braking torque is higher than that of the ECB, with a coil magnetic field source at the same current. This design has the disadvantage that there is still braking, even though no current flows to the coil, as a result of the existence of permanent magnets as the coil’s core. If one wants to eliminate the braking force, a current that is opposite the permanent magnetic pole must be provided. As a refinement of the design, permanent magnets are placed at the end of the coil core. Because the permanent magnet is at the end of the coil core, when the coil has no electricity, the resulting magnetic field flows to the coil core. When the coil is given a current, the permanent magnetic field will add to the strength of the magnetic field, which works to produce braking torque, because a parallel arrangement is arranged. The magnetic field is produced by an electrically wound coil. The amount of magnetic flux produced by a coil depends on the amount of flowing electric current. The amount of torque is regulated by changing the amount of flowing current. The greater the electric power given, the greater the magnetic field produced. Setting the frequency and shape of the current signal also affects the performance. Speed and torque settings are for variations in the shape of the sine and square step signals [10,13]. The difficulty setting is at low speeds. Circuits can be made parallel and in series. Performing a series of settings obtained the performance as needed [5]. The braking torque in the ECB is strongly influenced by the skin effect. The increase in speed results in the increase of this skin effect. According to Sharif et al. [20], the skin effect variable must be added when calculating the braking torque. The performance of the ECB for braking has been reviewed by Sharif et al. [21]. The skin effect shows different impacts, depending on the thickness of the conductor. Schieber [22] conducted an ECB study on thin plate sheets and found that the skin effect was not significant. Singh et al. [23] conducted research on thick plates and evaluated the impact of skin effects. In thick conductors, the skin effect greatly affects the braking torque produced at high speeds.

3. Modeling and Methods Modeling was carried out in order to evaluate the developed model and analyze the effects of operating parameters to the produced braking torque. The braking torque in the ECB system was obtained from the interaction between the main magnetic field and the magnetic field generated by the eddy currents. The amount of braking torque depends on the magnitude of the main magnetic field that produces the eddy currents. The eddy currents further produce a second magnetic field that affects the braking force. The main factors that influence the performance are the magnetic field strength in the air gap and the amount of eddy currents in the inductor. The amount of braking torque can be calculated by using several approaches, including the eddy currents loss equation. The amount of braking torque is proportional to the magnitude of eddy currents losses that occurs in the inductor material. The determination of the amount of braking torque can also be applied by using the Lorentz force equation and the Maxwell stress equation.

3.1. Developed Model Modeling was conducted using the ﬁnite element method (FEM) in a 3D model using Ansys: Maxwell 3D software. The 3D design of the braking system was performed by using the 3D Solid Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 16

Appl. Sci. 2020, 10, 4659 5 of 15 Works modeling software of SolidWorks, and the produced 3D modeling data were then exported to Maxwell 3D. Table 1 shows the basic specifications of the developed braking system, including the Worksmaterials modeling and geometry. software Throughout of SolidWorks, the numerical and the produced calculation 3D and modeling experimental data were test, then the exportedair gap and to Maxwelldisc thickness 3D. Table are fixed1 shows at 0.5 the and basic 4 mm. speciﬁcations of the developed braking system, including the materials and geometry. Throughout the numerical calculation and experimental test, the air gap and disc thicknessTable 1. Basic are specifications ﬁxed at 0.5 and of the 4 mm.developed compact ECB system, including material and geometry.

Table 1. Basic specificationsVariable of the developed compact ECB system, including Unit material and geometry.Value Pole shoe length mm 30 Pole shoeVariable width Unitmm Value 12.5 The total lengthPole shoeof the length coil core mm mm 30 248 Distance of polePole shoe shoe to width disk center mmmm 12.5 83.5 The totalAir length gap of the coil core mmmm 248 0.5 Distance of pole shoe to disk center mm 83.5 Disk thickness mm 4 Air gap mm 0.5 Relative permeabilityDisk thickness of aluminum mm- 4 1.000022 RelativeRelative permeability permeability of aluminumsteel - 1.000022- 400 ConductivityRelative permeability of aluminum of steel -Ωm 400 2.06 × 10−7 Conductivity of aluminum Ωm 2.06 10 7 × − Table 2 shows the parameters that are evaluated in this study, including the number of coil windings,Table2 the shows number the of parameters electromagnets, that are and evaluated the electrical in this current. study, These including parameters the number are considered of coil windings,to significantly the number influence of electromagnets, the generated magnetic and the electrical field intensity. current. During These parametersthe numerical are calculation, considered tothe significantly heat generated influence during the generatedbraking is magneticignored. fieldThe intensity.disk brake During is used the as numerical a rotor, calculation, while the theelectromagnet heat generated is adopted during brakingas the source is ignored. of magnetic The disk fields brake for is the used braking as a rotor, system. while In the order electromagnet to validate isthe adopted model, asa validation the source test of magneticis initially fieldsconducted, for the and braking its results system. are explained In order to in validate Section 4. the model, a validation test is initially conducted, and its results are explained in Section4. Table 2. Evaluated parameters during the modeling and numerical calculation. Table 2. Evaluated parameters during the modeling and numerical calculation. Parameters Unit Values Number Parametersof coil winding Unit Values- 360, 540, 720 NumberNumber ofof coilmagnet winding - 360, 540, - 720 1, 2, 4 ElectricalNumber current of magnet - 1, 2, A 4 20, 25, 30, 35, 40 Electrical current A 20, 25, 30, 35, 40 Figure 2 shows the basic designs of the used disk and coil core during the validation test and numericalFigure calculation.2 shows the These basic designs ofare the based used on disk the andgeneral coil coredesigns during of a themotorcycle validation disk test brake. and numericalThe disk brake calculation. uses non-ferrous These designs material, are which based onhas thea good general performance designs of as a an motorcycle ECB disk. diskThe size brake. of Thethe coil disk core brake is also uses adjusted non-ferrous based material, on the size which of th hase vehicle a good brake performance caliper; astherefore, an ECB disk.it can Thebe applied size of theto motorcycles. coil core is also The adjusted design basedof the onECB the uses size differ of theent vehicle brackets brake from caliper; the conventional therefore, it can brake be appliedcaliper. toTherefore, motorcycles. if the The conventional design of the brake ECB operates, uses diff erentthe conventional brackets from brake the conventional does not affect brake the caliper. ECB Therefore,parameter ifperformance the conventional in terms brake of operates,the position the conventionaland the air gap. brake does not affect the ECB parameter performance in terms of the position and the air gap.

(a) (b)

Figure 2. Designs of the used (a) disk and (b) magnet. Appl. Sci. 2020, 10, 4659 6 of 15

The numerical calculation was carried out with the objective of obtaining the data in the form of braking torque produced by the ECB under different evaluated parameters. The data are used to determine the characteristics of the variables toward the optimum design of the brake. Before conducting the analysis, the clarification of the required braking conditions and the calculation of the required braking torque were conducted. The braking conditions were determined as the braking required to reduce the speed of a vehicle with a weight of 100 kg (assumed as motorcycle) and two wheels from 17 to 5.6 m/s, in a deceleration distance of 50 m. In addition, the disc has a diameter of 120 mm [24]. From this braking condition, it is clarified that the required torque to reduce the speed from 17 to 5.6 m/s is 52.1 Nm.

3.2. Governing Equations The approach to producing the braking torque is to calculate the amount of braking force multiplied by the ECB radius. Because of the braking force that occurs due to electromagnetic interactions, the amount of braking force can be calculated using the Lorentz force. The magnitude of the force is calculated from the magnitude of the magnetic field and the current flowing at a certain volume and can be written in Equation (1): * Z * * F d = J BdV (1) v × Volume can also be calculated using the surface area with double integrals. The amount of braking torque can be written by multiplying the braking force with the radius of the drum, as Equation (2):

Tb = x rp (J B)dS (2) s × ×

The magnitude of the magnetic ﬁeld strength can also be written with the density of the magnetic ﬁeld per unit area, or B = Φ/A. Hence, the braking force equation can be written as Equation (3):

FR = IR,i . lR,e,i . ∅R,i/AR,m,i (3)

The amount of eddy currents is calculated by Lorentz’s force equation:

J = σ(E + v + B) (4)

The magnitude of the eddy currents is influenced by the presence of magnetic fields and electric fields [25–35]: Z 2t Iδp F = Re B2xB2y Dx (5) 2µ0 0 y=0 Z 1 2tp F = Re B1xB1y Dx (6) 2µ0 0 where µ0 is the air permeability. The magnitude of the magnetic field is different from that proposed by Jang et al. [11] because the magnitude of the magnetic field is sought using magnetic vector potential techniques: Z 2x X∞ ωpp Fab = Re BmxBmy dx (7) 2µ0 n=1, odd 0 The calculation of the braking system using the disk brake design can be shown by the equation as stated by Shin et al. [36] and Choi et al. [6]: Z Z 1 2π R0 h n oi = III( + ) III( + ) Fb Re Bθ r, θ, d g . Bz r, θ, d g edrdθ (8) 2µ0 0 Ri Appl. Sci. 2020, 10, 4659 7 of 15

The formulation to obtain the value of the braking torque is calculated from the magnitude of the magnetic field, which affects the conductor or the magnetic flux density of the air gap. The use of only mathematical analytics results in an inaccurate calculation because it does not take into account the geometry of objects. The advantage of using analytical methods is the speed in calculation and te simplicity of the equation used. On the other hand, numerical analysis using FEM produces results that have a good agreement to the real conditions, because the numerical analysis takes into account the object geometry.

4. Experimental Validation

4.1. Configuration of the Test Bed Experiments were carried out using a test table that has been designed as an ECB test tool. Figures3 and4 show a schematic of the experimental setup and a picture of the actual setup, respectively. Essentially, the test table had the same specifications as those in the numerical calculation. The current source used was a voltage rectifier (Lakoni, Falcon 120 E 900 W 120 E inverter, China). The given current was measured using AC/DC clamp meters (Krisbow, KW 0600491, Kawan Lama Grup, Indonesia), while braking force was identified by using a single point load cell. The brushless dc motor (qs motor, 3000 W 138 70H, China) was used as the driving source. The influence of the temperature was disregarded in this experimental work. Table3 shows the additional conditions used during the experimental test.

Figure 3. Experimental setup: (1) pulley; (2) disk; (3) bracket; (4) stopper; (5) load cell; (6) electromagnet; (7) power; (8) magnet.

Table 3. Additional conditions used during the experimental test.

Variable Unit Value Electrical current A 20 Number of coil windings - 360 Number of magnet - 1

The electric motor is set to operate at a given speed. The experimental test is carried out by activating the ECB with a current of 20 A. After the current is given, the braking test is conducted.

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During the braking process, the speed is kept constant at a certain value. It is intended that the load measuredAppl. Sci. 2020 by, 10 the, x FOR load PEER cell REVIEW matches the desired speed. 8 of 16

Figure 4. Picture of the used experimental test table.

Table 3. Additional conditions used during the experimental test.

Variable Unit Value Electrical current A 20 Number of coil windings - 360 Number of magnet - 1

The electric motor is set to operate at a given speed. The experimental test is carried out by activating the ECB with a current of 20 A. After the current is given, the braking test is conducted. During the braking process, the speed is kept constant at a certain value. It is intended that the load Figure 4. Picture of the used experimental test table. measured by the load cell matches the desired speed. Experiments were conducted using a relatively simple principle. Figure 5 provides a schematic Experiments wereTable conducted 3. Additional using conditions a relatively used simple during principle.the experimental Figure test.5 provides a schematic ﬂowflow of the experimental process. The The rotation rotation sp speedeed is is set at the desginated speed (150–750 rpm), and the motor torque is deliveredVariable to the disk brake through the shaft. During Unit the braking, Value an electric current is passed to the Electrical winding coilcurrent in the electromagnet.electromagnet. The generated A magnetic field ﬁeld will 20 interact with the disc and create Number the braking of coil windings force. The The resulting resulting braking force is measured measured- using the 360 load load cell, cell, and its obtained data areNumber then displayed of magnet on the LED display.display. - 1

The electric motor is set to operate at a given speed. The experimental test is carried out by activating the ECB with a current of 20 A. After the current is given, the braking test is conducted. During the braking process, the speed is kept constant at a certain value. It is intended that the load measured by the load cell matches the desired speed. Experiments were conducted using a relatively simple principle. Figure 5 provides a schematic flow of the experimental process. The rotation speed is set at the desginated speed (150–750 rpm), and the motor torque is delivered to the disk brake through the shaft. During the braking, an electric current is passed to the winding coil in the electromagnet. The generated magnetic field will interact with the disc and create the braking force. The resulting braking force is measured using the load cell, and its obtained data are then displayed on the LED display.

Figure 5. Schematic ﬂow of the experimental test. Figure 5. Schematic flow of the experimental test. 4.2. Results of Experimental Test

Figure 5. Schematic flow of the experimental test. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 16

4.2. Results of Experimental Test

4.2.1. Results of Experiments Figure 6 shows the correlation of the rotation speed and the torque obtained from the experimental test. The rotation speed is set in the range of 150–750 rpm with an interval of 100 rpm. Appl.As can Sci. 2020be ,observed10, 4659 in Figure 6, the rotation speed influences the required braking torque, and9 of 15a higher rotation speed tends to require a higher braking torque. However, the required braking torque is maximum at 600 rpm and tends to decrease slightly when the rotation speed is increased. This speed tends to require a higher braking torque. However, the required braking torque is maximum phenomenon is considered due to the skin effect, which influences the sensing during measurement. at 600 rpm and tends to decrease slightly when the rotation speed is increased. This phenomenon is The characteristics of the ECB using aluminum conductor material are such that, if the air gap considered due to the skin effect, which influences the sensing during measurement. The characteristics changes, there will be a shift in the value of the critical velocity caused by changes in current density. of the ECB using aluminum conductor material are such that, if the air gap changes, there will be a When the critical speed point is exceeded, brake torque is reduced. This is caused by the emergence shift in the value of the critical velocity caused by changes in current density. When the critical speed of the magnetic field concentration on the outside (skin effect). point is exceeded, brake torque is reduced. This is caused by the emergence of the magnetic field concentration on the outside (skin effect). Torque (Nm)

Rotation speed (rpm)

Figure 6. Correlation between the rotation speed and torque during the validation test. Figure 6. Correlation between the rotation speed and torque during the validation test. 4.2.2. Comparison of Experimental Data and Numerical Calculation 4.2.2.Table Comparison4 shows theof Experimental comparison ofData experimental and Numerical data andCalculation numerical calculation conducted based on FEM.Table In 4 general,shows the the comparison average errors of experimental of the braking data torque and numerical for each calculation rotation speed conducted between based the experimentson FEM. In general, and numerical the average calculation errors are of verythe braking small (almost torque below for each 5%). rotation These dispeedfferent between values arethe consideredexperiments due and to numerical several aspects, calculation including are very the smal coilltemperatures (almost below and 5%). heat These generation different during values theare brakingconsidered process. due to In several addition, aspects, it is di includingfficult to maintainthe coil temperatures the rotation speedand heat of thegeneration electric motorduring as the a constantbraking process. when braking In addition, occurs. it Moreover,is difficult theto main vibrationstain the aff rotationect the load speed cell of readings, the electric as wellmotor as as the a materialconstant ofwhen the bracketsbraking occurs. used in Moreover, the electromagnet. the vibrations As the affect error the value load is cell very readings, small (below as well 5%), as itthe is believedmaterial thatof the the brackets data generated used in bythe theelectromagnet. numerical calculation As the error are value valid. is very small (below 5%), it is believed that the data generated by the numerical calculation are valid. Table 4. Comparison of experimental data and numerical calculation conducted based on the finite element method (FEM). Table 4. Comparison of experimental data and numerical calculation conducted based on the finite elementTorque method (Nm) (FEM). Rotation Speed (rpm) Error Difference % ExperimentTorque (Nm) FEM Rotation Speed (rpm) Error Difference % 150Experiment 7.3 FEM 6.97 4.52 150300 10.37.3 10.926.97 6.014.52 300450 11.510.3 12.0310.92 4.66.01 600 12 11.67 2.75 450 11.5 12.03 4.6 750 11.2 10.69 4.5 600 Average error (%)12 11.67 4.4762.75 750 11.2 10.69 4.5 5. Calculation Results and Discussion

5.1. Corelation of Braking Torque and Rotation Speed The braking torque at the ECB is strongly influenced by the skin effect, which is also increased by the increase in rotation speed. This is indicated by the tendency of the stronger generated magnetic field, resulting in the changes in flux density. The skin effect can be described as the phenomenon of Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 16

5. Calculation Results and Discussion

5.1. Corelation of Braking Torque and Rotation Speed Appl. Sci. 2020, 10, 4659 10 of 15 The braking torque at the ECB is strongly influenced by the skin effect, which is also increased by the increase in rotation speed. This is indicated by the tendency of the stronger generated magnetic electron accumulationfield, resulting in in the a conductor,changes in flux which density. is concentratedThe skin effect can in thebe described conductor’s as theskin. phenomenon The presence of of a electron accumulation in a conductor, which is concentrated in the conductor’s skin. The presence of skin eﬀecta resultsskin effect in results an increase in an increase in current in current density dens inity the in farthestthe farthest part part of of the the conductor conductor core, core, as as shown in Figure7shown. in Figure 7.

(a)

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 16

5.6 m/s, and it is 52.1 Nm. Therefore, the use of the ECB can reduce 23% of the braking torque required (b) by the braking system. It is considered that there are several ways to improve the performance of the ECB and still retain the compactness of the system, including the number of coil windings, the number of electromagnets, and the electric current; therefore, the ECB can be applied for light vehicles including motorcycles. The influences of these parameters are analyzed and shown in this sub-section. The numerical simulation for analyzing these parameters was conducted under the conditions of an air gap and rotation speed of 1 mm and 450 rpm,( crespectively.)

The magnetic field is generated by using a coil that is powered by an electric current. The amount of magneticFigure 7. fluxMagnetic produced flux density by a coil change depends at different on th rotatione amount speeds: of electric (a) 150 rpm; current (b) 450 given. rpm; (cTherefore,) 750 rpm. the amount of required torque can be regulated by changing the amount of electric current. From Figure8, it is clear that a higher rotation speed results in an increased skin e ffect. This skin Figure 8 shows the correlation of the number of coil windings and the generated braking torque. effect causesFigure a decrease 7. Magnetic in theflux density braking change torque at different at high rotation speed. speeds: When(a) 150 rpm; the (b rotation) 450 rpm; speed(c) 750 is 150 rpm,Commented [M1]: Please use Scientific notation,eg × 101 An increase in coil windings results in a higher braking torque. However, this increase in coil the magneticrpm. field generated on the disk is still in the area close to the source of the magnetic field. windings also resulted in a larger system size. In addition, Figure 9 shows the correlation of electric However, as the rotation speed is increased to 450 and 750 rpm, the generated magnetic fields are current and generatedFrom Figure braking 8, it is clear torque. that a higher Similar rotation to th speede effect results of in the an increasednumber skin of coileffect. windings, This skin a high distributedeffect at thecauses area a decrease outside in of the the braking magnetic torque field at high source. speed. ItWhen seems the thatrotation the speed rotation is 150 speed rpm, the of 450 rpm electric current also leads to a higher braking torque. Unfortunately, a higher electric current is the criticalmagnetic speed field in generated this developed on the disk design. is still Thein the secondary area close to magnetic the source fieldof the for magnetic this rotation field. speed potentially causes an increase in heat during braking, leading to a risk of overheating. Both the is higherHowever, than the as one the ofrotation lower speed rotation is increased speed to but 450 still and centralized750 rpm, the generated near to the magnetic magnetic fields fieldare source. number ofdistributed coil windings at the area and outside the electric of the magnetic current field clearly source. influence It seems thatthe thebraking rotation torque speed of[30], 450 and it is Meanwhile, at a rotation speed of 750 rpm, the braking torque begins to decrease due to the distribution importantrpm to isset the optimum critical speed values in this for developed each braking design. The system. secondary magnetic field for this rotation speed of the secondaryis higher than magnetic the one field. of lower rotation speed but still centralized near to the magnetic field source. Meanwhile, at a rotation speed of 750 rpm, the braking torque begins to decrease due to the distribution40 of the secondary magnetic field. 37.71 5.2. Increase35 in Magnetic Field

Based30 on experimental tests and numerical calculations, the highest torque value was obtained at 12 Nm from the experimental process using a reference-based design. In reality, the braking 25 21.26 20

15 9.42 10 Braking Torque (Nm) 5

0 0 100 200 300 400 500 600 700 800 Number of coil winding

Figure 8. Eﬀects of the number of coil windings on the braking torque.

Figure 8. Effects of the number of coil windings on the braking torque.

40 37.71 35 28.87 30 25 21.21 20 14.73 15 9.42 10

Braking Torque (Nm) 5 0 0 1020304050 Electric Current (A)

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 16

5.6 m/s, and it is 52.1 Nm. Therefore, the use of the ECB can reduce 23% of the braking torque required by the braking system. It is considered that there are several ways to improve the performance of the ECB and still retain the compactness of the system, including the number of coil windings, the number of electromagnets, and the electric current; therefore, the ECB can be applied for light vehicles including motorcycles. The influences of these parameters are analyzed and shown in this sub-section. The numerical simulation for analyzing these parameters was conducted under the conditions of an air gap and rotation speed of 1 mm and 450 rpm, respectively. The magnetic field is generated by using a coil that is powered by an electric current. The amount of magnetic flux produced by a coil depends on the amount of electric current given. Therefore, the amount of required torque can be regulated by changing the amount of electric current.

Appl. Sci.Figure2020, 810 shows, 4659 the correlation of the number of coil windings and the generated braking torque.11 of 15 An increase in coil windings results in a higher braking torque. However, this increase in coil windings also resulted in a larger system size. In addition, Figure 9 shows the correlation of electric 5.2.current Increase and ingenerated Magnetic braking Field torque. Similar to the effect of the number of coil windings, a high electricBased current on experimental also leads teststo a andhigher numerical braking calculations, torque. Unfortunately, the highest torque a higher value waselectric obtained current at 12potentially Nm from thecauses experimental an increase process in heat using during a reference-based braking, leading design. to In a reality,risk of the overheating. braking requirement Both the cannumber be defined of coil by windings using the and braking the electric torque incurrent order toclearly reduce influence the vehicle the speed. braking In thistorque case, [30], the and braking it is torqueimportant is calculated to set optimum as the values torque for required each braking to reduce system. the vehicle speed from 17 to 5.6 m/s, and it is 52.1 Nm. Therefore, the use of the ECB can reduce 23% of the braking torque required by the braking system. It is considered40 that there are several ways to improve the performance of37.71 the ECB and still retain the compactness of the system, including the number of coil windings, the number of electromagnets, 35 and the electric current; therefore, the ECB can be applied for light vehicles including motorcycles. The influences of these30 parameters are analyzed and shown in this sub-section. The numerical simulation for analyzing these25 parameters was conducted under the conditions21.26 of an air gap and rotation speed of 1 mm and 450 rpm, respectively. 20 The magnetic ﬁeld is generated by using a coil that is powered by an electric current. The amount of magnetic ﬂux15 produced by a coil depends on the amount of electric current given. Therefore, 9.42 the amount of required10 torque can be regulated by changing the amount of electric current.

Figure8 showsBraking Torque (Nm) the correlation of the number of coil windings and the generated braking torque. 5 An increase in coil windings results in a higher braking torque. However, this increase in coil windings also resulted in a larger0 system size. In addition, Figure9 shows the correlation of electric current and generated braking torque.0 Similar 100 to 200 the eﬀect 300 of the 400 number 500 of coil windings, 600 700 a high 800 electric current also leads to a higher braking torque. Unfortunately,Number of coil a higher winding electric current potentially causes an increase in heat during braking, leading to a risk of overheating. Both the number of coil windings and the electric current clearly inﬂuence the braking torque [30], and it is important to set optimum values for each braking system.Figure 8. Effects of the number of coil windings on the braking torque.

40 37.71 35 28.87 30 25 21.21 20 14.73 15 9.42 10

Braking Torque (Nm) 5 0 0 1020304050 Electric Current (A)

Figure 9. Eﬀect of electric current on the braking torque (where the number of coil windings is ﬁxed at 360).

The number of electromagnets is also considered to potentially improve the braking performance. In this study, difference numbers of electromagnets were analyzed, starting from one to four electromagnets. Figure 10 shows the configuration of the magnet used during numerical calculation. Figure 10 shows the correlation between the number of electromagnets and the generated braking torque. It is clear that the increase in the number of magnet significantly improves the braking torque. Figure 11 shows the obtained contours of the magnetic field for two- and four-magnet configurations. By increasing the number of electromagnets, the distribution of the magnetic field throughout the disc brake becomes larger. By increasing the number of electromagnets, the size of each braking unit can be kept small and compact, while fulfilling the required braking torque. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 16

Figure 9. Effect of electric current on the braking torque (where the number of coil windings is fixed at 360).

The number of electromagnets is also considered to potentially improve the braking performance. In this study, difference numbers of electromagnets were analyzed, starting from one to four electromagnets. Figure 10 shows the configuration of the magnet used during numerical calculation. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 16

Figure 9. Effect of electric current on the braking torque (where the number of coil windings is fixed at 360).

The number of electromagnets is also considered to potentially improve the braking performance. In this study, difference numbers of electromagnets were analyzed, starting from one toAppl. four Sci. 2020electromagnets., 10, 4659 Figure 10 shows the configuration of the magnet used during numerical12 of 15 calculation.

(a) (b)

Figure 10. Developed compact ECB designs with (a) two electromagnets and (b) four electromagnets.

Figure 10 shows the correlation between the number of electromagnets and the generated braking torque. It is clear that the increase in the number of magnet significantly improves the braking torque. Figure 11 shows the obtained contours of the magnetic field for two- and four-magnet configurations. By increasing the number of electromagnets, the distribution of the magnetic field throughout the disc brake becomes larger. By increa sing the number of electromagnets, the size of each braking unit can be kept(a) small and compact, while fulfilling the required(b) braking torque.

FigureFigure 10. 10. DevelopedDeveloped compact compact ECB ECB designs designs with with ( (aa)) two two electromagnets electromagnets and and ( (bb)) four four electromagnets. electromagnets. 45 Figure 10 shows the correlation between the number of electromagnets38.22 and the generated 40 braking torque. It is clear that the increase in the number of magnet significantly improves the 35 braking torque. Figure 11 shows the obtained contours of the magnetic field for two- and four-magnet 30 configurations. By increasing the number of electromagnets, the distribution of the magnetic field 25 throughout the disc brake becomes larger. By18.89 increasing the number of electromagnets, the size of 20 each braking unit can be kept small and compact, while fulfilling the required braking torque. 15 9.42 10 Braking Torque (Nm) Torque Braking 455 38.22 400 35 012345 30 Magnets 25 Figure 11. Correlation between the number of18.89 electromagnets and the braking torque with 20A and 360 coils. 20 Figure 11. Correlation between the number of electromagnets and the braking torque with 20A and 15 9.42 360It is coils. important 10 to note that the increase in the number of electromagnets should avoid any Braking Torque (Nm) Torque Braking interaction risk of the5 magnetic field generated by each magnet. In addition, it is necessary to consider the space left0 for the installation of conventional braking systems. As shown in Figure 12, although using four electromagnets,012345 the interaction of the magnetic field among the poles can be avoided. The interactions of the magnetic field canMagnets result in magnetic field leakage. The use of four electromagnets can generate a 48.8-Nm braking torque. In addition, reducing the vehicle’s speed from 60 to 20 km/h requires 52.1 Nm (a vehicle weight of 100 kg). The use of the ECB with this design can reduceFigure 93.66% 11. Correlation of the braking between torque the number required of byelectromag the brakingnets and system the braking compared torque to with without 20A and an ECB. By flowing360 coils. an electric current of 30 A, setting the number of coil windings to 540, and using two electromagnets, a higher braking torque than is required can be generated. Looking at Figure 13, it is important to note that the ECB does not replace conventional brakes. The ECB is used as an additional part for the braking system of a lightweight vehicle. The braking distances will be relatively similar whether an ECB is used or not. It is important that the ECB will be used in the speed range of medium to high. Therefore it will decrease the use of conventional brakes and thus will increase its life since the heat during braking operation, and the wear of the brake pads will be reduced. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 16

(a) (b)

Figure 12. Obtained contours of the magnetic field: (a) two electromagnets and (b) four electromagnets.

It is important to note that the increase in the number of electromagnets should avoid any Appl. Sci. 2020, 10, 4659 13 of 15 Appl.interaction Sci. 2020, 10 risk, x FOR of the PEER magnetic REVIEW field generated by each magnet. In addition, it is necessary to consider13 of 16 the space left for the installation of conventional braking systems. As shown in Figure 12, although using four electromagnets, the interaction of the magnetic field among the poles can be avoided. The interactions of the magnetic field can result in magnetic field leakage. The use of four electromagnets can generate a 48.8-Nm braking torque. In addition, reducing the vehicle’s speed from 60 to 20 km/h requires 52.1 Nm (a vehicle weight of 100 kg). The use of the ECB with this design can reduce 93.66% of the braking torque required by the braking system compared to without an ECB. By flowing an electric current of 30 A, setting the number of coil windings to 540, and using two electromagnets, a higher braking torque than is required can be generated. Looking at Figure 14, it is important to note that the ECB does not replace conventional brakes. The ECB is used as an additional part for the braking system of a lightweight vehicle. The braking distances will be relatively similar whether an ECB is used or not. It is important that the ECB will be used in the speed range of medium to high. Therefor e it will decrease the use of conventional brakes and thus will increase its life(a )since the heat during braking operation,(b) and the wear of the brake pads will be reduced. FigureFigure 12. 12.ObtainedObtained contours contours ofof the the magnetic magnetic ﬁeld: field: (a) two (a) electromagnets two electromagnets and (b) four and electromagnets. (b) four electromagnets.

It is important to note that the increase in the number of electromagnets should avoid any interaction risk of the magnetic field generated by each magnet. In addition, it is necessary to consider the space left for the installation of conventional braking systems. As shown in Figure 12, although using four electromagnets, the interaction of the magnetic field among the poles can be avoided. The interactions of the magnetic field can result in magnetic field leakage. The use of four electromagnets can generate a 48.8-Nm braking torque. In addition, reducing the vehicle’s speed from 60 to 20 km/h requires 52.1 Nm (a vehicle weight of 100 kg). The use of the ECB with this design can reduce 93.66% of the braking torque required by the braking system compared to without an ECB. By flowing an electric current of 30 A, setting the number of coil windings to 540, and using two electromagnets, a higher braking torque than is required can be generated. Figure 13. Braking distance of the conventional brakes and the conventional brakes with ECB. LookingFigure at 13.Figure Braking 14, itdistance is important of the conventional to note that brakes the ECB and does the conventional not replace brakes conventional with ECB. brakes. The6. ECB Conclusions is used as an additional part for the braking system of a lightweight vehicle. The braking 6. Conclusions distancesIn will this be work, relatively braking similar torque whether analysis an using ECB theis used multi-polar or not. It ECB is important was performed that the with ECB the will objective be usedof in developingIn the this speed work, arange compact braking of medium ECB torque system to analysishigh. and Therefor evaluating usinge theit severalwill multi-polar decrease parameters, theECB us includingewas of conventional performed the number with brakes ofthe coil andobjectivewindings, thus will of increasethedeveloping number its life of a electromagnets, compactsince the ECBheat duringsystem and the brakingan electricd evaluating operation, current. Numericalseveral and the parameters, wear calculation of the including wasbrake conducted pads the willnumberusing be reduced. 3Dof FEM.coil Anwindings, experimental the number study was of alsoelec conductedtromagnets, to and validate the theelectric developed current. model. Numerical From the calculationexperimental was test,conducted the higher using rotation 3D FEM. speed An tendsexperi tomental require study a higher was brakingalso conducted torque, although,to validate due the to developedthe skin e model.ffect, it slightlyFrom the decreases experimental when thetest, rotation the higher speed rotation is higher speed than tends the critical to require value. a higher The number of coil windings and the electric current also significantly influence the generated braking torque. A higher number of coil windings and an electric current generally increases the generated braking torque. In addition, the number of electromagnets also influences braking performance. By distributing the required braking torque into several braking units (electromagnets), the size of each braking unit can be kept compact, so it is applicable to any light vehicle, including motorcycles. As a further study, research related to efforts to clarify how the ECB can effectively and completely stop vehicles is required, in correlation with several parameters, to achieve the optimum design and performance of the ECB system.

Author Contributions: Conceptualization, M.R.A.P., M.A. and M.N.; methodology, M.R.A.P.; software, M.R.A.P.; validation, M.R.A.P.; formal analysis, M.R.A.P., M.N. and D.D.D.P.T.; investigation, M.R.A.P.; resources, M.R.A.P.; dataFigure curation, 13. M.R.A.P.;Braking distance writing—original of the conventional draft preparation, brakes and M.R.A.P., the conventional M.A. and brakes A.R.P.; with writing—review ECB. and editing, M.R.A.P., M.A. and M.N.; visualization, M.R.A.P.; supervision, M.N.; project administration, M.N.; 6. Conclusionsfunding acquisition, M.N. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. In this work, braking torque analysis using the multi-polar ECB was performed with the objectiveAcknowledgments: of developingThis a workcompact was fundedECB system by the Ministryand evaluating of Research, several Technology, parameters, and Higher including Education the of the Republic of Indonesia, through contract no. 208/SP2H/AMD/LT/DRPM/2020. This research was partially funded numberby the of Indonesian coil windings, Ministry the of Research, number Technology, of electromagnets, and Higher and Education, the electric under thecurrent. World ClassNumerical University calculationProgram, was managed conducted by the using Institute 3D of FEM. Technology An experi Bandung,mental Indonesia. study was also conducted to validate the developed model. From the experimental test, the higher rotation speed tends to require a higher Appl. Sci. 2020, 10, 4659 14 of 15

Conﬂicts of Interest: The authors declare that there is no conﬂict of interest.

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