1. Introduction
A motorcycle is the most popular vehicle in Thailand and ASEAN countries. In Thailand, more than 20 million units, or 50% of all on-road vehicles, registered to the Department of Transportation are motorcycles. Furthermore, there has been a rapidly increasing trend in the number of motorcycles during the pandemic for delivery services and urban commuting, which has caused more serious air pollution and health problems. In order to tackle this problem, converting existing old motorcycles or using new electric motorcycles is considered to be an interesting policy of government agencies [
1]. Recently, some electric motorcycles and small electric scooters have been exported. The propulsion systems of those vehicles mainly use the motors that contain permanent magnets inside, which causes some difficulties for local mass production and uncontrollable price fluctuation. From the literature [
2,
3], the switched reluctance motor (SRM) is one type of motor that does not contain any rare earth materials or permanent magnets inside. It presents many advantages, opportunities, and challenges over the other type of electric motors for electric vehicle applications. Various structures and winding configurations of the SRM as well as various driving topologies have been reported [
4]. It has been widely studied and installed for traction drives in various hybrid [
5,
6,
7] and electric vehicles, including bicycles [
8], forklifts [
9], and light electric vehicles [
10]. The design, production, and verification of a complete running prototype based on SRM wheel hub drive train has been reported [
11]. A control method in the drive systems of electric motorcycles using SRMs [
12] has also been previously reported. Some previous publications focused only on the performance improvement of the SRM drive system by optimizing the turn on and turn off angle at each rotating speed and vehicle load torque [
13], or by simultaneously adjusting the geometry and commutation angles [
14].
The SRM has a special structure and several advantages, such as rugged and low-cost construction, a simple stator and rotor structure, an easy cooling system, high reliability, and good performance over a wide speed range. These characteristics make it a low-cost and rugged alterative for industry applications, a worthy competitor to other drive systems, and suitable for various applications. Additionally, its performance can be easily adjusted for various load profiles. Thus, it is also one of the most interesting applications for electric and hybrid vehicle drive systems. Unlike permanent magnet machines, the SRM is easy to manufacture with a cheaper cost, since it does not need a permanent magnet. Moreover, the SRM is very suitable for harsh and high-temperature environments. For the sake of safety in a traction drive system [
2], very high torque in the low-speed region is required. Furthermore, for the wide constant power region during high vehicle speeds, the power train must be mostly operated in the constant power region. It is reported in [
3] that the three-phase 6/4 pole SRM has an overload capacity and constant power region with a longer range, which is suitable for the traction drive application. However, it has a bigger torque ripple and some vibration. Therefore, some papers overlook this problem by using a four-phase motor [
7] or increasing the number of stator and rotor poles [
5], such as 8/6, 12/8, or 6/10 pole [
8] SRMs. However, the increasing number of components for increasing phases also causes the drive circuit to be more expensive and have more complexity.
Until now, there has been a variety of literature on the design of SRMs. The design in each paper focuses on different points of view. In the case of electric motorcycle applications, it is very challenging to design a drive system and a high torque density motor (per volume or weight) with limited space, low cost, and high reliability in order to meet high performance and high efficiency requirements and to make it acceptable for many customers. In this paper, the first task to be accomplished is to design a motor to satisfy the performance specifications and requirements of the vehicle. A high starting torque motor and drive system was designed in order to reduce the acceleration and deceleration time to an acceptable value. The motor design mainly focuses on the starting torque, whereas the other criteria, such as the good performance and high efficiency of the motor for longer driving distances, are also taken into consideration. The geographical dimensions for the design guidelines of the stator, rotor core, and winding parameter for the four-phase 8/6 pole and three-phase 6/4 pole SRM were considered. The comparison of both types of SRMs was investigated by the simulation results using a static performance analysis tool. The dynamic performance analysis of the four-phase 8/6 pole and three-phase 6/4 SRMs, including the control methodology in the drive system, were also investigated using the dynamic simulation tool. Furthermore, the developed torque and flux density of different winding configurations for the three-phase 6/4 SRM were also investigated. In order to implement a low-cost and high-reliability drive system, an indirect toque control method for optimal torque and efficiency operation and higher accuracy of rotor position detection based on a combination of a magnetic sensing circuit and an initial rotor position estimation method are proposed. The construction and the measured performance on the test bench of the selected three-phase 6/4 pole SRM and drive system are described. Finally, the measured results for the vehicle performance on a dynamometer and during on-road testing are also considered.
3. Construction of the Motor and Drive System
Figure 13 shows the constructed three-phase 6/4 pole SRM prototype and drive system. In total, 30 units of the three-phase 6/4 pole SRM were manufactured by the private company in order to control the quality of the prototypes.
The motor drive system employs the conventional three-phase, asymmetric half-bridge inverter. This topology has the advantages of simplicity, robustness, and fault tolerance, where each phase can be controlled independently with unidirectional current flow. These greatly enhance the safety features of the electric motorcycle drive system. The control block diagram for the SRM drive system is shown in
Figure 14, where the user turns the accelerator to define the torque command as an input. The motor torque is estimated based on the torque to current block. The optimal turn-on angle, turn-off angle, and current command for maximum efficiency operation (optimal efficiency operation) at each torque and speed level will be calculated off-line by FEA and put into the 3D look-up table, as shown in
Figure 15. At each torque command and detected motor speed, the corresponding current command and on/off angles will be calculated by the following Equation.
where
and
are the torque command and detected motor speed, respectively.
The rotor position is also a necessary parameter for the indirect torque controller and current controller in the drive system. For the electric motorcycle application, especially when the vehicle with maximum payload starts from the standstill condition on the slope, the initial rotor position is very necessary for high starting torque development. Some low-cost, incremental optical encoders may be used for rotor position detection. However, they cannot detect the initial rotor position at a standstill condition, leading to torque jerk [
20], which seriously affects the safety of the electric motorcycle. Furthermore, due to the harsh environment on the road, with dust, water, and vibration, a more reliable detection system for a non-contact rotor position is required. In this paper, the contactless magnetic-based position sensor was installed on the rotor shaft for position and speed feedback, as shown in
Figure 16. It is a very low-cost and high-reliability solution. With the proposed mechanical coupling device using a bolted shaft and ball bearing, the effect of the deviated distance between the permanent magnet and sensing circuit caused by speed variation and vibration could be eliminated effectively. A resolution of the rotor position detection of about 0.87 degrees could be achieved from 0 to 8000 rpm. The proposed initial rotor position estimation method [
20] based on pulse injection and phase current profile has a maximum error of about 2.5 degrees, but it could identify the sector of the rotor position. Therefore, the combination of the proposed rotor position estimation and magnetic sensing system could increase the reliability of the electric motorcycle.
The motor phase current is sensed by current sensor model ACS758ECB-200U which can be mounted on a PCB. The current regulation is also performed by an inner digital hysteresis controller. A 32-bit micro-controller, STM32F103RET6, and a power MOSFET, IRF4468, were used.
Figure 17 shows the battery pack and circuit board in the battery management system. In one battery pack, a total of eight cells of LiFePO4 model NLC36130185PF (3.2 V 63 Ah) were connected in a series and two sets of battery packs were installed in each electric motorcycle. The voltage of each cell is monitored by the cell monitoring board and it is transmitted to the master board. The master board will perform passive balancing by bypassing the current to the cell that has the lower capacity and it will make a decision for communication to the quick charger and interlock circuit for safe operation.