Journal of World s Electrical Engineering and Technology J. World. Elect. Eng. Tech. 3(2): 67-73, 2014

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1 ORIGINAL ARTICLE Received 03 May Accepted 20 May , Scienceline Publication Journal of World s Electrical Enineerin and Technoloy J. World. Elect. En. Tech. 3(2): 67-73, 2014 JWEET Desin of Slotless BLDC Motor for Eliminatin Coin Torque Reza Ilka, Yousef Alinejad-Beromi, Hamid Yahobi, Hossein Asharpour-Alamdari Department of Electrical Enineerin, Semnan University, Semnan, Iran *Correspondin author's Ilka@sunliht.semnan.ac.ir Abstract Desin of a coin torque free motor is presented in this paper. Coin torque is a disturbin phenomenon in electrical machines that causes certain problems in performance of them. This unwanted component in output torque could be reduced or eliminated by some methods i.e. skewin the stator slot, skewin the permanent manets (PMs) of rotor, employin fractional pitch windin and buildin slotless motor. In this paper, firstly these ethods are discussed and then the effect of skewin PMs is investiated which is distinuished by a desin parameter i.e. offset. After that, slotless structure of a brushless DC motor (BLDC) is considered. In a slotless BLDC motor, coin torque would be eliminated and there is no major component of coin torque in slotless structure of a BLDC motor. These types of motors are very convenient due to excellent features such as zero-coin torque, minimal vibration, hih speed capability, excellent power-to-weiht ratio, compact desin, liht weiht and etc. At the end, finite element analysis (FEA) is employed to validate the topics. Keywords: Slotless, Brushless DC Motor, Coin Torque, Skewin, Fractional Pitch Windin, Finite Element Analysis, Ansys Maxwell INTRODUCTION One of the main problems of electrical motors is coin torque. This unwanted and disturbin torque reduces the nominal performance of electrical motors. Hence, researchers attention has paid to overcome this problem. Coin torque is caused by an uneven air-ap permeance resultin in the manets constantly seekin a position of minimum reluctance. Several methods have been proposed to reduce coin torque includin skewin the stator slot, skewin the permanent manets (PMs) of rotor, employin fractional pitch windin and buildin slotless motor [1-5]. Emerence of slotless motors leads to coin torque free motors. Aim of this paper is to desin a BLDC motor. These types of motors are becomin increasinly prominent because of their ood merits over conventional motors. Main advantaes of slotless brushless DC motors can be listed as: Zero-coin torque for smooth operation, minimal vibration, hih speed capability of up to 100,000 RPM, excellent power-to-weiht ratio enablin the desin of compact, liht-weiht devices, smooth hih speed operation and lower audible noise [6-10]. This paper first discusses methods for coin torque reduction. After that, equations and structure description of slotless BLDC motor are presented. Then, characteristics of motor are investiated usin finite element analysis (FEA). Main output performance characteristics of a slotless BLDC motor are discussed and compared with a slotted BLDC motors. COGGING TORQUE Coin torque is an unwanted phenomenon in electrical machines which is produced as a result of reluctance variations between stator tooth and rotor manetic poles. In other words, this torque is occurred because rotor tends to alin with stator in some specific directions. Simply, Coin torque is the torque created when the rotor permanent manets attempt to alin themselves with a maximum amount of ferromanetic material. Coin torque is a pulsatin torque with zero averae value [1-5]. Coin torque describes the interaction of the rotor manets actin on the stator teeth or poles independent of any current. While this torque is often considered beneficial in step motors, it is considered detrimental in brushless permanent manet motors [1]. Coin torque produces noise and destructible pulsations in electrical machines and in some cases mechanical resonance would happen that leads to serious problems. However, certain amount of this torque is required in some specific industrial applications. Therefore, coin torque is an important characteristic of machine which should be considered with a lot of care in desin process. Peak value for coin torque is determined by some parameters like slot width to slot pitch ratio, permanent manet enery and air ap lenth. Coin torque characteristic could be corrected by chanin the pole arc to pole pitch ratio. In practice, this disturbin torque is minimized by accurate selection of abovementioned parameters. To cite this paper: Ilka R, Alinejad-Beromi Y, Yahobi H, Asharpour-Alamdari H Desin of Slotless BLDC Motor for Eliminatin Coin Torque. J World Elec. En. Tech., 3 (2):

2 Ilka et al., 2014 Coin torque can be represented by 1 2 dr Tco (1) 2 d where ϕ is the air-ap flux by PMs, R is the air-ap reluctance, and θ is the position of the rotor [1]. Coin torque has a periodic nature because air-ap reluctance varies periodically. In addition, coin torque is independent of flux direction because the manet flux ϕ is squared. A. Coin Torque Reduction/Elimination There are some methods for reduction/elimination of coin torque based on applications and considerin technical and economic aspects: Skewin the stator slots Skewin the permanent manets of rotor or optimizin the manet pole arc Employin fractional pitch windin Buildin slotless motor Amonst abovementioned methods, the two earlier are more common. Besides, there are some new methods that could reduce coin torque [5]. For further clarification, a brief description is presented about these methods [1]. The first method is to skew the stator slots. The net chane in reluctance can be minimized, despite the slot openins, if the slot openins are spread out over the surface area of the manet as depicted in Fiure 1. In Fiure 1, the slots are skewed so that each manet sees a net reluctance that stays the same or nearly the same as slots pass by. In this way, chanes alon the axial dimension are used to diminish the effect of chanes alon the circumferential dimension. As a result, the dr/dθ experienced by the entire manet decreases and the coin torque decreases [1]. The third method is associated with employin fractional pitch windin. Since each manet produces coin torque as it passes by stator slots, the relationship between the number of manet poles and the number of stator slots influences coin torque. In interal slot motors, each manet appears in the same position relative to the stator slots. As a result, the coin torque created by all manets are in phase with each other, and the net coin torque is equal to the product of the number of manet poles and the coin torque created by one manet. That is, the coin torque from each manet simply adds to create the net result. On the other hand, in fractional slot motors, each manet appears in a different position relative to the stator slots. As a result, the coin torques created by all manets are out of phase with each other, and the net coin torque is reduced since the coin torque from each manet adds toether and at least partially cancels the coin torque from other manets. This fact is one of the primary reasons for choosin a fractional slot motor [1]. The last way to minimize coin torque is buildin up a slotless BLDC motor which is discussed in the next section. SLOTLESS BLDC MOTOR Slotless BLDC motors are developed for eliminatin coin torque in conventional slotted BLDC motors (Fiure 2). Reardin Fiure 2, as each manet in the motor rotates past the stator teeth, the reluctance experienced by the manet under the slot openins chanes because of the loner flux path lenth into the slots terminatin on the shoes. Therefore, the slot openins create a varyin reluctance for the manet flux, thereby creatin coin torque. If the stator teeth did not have shoes, the reluctance variation and resultin coin torque would be much reater. Thus, the primary purpose for shoes is coin torque reduction [1]. Shoe desin represents a fundamental tradeoff. The narrower the slot openin, the smaller the coin torque becomes. In the limitin case, if there was no slot openins, coin torque would be zero. Therefore, slotless BLDC motors are devised for applications where zero coin torque is the major concern. Fi. 1. Skewed stator slots [1] The second method is related to permanent manets on rotor. It is a well-established fact that the manet skewin and its pole arc can have a lare effect on the manitude of the coin torque. By doin this, the reluctance seen by manets will chane and coin torque will reduce accordin to well adjustin PMs shape and position. Fi. 2. Structure of conventional slotted BLDC motor [1] 68

3 J. World. Elect. En. Tech., 3(2) 67-73, 2014 Fiure 3 shows a diaram of slotless BLDC motor. Separated permanent manets are united to rotor core and distributed windins are fixed on the stator core. Flux path and manetic circuit of slotless BLDC motor is shown in Fiure 4. From eometric and material parameters of model, air ap manetic flux is calculated. In manetic circuit model takin into account one pole pair in Fiure 4, the manetic flux Φ can be expressed as 2Rm 1 r r (2) 2R m 2K R r R 1 K r R m where R m and R are the manet and air ap reluctance, respectively, Φ r is the flux source, and K r is the reluctance factor which increases air-ap reluctance slihtly to compensate for the missin steel reluctance [7]. The air ap flux can be written as K l K l r (3) ram 1 K r l A m where Kl 1.5p (4) D w is the flux leakae factor considerin manet flux without passin into the stator core, and p, β, D w are pole pair number, polar arc to polar pitch ratio, inner diameter of windin part, respectively [7, 8]. Fiure 5 shows a simplified model of manetic circuit for slotless BLDC motor. Fi. 3. Structure of slotless BLDC motor Fi. 4. Flux path and manetic circuit of slotless BLDC motor Fi. 5. Simplified manetic circuit model for slotless BLDC motor RESULTS AND DISCUSSION In this section, finite element analysis (FEA) is occupied to investiate the discussed issues. Ansys Maxwell is used for this purpose. Firstly, a brief description is presented about the FEA and Ansys Maxwell. Then, the effect of skewin the PMs is analyzed and after that slotless structure for BLDC motor is considered. A. FEA Validation The FEA found its way into electrical enineerin almost 30 years ao. The advantae of numerical methods like the FEA is to analyze the arbitrary shapes, arbitrary boundary conditions and complicated or distributed sources. First implementation of FEA was in the analysis of DC motors, SR motors and etc. In this method, the space of interest is divided into small (but finite) reions called elements, which completely cover the space but does not overlap. Additionally it is required that an individual element does not cross a material boundary. Then it is assumed that over this small reion, the unknown quantity can be described by a simple function, polynomials are usually chosen because the formulation involves differentiatin and interatin these functions. The order of polynomial determines the order of the element. Maxwell software which is based on FEA is one of the most important and efficient tools for FEA validation. Desirable output quantities can be extracted usin this software by doin below steps: Drawin the motor Assinin materials and boundary to the motor parts Performin mesh operation Settin up an analysis to solve Extractin output data and plots. For this motor, stator and rotor core are composed of steel Surface-mounted permanent manets are NdFeB with: B r : 1.23 T, H c : 890 ka/m. 69

4 CoinTorque [mnewtonmeter] CoinTorque [mnewtonmeter] CoinTorque [mnewtonmeter] Ilka et al., 2014 Simulation is carried out for 70 msec. B. Skewin the PMs Coin torque is reduced by skewin the edes of PMs. This is distinuished in the Maxwell software by a desin parameter named offset. Offset determines the deree of skewin for PMs. Fiure 6 shows BLDC motor with surface-mounted PMs. As shown in Fiures 7 to 9, coin torque is reduced by increasin the offset. However it has to be considered that when offset is increased, air ap flux density is reduced and as a consequence, output power of motor is decreased. Therefore there should be a trade-off between these two parameters. Table 1 compares the coin torque of three cases with different offset. As shown in Table I, coin torque is reduced about three times by skewin the PMs. Fi. 6. Surface-mounted PMs XY Plot 26 RMxprtDesin1 CoinTorque Setup1 : Performance ElectricalDeree [de] Fi. 7. Coin torque diaram (Offset=0) 50 XY Plot 27 RMxprtDesin1 CoinTorque Setup1 : Performance ElectricalDeree [de] Fi. 8. Coin torque diaram (Offset=2) XY Plot 31 RMxprtDesin1 CoinTorque Setup1 : Performance ElectricalDeree [de] Fi. 9. Coin torque diaram (Offset=4) 70

5 J. World. Elect. En. Tech., 3(2) 67-73, 2014 TABLE 1 Comparison of coin torque for different offset Offset (mm) Coin Torque (m N.m) C. Buildin Up an Slotless BLDC Motor In this section, FEA is employed to investiate the effect of slotless desin in coin torque. For this purpose, a comparison is carried out between two types of BLDC motors i.e. conventional slotted BLDC and slotless BLDC motor. Nominal parameters and dimensions of the two motors are identical except the fact that in slotless desin, there is no slot. Main characteristics of the desined motor are listed in Table 2. Fi. 10. Mesh produced by FEA TABLE II Main characteristics of motor Parameter Value Power (kw) 1 Rated voltae (V) 220 B ys, B yr (T) 1.5 B t (T) 1.5 B r (T) 1.2 Number of slots (for slotted motor) 30 Mesh diaram produced by finite element analysis is shown in Fiure 10. Flux lines and flux density of the proposed slotless BLDC motor are shown in Fiures 11 and 12, respectively. As shown in these fiures, first assumptions for motor are fulfilled. Fiure 13 shows coin torque for slotted BLDC motor while no coin torque is observed for slotless BLDC motor. It is obvious that coin torque for slotted motor is N.m while in slotless BLDC motor coin torque is equal to zero. Fiures 14 and 15 illustrate the output torque of motor for slotted and slotless BLDC motors, respectively. Averae torque of slotted BLDC confiuration is N.m and for slotless confiuration is equal to N.m. Table III compares output and coin torque of the two mentioned confiurations. Averae torque of sloless confiuration is about 11 % less than the averae torque of slotted one because of the structure difference. Therefore, it can be said that slotless confiuration is a ood candidate for precise applications which demand ripple-free torque. Fi. 11. Flux lines distribution Fi. 12. Flux density of motor 71

6 Movin1.Torque [NewtonMeter] Movin1.Torque [NewtonMeter] Torque (N.m) Ilka et al., Coin Torque Air-Gap Position (elec. derees) Fi. 13. Coin torque of slotted BLDC motor Torque Maxwell2DDesin1 Movin1.Torque Setup1 : Transient av Time [ms] Fi. 14. Output torque of slotted BLDC motor Torque Maxwell2DDesin2 Movin1.Torque Setup1 : Transient av Time [ms] Fi. 15. Output torque of slotless BLDC motor 72

7 J. World. Elect. En. Tech., 3(2) 67-73, 2014 TABLE III Comparison of BLDC motor confiurations Averae Coin Confiuration Torque Torque (N.m) (N.m) Slotted BLDC motor Slotless BLDC motor CONCLUSION In this paper, methods for reducin coin torque were investiated and after that a coin torque free motor i.e. slotless BLDC motor was presented. There are some methods for coin torque reduction/elimination such as skewin the stator slot, skewin the permanent manets (PMs) or optimizin manet pole arc of rotor, employin fractional pitch windin and buildin up a slotless motor. At first, these methods were discussed and slotless confiuration of BLDC motor was surveyed. After that, the effect of skewin PMs was expressed by a desin parameter i.e. offset. This parameter determines the deree of skewin for PMs. It was shown that increasin offset leads to coin torque reduction. Then, slotless structure of a brushless DC motor (BLDC) was considered. Theoretically, a slotless BLDC motor has no major component of coin torque. FEA results showed that coin torque reduces to zero. Besides, while averae torque of slotless motor has less manitude in comparison with slotted confiuration, but output torque of slotless confiuration has less variation due to zero coin torque. Future works may be devoted to optimal desin of manet pole arc and usin fractional pitch windin in BLDC motors in order to reduce coin torque. [7] J.M. Seo, J.H. Kim, I.S. Jun, and H.K. Jun, Desin and Analysis of Slotless Brushless DC Motor, IEEE Trans. Industry Applications, VOL. 47, NO. 2, March/April [8] J. M. Seo, Y. K. Kim, S. H. Rhyu, I. S. Jun, H. K. Jun, A Desin of Slotless BLDC Motor for Robot Usin Equivalent Manetic Circuit Model, The 8th International Conference on Ubiquitous Robots and Ambient Intellience (URAI 2011) Nov , 2011 in Sondo ConventiA, Incheon, Korea. [9] S.M. Jan, S.S. Jeon, D.W. Ryu, and S.K. Cho, Desin and Analysis of Hih Speed Slotless PM Machine with Halbach Array, IEEE Trans. Manetics, VOL. 37, NO. 4, July [10] R. P. Praveen, M. H. Ravichandran, V. T. Sadasivan Achari, V. P. Jaathy Raj, G. Madhu, and G. R. Bindu, A Novel Slotless Halbach-Array Permanent- Manet Brushless DC Motor for Spacecraft Applications, IEEE Trans. Industrial Electronics, VOL. 59, NO. 9, September REFERENCES [1] D. C. Hanselman, Brushless permanent manet motor desin, Mana Physics Publishin, 2nd edition, [2] J. F. Gieras, M. Win, Permanent Manet Motor Technoloy: Desin and Applications, CRC Press, 2 nd edition, [3] H. A. Toliyat, G. B. Kilman, Handbook of Electric Motors, CRC Press, 2 nd Edition, [4] J. Pyrhonen, T. Jokinen and V. Hrabovcova, Desin of Rotatin Electrical Machines, John Wiley & Sons, [5] Luke Dosiek, Praasen Pillay, Coin Torque Reduction in Permanent Manet Machines, IEEE Trans. Industry Applications, VOL. 43, NO. 6, November/December [6] A. Rahideh, T. Korakianitis, P. Ruiz, T. Keeble, M.T. Rothman, Optimal brushless DC motor desin usin enetic alorithms, Journal of Manetism and Manetic Materials, 2010, pp