Design and Analysis of a Novel Speed-Changing Wheel Hub with an Integrated Electric Motor for Electric Bicycles

Hindawi Publishing Corporation Mathematical Problems in Engineering Volume 2013, Article ID 369504, 8 pages http://dx.doi.org/10.1155/2013/369504 Research Article Design and Analysis of a Novel Speed-Changing Wheel Hub with an Integrated Electric Motor for Electric Bicycles Yi-Chang Wu and Zi-Heng Sun Department of Mechanical Engineering, National Yunlin University of Science & Technology, Yunlin 640, Taiwan Correspondence should be addressed to Yi-Chang Wu; Received 9 September 2013; Accepted 10 October 2013 Academic Editor: Teen-Hang Meen Copyright © 2013 Y.-C. Wu and Z.-H. Sun. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The aim of this paper is to present an innovative electromechanical device which integrates a brushless DC (BLDC) hub motor with a speed-changing wheel hub stored on the rear wheel of an electric bicycle. It combines a power source and a speed-changing mechanism to simultaneously provide functions of power generation and transmission for electric bicycles. As part of the proposed integrated device, the wheel hub consists of a basic planetary gear train providing three forward speeds including a low-speed gear, a direct drive, and a high-speed gear. Each gear is manually controlled by the shift control sleeve to selectively engage or disengage four pawl-and-ratchet clutches based on its clutching sequence table. The number of gear teeth of each gear element of the wheel hub is synthesized. The BLDC hub motor is an exterior-rotor-type permanent-magnet synchronous motor. Two-dimensional finiteelement analysis (FEA) software is employed to facilitate the motor design and performance analysis. An analysis of the power transmission path at each gear is provided to verify the validity of the proposed design. The results of this work are beneficial to the embodiment, design, and development of novel electromechanical devices for the power and transmission systems of electric bicycles. 1. Introduction An electric bicycle is a bicycle with an integrated electric motor which can be used for urban transportation, propulsion, and recreation. Except for those with direct-driven hub motors, electric bicycles are typically equipped with speed-changing devices to mechanically adjust the rotational speed of the rear wheel. Since the efficiency of an electric motor is related to its rotational speed, an electric bicycle further provides a mechanical speed-changing device for transmission enabling the electric motor to operate in its most efficient state and, hence, leads to longer periods of use before the battery needs to be charged. As can be seen in the existing products [1], the electric motor and the speedchanging device of an electric bicycle, which are, respectively, used for power generation and transmission, make up individual electrical and mechanical devices. The driving power generated by the electric motor is mostly transmitted from the front chain-wheel at the crankset to the rear sprocket installed on the rear wheel via a chain mechanism. The main drawback of such a conventional design is the lengthy power transmission path from the electric motor to the speedchanging device using a sprocket and chain mechanism, which may cause additional mechanical energy losses due to friction. A second drawback is the cumbersome workspace arrangement due to the individual design of the electric motor and the speed-changing device. Hence, the integration of the electric motor and the speed-changing device for electric bicycles is worth studying. Upon investigating existing electric bicycles, we found that they usually employ the rear derailleur system as the mechanical speed-changing device to provide a set of speed ratios. Unfortunately, the derailleur, the chain, and sprockets are all exposed to the elements. Rain and musk usually stick to the shifter and may damage the derailleur. Therefore, the derailleur mechanism needs to be regularly maintained. In contrast, the speed-changing wheel hub, which is implemented with a planetary gear train and a speed-changing control mechanism, is immune to contamination due to the protection of the hub shell. Such a kind of internal transmission hub for electric bicycles 2 has the unique advantages of compact size, good reliability, and high efficiency [2–4]. One special feature of the speedchanging wheel hub is that it can change gear ratios when the rear wheel is not rotating. This can be very useful for a commuter with frequent stop-and-go riding in urban areas. Because the speed-changing wheel hub generally has a long maintenance-free life, it is the subject of ongoing research by commercial organizations and academic institutions. As for the power source of electric bicycles, several types of electric motors, including brush DC motors, induction motors, reluctance motors, and brushless permanent-magnet motors, are employed in existing products for traction. Among these electric motors, brushless DC (BLDC) motors have attracted increasing interest due to the characteristics of high efficiency, low cost of maintenance, light weight, easy speed control, and low noise and vibration [5, 6]. Due to these reasons, the BLDC motor and the speed-changing wheel hub are designated as the objects of this study. In addition, the integration of the BLDC motor and the speed-changing wheel hub for electric bicycles may offer new opportunities to overcome the above shortcomings of existing products. The purpose of this paper is to develop a novel electromechanical device by combining an electric motor with a speed-changing wheel hub for electric bicycles to overcome the drawbacks of traditional designs. An integrated design that combines an exterior-rotor BLDC hub motor within a three-speed wheel hub resulting in a compact power generation and transmission device is introduced. The configuration, operational principles, and qualitative features of the proposed design are addressed. A clutching sequence table is synthesized to provide three forward speeds. The embodiment design of a speed-changing wheel hub, that comprises a basic planetary gear train and a speed-changing control mechanism, is presented. Besides, a 350 W, 3-phase, 12-pole/18-slot BLDC hub motor with an exterior-rotor configuration is designed as part of the integrated device, and the electromagnetic torque of this motor is calculated by finiteelement analysis (FEA). Finally, the power transmission path at each speed is illustrated to verify the feasibility of the integrated device. 2. A Novel Design Concept By integrating a 3-phase, 12-pole/18-slot exterior-rotor BLDC hub motor within a three-speed wheel hub, an electromechanical device with a compact structure is proposed. This integrated device is stored on the rear wheel of the electric bicycle. Figures 1(a), 1(b), and 1(c), respectively, show an e (...truncated)