A High Torque Segmented Outer Rotor Permanent Magnet Flux Switching Motor for Motorcycle Propulsion

Electric scooters also known as electric motorcycle are viable and personal means of road transportation have been making their ways into the world markets now because in them, combustion engine with the use of fuel oil for propulsion have been completely eliminated for economic and environmental imperatives. Electric motor which converts electrical energy into mechanical energy is used to overcome the complication of combustion engine. As it is, everyone is opting for combustion engine free and fuel-less type of vehicle. For this reason, manufacturers have exhibited interest, making research on electric motor very attractive. Meanwhile, surface permanent magnet synchronous motor (SPMSM) has been successfully developed having output torque of 110 Nm, the assembly of motor lacked mechanical strength between the rotor yoke and the mounted permanent magnet (PM) which heats up during speed operation, resulting to poor performance. To overcome the challenges laced with SPMSM, this paper presents a novel design of 24 stator 14 pole outer rotor-permanent magnet flux switching motor (SORPMFSM) capable of high torque and high performance. It employs an unconventional segmented rotor which has short flux path flow. It also embraces alternate stator tooth windings to reduce material cost. Design specifications and restriction with input DC current are the same with SPMSM. The 2D-FEA by JMAG, version 14 is used to examine the performance of the proposed motor in terms of cogging torque, back-emf, average torque, power and efficiency. Preliminary results showed that torque, power output and efficiency of the proposed motor are 1.9Nm times, 5.8kW times more than SPMSM and efficiency of 84% thus, can sustain acceleration for long distance travel.


Introduction
For more than a century now, combustion engine (CE) vehicles which include conventional motorcycles or scooters have used fuel oil to create energy for propulsion for road transportation [1][2]. As it is, cost of maintaining these complex engines with the gloomy fuel crises has been a major concern [3][4]. To reduce fuel consumption in conventional scooters, hybrid electric scooters (HES), also known as hybrid electric scooters have combined combustion engine and electric motor in inner rotor structure, to greatly minimize the intake of oil [5][6].
The four electric motor candidates viable for HES are permanent magnet direct current motor (PMDC), induction motor (IM), switched reluctance motor (SRM) and permanent magnet synchronous motor (PMSM) respectively. Meanwhile, there are different types of HES with various speed operating ranges from 40 to 65 kmph [7]. Obviously, these speed ranges are not suitable for long distance travels.
PMDC was used because of the advantages of simple control which was developed with the tendency of high torque but due to the problems of commutator and regular maintenance of brushes renders it not reliable for driving operation [8][9]. Induction motor has been employed in electric scooter propulsion due to its robustness, low maintenance and ability to operate in unfriendly environments. Unfortunately, it has fundamental limitation which lies on low efficiency at low speed-light load region, low starting torque and speed which cannot be easily changed [10].
Obviously, SRM does not contain permanent magnets, the operation of stators is the same as brushless DC motors. SRM is reliable, rugged and low cost with advantage of low maintenance [11][12]. SRMs have lower torque capability and are generally noisy. They are suitable for low cost, medium to high speed applications where controllability and noise/shaft torque ripple are not critical. The torque is produced by the predisposition of the rotor to adjust with excited stator pole. The path of torque created is an element of the rotor position with respect to the energized phase, and is independent of the current flow direction through the phase winding. Constant torque can be produced by aligning stator pole and phase's excitation with the rotor rotor position [13].
For PMSM, especially IPMSM, has the advantage of high torque density and power density, its design tends is very difficult as permanent magnet (PM) is inserted inside the rotor with assembly lacking mechanical strength thus, degrading the performance of motor for driving system [14][15].
While each of these electric motors has assisted in reducing fuel oil intake, emission of hazardous gas remains worrisome [16][17]. As a viable solution for complete elimination of the complex combustion engine with fuel oil together, high torque electric motor is crucially necessary due to economic imperatives. Meanwhile, surface mounted permanent magnet synchronous motor (SPMSM) [18][19] has been developed that has completely eliminated CE and the use of fuel oil from HES, thus pure electric scooter vehicle (ESV) in the following scooters: Kingday electric scooters, Aima electric scooters, Mobility electric scooter and Terra motors with operating ranges between 35-60 km/hr and power output of 3kW-5kW respectively. However, the torque capability of the motor operates at low speed range thus, the continuous search for a high torque motors for high performance suitable for long distance travels. Figure 1 illustrates the growth demand of ESVs from 2014 -2024 [20].
For this reason, research for a new motor with high torque capability is of importance. Meanwhile, surface mounted permanent magnet synchronous have been successfully developed with torque output of 110 Nm and speed range of 100km/hr with 6kW [21]. It consisted of 24 stator slots with single tooth armature windings but embracing surface mounted permanent magnet rotor. While SPMSM has exhibited longer speed range of travel, motor's torque of 110 Nm cannot sustain acceleration for long distance travels as presented in figure 2.
Meanwhile, the development of flux switching motor (FSM) [22][23], has turned around motor performance. FSM is a combination of the structures of SRM and induction machine (IM) but locates all active parts on the stationary stator only. It encompasses a new class of brushless motor that does not locate any active part on the rotating rotor, thus immaterial rotor. Obviously, FSM is the electric motor for high torque, high speed, high performance, constant power, simple cooling option, light-weight, high efficiency and cost competiveness.
In the main time, FSM has been applied in an aerospace, domestic appliances and automotive [24][25]. Furthermore, FSM is categorized into three sources of flux excitations namely, permanent magnet FSM, field excitation FSM and hybrid excitation FSM respectively.
To overcome the problems of SPMSM and its limitations, segmented outer rotor permanent magnet flux switching motor (SOR-PMFSM) for high torque electric scooter propulsion is presented.

Research method
The design restriction and parameter specifications of the proposed motor for electric scooter propulsion are the same with SPMSM as listed in Table 1. In the same vein, the electrical restrictions related with the inverter are set the same. Furthermore, the limit of the current density is maximum of 30 A/mm2 for the armature coil. The outer radius of motor, stack length, and shaft radius of the target motor are the same with PMSM.
The motor design employed the commercial 2D-FEA package, JMAG-Software version 14.1 released by Japan Research Institute (JNI). At first, the JMAG Geometry Editor to draw the rotor, stator, armature coil and permanent magnet respectively. Flow chart for design process is shown in figure 3.
At the completion of the entire drawing, the motor is uploaded on the JMAG-Designer where materials, conditions, circuits and properties are set. The PM material used is NEOMAX-35AH having the residual flux density and coercive force at 20ᴼC are 1.2T and 932kA/m, the electrical steel 35H210 is used for rotor and stator parts. Additionally, the external rotor shaft is a non-ferromagnetic material of aluminium employed to secure the isolated segmented rotor members. Coil arrangement test was carried to affirm the operating principle of SOR-PMFSM. Furthermore, flux lines, flux distribution, cogging torque, induced back-emf voltage and finally, torque characteristics and efficiency were all examined.

Design motor parameters/rotor segment
Each of the motor components such as rotor thus dimensioned as D1-D3, stator, PM D4-D5 and armature coil D6-D7 respectively were sketched in the JMAG Geometry Editor followed the process which was arranged as region mirror and region redial respectively as shown in figure 4. Stator topology with 12 slots limited to 7, 8 and 10 segmented rotor pole and 24 stator slots limited to 10, 14 and 22 segmented rotor pole respectively were investigated for three-phase to embrace two or four set of armature windings for the purpose of selecting the topology with optimal torque capability.
Moreover, the choice of rotor segment is significant to allow an appropriate permanence for the flow of flux. Already, some segmented rotor geometries have been considered as illustrated in figure 5. The shape in (a) shows an earliest circumferential extending pole applied in inner rotor synchronous reluctance machine [26]. Though circumferential extending pole is ideal in terms of performance in inner rotor configuration, it needs notable modifications for in-wheel/outer rotor application. However, to retain these segments for speed operation, design provision is necessary so that nonferromagnetic material is used to secure them without adding excessive load that will affect motor's performance. The modification led to dovetail structure in part (b) and the transerving design -  [27]. While the dovetail segment is applied in the FSM as compromise, it lacks the mechanical strength to be used for speed application. In order to overcome it, this proposed segment geometry provides a feature so that the segmented rotors will be properly secured using an external stator. Therefore, achieving this geometry follows the relationship that exists between length of an arc and segment of a circle using the convention given in Eqn. (1). Segment span, θ is calculated as: (mm) radius rotor inner x π

Results and discussions
This section explained the results of research and at the same time, given the comprehensive discussion. Results are presented in figures, graphs that make the reader to follow through the process. The discussions are made in the following sub-topics.

Coil test and operating principle
Coil test was conducted on the proposed motor to properly set the correct position of each armature coil phase so as to confirm the operating principle of SOR-PMFSM. To accomplish this, a coil arrangement test examination was conducted on each armature coil separately to witness whether they were wound in the same direction or in opposite direction so that each coil phase would be correctly set in the same way. After this, the zero rotor position was clarified. By comparing the flux linkages of different coils, the armature coil phases were defined according to conventional three-phase system. Figure 7

Cogging torque
Cogging torque is the undesirable component that results to jerking of motor when value is high. For the proposed designs, the cogging torques with the PM flux source in open circuit are shown in figure 8. Taking a cursory look them all, 12S-10P has the highest peak-to-peak cogging torque with 48 Nm and followed by 12S-8P with 24 Nm. Other topologies have low cogging torque but 24S-14P has favorable sinusoidal waveform which is considered more acceptable.

Induced back-emf
The motors induced emf presents the voltage generated due to PM only at the speed of 1900 rev/min are depicted in figure 9. From the plot, all the motors have induced back-emf which appeared distorted. Meanwhile, 24S-10P has the highest induced emf of 148V followed by 24S-14P of 110V. While 12S-7P has emf amplitude of 48V, 12S-8P and 24S-22P could not achieve sinusoidal waveform.

Load analysis
To establish the best configuration, load analysis is conducted to examine torque performances of all the motors. At first, the maximum armature current density of 30 Arms/mm 2 is injected into the circuit and the torque is investigated and analyzed. The results obtained for torque at various armature current density are plotted in figure 10. It is obvious that torque production of 24S-14P is the highest with 209 Nm at the maximum   Table 2.

Torque and power versus speed characteristics
24S-14P topology of SOR-PMFSM was selected to examine the torque and power versus speed characteristics for securing the highest torque among the motors. Figure 11 shows the torque and power versus speed characteristics of the motor in which the power has increased from to almost 30kW and remained constant throughout the speed operation at the base speed of 1374 rev/min.
As shown in the plot, electric motors generate maximum torque at zero speed to pull away from a stop, for it uses a magnetic field to rotate. If applied a set current, a set torque is generated, thus maximum torque at zero speed. Furthermore, the power output is seen to be constant throughout the operating speed region.

Losses analysis and efficiency of 24S-14P
Motor losses such as iron loss in all the laminated cores and copper loss in armature coil and the efficiency of 24S-14P have been conducted and calculated on the basis of 2D-FEA. The specific operating points such as the maximum torque and highest speed at the operating points are chosen for motor's losses and efficiency analysis. The points are outlined in Figure 12. While points 1 and 2 are selected at maximum torque and highest speed, other operating points of 3 to 8 are selected at the speeds of 1000 rev/min, 2000 rev/min and 3000 rev/min respectively.    high performance. This result is expected to improve during design optimization which will be reported soon. Therefore, SOR-PMFSM, serves a better motor for high torque and high performance for electric scooter propulsion.