Enhanced Torque Motor for Electric Vehicles
Field of invention: The present invention relates to the electric vehicles. Now a days, electric vehicles are becoming very popular and their proportion in the commercial market is getting increased day by day for the past few decades as it helps in reducing the air pollution in the cities. However, there is a need to increase the efficiency of such electric vehicles which further requires optimization of various components thereby increasing the efficiency of the system and increase in the robustness of the whole system. Therefore, the present invention discloses a novel enhanced torque In-wheel Switched Reluctance Motor, hereinafter referred to as SRM, with divided teeth for electric vehicles.
Motors are the major part of EVs to convert electrical energy to mechanical energy. Therefore, a high-performance motor is needed. Switched reluctance motors (SRMs) have been applied to EVs due to their features of low cost, fault tolerance, wide speed range, simple and robust structure [2]. The in-wheel version of SRMs, which is an inner stator and outer rotor structure, has been utilized for direct drive to eliminate the transmission loss.
However, because of high magnetic saturation in the stator and rotor steel, the flux characteristics are nonlinear, causing lower efficiency and worse torque performance for the machine and making an SRM a less favored candidate than a permanent magnet synchronous motor [4]. Several SRMs utilize multi-teeth per stator pole to enhance torque characteristic [5]. But there is high magnetic saturation in the stator pole because of doubled overlapping arc angle, causing large torque ripple, small slot area for windings and increased difficulty in control, thus weakening motor performance [6]. Another effective way to enhance output torque is to increase the number of rotor poles to obtain the increased slot area for more number of turns in each phase [7]. Nevertheless, it is at the expense of increasing copper weight and cost.
In this paper, an SRM incorporating a new design formula has been proposed. The topology contains two features: two teeth per stator pole and rotor poles more than stator teeth. A comprehensive analysis of magnetic flux distribution and energy conversion is conducted firstly. After that, the number of rotor poles is defined based on the new design strategy. According to the principle of energy conversion and design formula, a 6/16 SRM is proposed. The geometric parameters of the proposed topology are optimized by genetic algorithm (GA) method prior to characteristics analysis. Then, the static and dynamic torque performances are calculated and analyzed by using finite element method (FEM). The results are compared with three phase SRMs with more rotor poles of the same size whose parameters are also optimized.

Brief Description of the accompanying drawings:
Figure 1(a) represents the flux distribution in the novel enhanced torque In-wheel switched reluctance motor with divided teeth for electric vehicles
Figure 1(b) represents the flux distribution in the conventional motors.
Figure 2(a) represents the output for the SRM with different teeth per stator pole.
Figure 2(b) represents the core loss characteristics for SRMs with different teeth per stator pole.
Figure 3(a) represents the topology of the novel SRM.
Figure 3(b) represents the half cross section of the novel 6/8 SRM.
Figure 3(c) represents the half cross section of novel 6/10 SRM.
Figure 4 represents the fitness value variation during optimization.
Figure 5(a) represents the flowchart of the optimization procedure.
Figure 5(b) represents the flux density in the airgap of different positions.
Figure 6 represents the Phase torque characteristics of the three motors.
Figure 7 represents the full phase torque waveform characteristics of the three motors.
Detailed description with reference to the accompanying drawings:
I. Design Process of the SRM
Since the performance of the SRM depends greatly on the magnetic structure, the theory of magnetic flux distribution needs to be explained initially. Then, the number of rotor poles is calculated on the basis of the new pole design formula.
A. Magnetic principle of multi-tooth SRM
Conventional SRMs have only one tooth each stator pole to allow magnetic flux to pass through while the proposed topology has two teeth per pole to distribute the flux. Two couples of teeth together make the flux variation larger and the variation period shorter.

A comparison between conventional and two teeth topology is shown in Fig. 1 (a) and (b). To prove that the topology can provide higher transient output torque, a theoretical analysis on the basis of equivalent magnetic circuit is conducted in the following calculation. The transient torque of SRM is expressed as
:°£ CD
m
where Wm is the co-energy of the field and 0 is the position of the rotor. By setting the unaligned position 6unalign=0, the rotor position can be calculated as
n=^+^QaGa (2)
r Nr 2 2
where larc is the overlapping length between stator and rotor poles, Nr is the number of rotor poles, r is the outer radius of the stator and ds and dr is the stator and rotor pole arc angle, respectively. The co-energy can be computed as
wm = QO di. (3)
Because of Rg>>Rs+Rr. ,
D=^ D-^. (4)
Rs+Rr+2Rg 2Rg
Due to

/
«i«'-r a

(5)

the co-energy is expressed as
w BN>I*ml„U
- U 21
By inserting the(2) and (6) into equation (1) where the only variable is larc, the electromagnetic torque result is

1 Xt
where m is the number of teeth per pole, /g and / is the airgap and stack length, respectively; N is number of turns per phase; 0 is magnetic flux; / is the current of each phase; Rg, Rs, Rr is the reluctance of airgap, stator and rotor, respectively. Therefore, with the increase in m, the integrand has a higher value indicating two-teeth per pole can improve torque quality.
If m continues rising, Nr increases as well, causing much higher core loss, smaller commutation angle and smaller slot area for windings. Fig. 2 (a) and (b) show the torque output and core loss comparison of the motors with various number Qf teeth each stator pole at the same excitation current by using FEM. Therefore, the design of two teeth per stator pole has the maximum energy conversion capability
B. Design consideration for pole arc angles & number of rotor poles
From the working principle of 3-phase SRM with 6 stator poles, the angle equations of the two teeth topology can be expressed as
fe, . e, 360'
2 2 N- (8)
1 ... 120 , 360'
160 + = k
N N
where dslot is the arc angle of the stator inner slot and k is an integer (k>2). In (8), the first equation can be understood easily from the arc angle between two dash lines in the proposed prototype in Fig. 3 (a); the second equation, where 60°stands for the stator pole pitch arc angle and 220 77Vr represents the commutation angle, can be explained based on the operating principle of the SRM. To keep the number of rotor poles more than that of stator teeth and the commutation angle at a suitable value, k is set as 3. Hence, &slot+&s=22.5 ? Nr=16 and the commutation angle is 7.5 degree. Current conducting mode is the same as traditional three phase SRMs. And the material of stator and rotor cores is DR510 steel.
II. Design optimization of the SRM
The optimization of the two teeth topology is conducted on the basis of genetic algorithm (GA) and equivalent magnetic circuit (EMC) model. According to the EMC model mentioned in [8], an analogical approach of obtaining various magnetic flux lines can be carried out to analyze the proposed topology. For EV applications, the most important factors are lighter weight and higher efficiency with the same torque performance. Therefore, the fitness function of GA is a combination of torque per unit of weight and torque per copper loss

based on weight coefficients. Then, GA process is utilized to calculate the maximum of the function.
The weight coefficients of specific torque and torque per copper loss are set as col and co2, respectively. So the fitness function can be expressed as,
T T
F = 1000 (CO, ^_ + GV^-) (9)
weight loss
where Tavg is the average torque for one phase during half an electric period, weight is composed of that of steel and copper, loss is the copper loss during the above conducting period and 1000 is the multiple for larger fitness value. Some parameters of the motor are kept as constants, including the outer diameter of rotor 382 mm, air gap length 0.5 mm, shaft diameter 100 mm and stack length 74 mm. The rotor inner diameter Drl, rotor pole height hr, stator inner diameter Dsl, stator teeth arc angle 9s, slot depth pole hsl, rotor pole arc angle 8r are set as the variable to be optimized.
Moreover, some constraints need to be considered. Some design limitation of the SRM structure are needed based on [8]. To avoid high saturation in the stator pole causing large torque ripple, a minimum restriction of the width of stator pole is required. Also, the slot area for windings is large enough and a minimum constraint has to be set. In addition, for in-wheel
motors, because of natural cooling condition, the current density is limited to 3A/mm^ and the slot packing factor is set at about 0.44.
Tavg is calculated according to the self-inductance variation with position. Mutual inductance is neglected compared with self-inductance. The value of self-inductance is a sum of individual inductances obtained from different flux lines in the EMC model. The weight coefficients col and u>2 are both evaluated as 0.5.
For GA parameters, the population number is set as 20, maximum generation is limited to 20, crossover probability is 0.8 and mutation is 0.04. To improve the GA performance, rank fitness scaling is utilized to remove the effect of initial value boundary. For selection, stochastic uniform is preferred. Elite count can directly go into the next generation to accelerate the optimization and the number is set as 2. The variation of mean and best fitness function during optimization process can be obtained from Fig. 4. Moreover, the optimization procedure is shown in the flowchart in Fig. 5 (a). With the constant value of some parameters considered, the final optimized parameters are listed in Table I.

TABLE I
OPTIMIZED VARIABLE PARAMETERS
Parameter Value Parameter Value
Dw(mm) 299 h„ (mm) 19
A, (mm) 10 0, (degree) 8
Dsl (mm) 163 0r (degree) 9.5
III. Performance analysis and Comparison
By using FEM, the magnetic flux distribution of the proposed topology can be obtained from the model in Fig. 3(a). It shows that the flux passes through two teeth and goes into two individual rotor poles, in agreement with its working principle. The characteristics of the motor, including torque and core loss, are also analyzed by FEM. For the rated operation condition of motors, flux densities of different positions when phase A is conducting are shown in Fig. 5 (b).
To show the superiority of the proposed motor in high performance, a 6/8 and a 6/10 SRMs in Fig. 3 (b) and (c) are chosen for comparison. They are of the same outline size with the proposed topology. The geometric parameters of the counterparts are also optimized by GA method for persuasive comparison. The objective function considers torque output, weight and energy loss as well. The basic parameters of the three optimized motors can be found in Table II.
TABLE II BASIC PARAMETERS OF THE SRMS

Dimensions 6/8 6/10 Proposed
SRM SRM SRM
Rotor outer diameter Dr2 (mm) 382 382 382
Stack length / (mm) 74 74 74
Airgap length lg (mm) 0.5 0.5 0.5
Stator pole/teeth arc angle 0S (degree) 22 18 8
Rotor pole arc angle dr (degree) 21 16 9.5
Number of turns per phase N 184 248 204
Coil end length (mm) 8.75 9.87 7.47
Copper wire diameter (mm) 1.12 1.12 1.12
Number of parallel windings 8 8 8
Slot fill factor 0.44 0.44 0.44
All the three motors are running at the same suitable total energy loss according to the temperature rise effect. Because of higher frequency of commutation, core loss of the proposed topology is higher than that of its counterparts under similar saturation condition.

Therefore, the copper loss is at a lower value for the proposed topology, resulting in lower current density. But for 6/10 SRM, the number of turns per phase is the largest. Considering the same loss, its current density is limited to a lower value than 6/8 SRM, only slightly higher than the proposed motor. With the same diameter of copper wire, the RMS current of 6/10 SRM is only slightly larger than the proposed one. The value of loss is on the basis of the motor of the same size in [3]. And the torque performance comparisons of one phase conducting in half an electric period with ideal square wave current and full phase conducting are shown in Fig. 6 and Fig. 7, respectively with the rated speed for EVs at lOOOrpm based on FEM computation. The detailed data of motor performance can be obtained from Table III.
TABLE III TORQUE CHARACTERISTICS COMPARISON OF THREE SRMS

Dimensions 6/8 6/10 Proposed
SRM SRM SRM
Current density (A/mm2) 3.00 2.60 2.54
RMS current(A) 23.6 20.4 20.0
Winding weight (kg) 13.2 17.4 15.2
Core weight (kg) 42.6 39.1 41.8
Total weight (kg) 55.8 56.5 57.0
Core loss (W) 45 47 90
Winding loss (W) 274 272 229
Total loss (W) 319 319 319
Torque (Nm) (05 123 159
Torque ripple (%) 63 67 36
Specific Torque ratio (Nm/kg) 1.88 2.18 2.79
Fig. 6-7 and Table III indicate that for the same total loss, the novel topology can provide higher output torque and less torque ripple. Also, the specific torque is much higher than the conventional ones, about 48% more than 6/8 and 28% more than 6/10 SRM. This result shows that the proposed topology has its potential in the application to EVs for higher propulsion force, higher efficiency and lighter weight. Although the 6/10 SRM provides 77% torque output of the proposed topology, it is at the expense of increasing 14% more copper wires, which means higher cost and heavier weight.
Therefore, the new in-wheel SRM topology is presented in this invention for direct drive, high propulsion force and light weight application in EVs. The proposed motor combines the advantage of multi- teeth per stator pole and more rotor poles than stator teeth to achieve the purpose of enhancing the specific torque. The structure of two teeth can improve the output torque. Increasing the number of rotor poles results in larger slot area for winding and allows larger excitation current. The principle of multiple teeth per stator pole is proved by equivalent magnetic circuit and a new design formula is put forward to select the number of rotor poles.

Then, GA method is carried out to make optimization for the geometric parameter. Moreover, the characteristics of the motor are calculated and analyzed by utilizing FEM.The performance is compared with that of 6/8 and 6/10 three phase SRMs to prove its higher specific torque, efficiency and lower torque ripple. In addition, the shape of the stator is convenient for automatic winding and the topology may have prospects for mass production. In sum, the proposed motor shows strong competitiveness for its application to future EVs.

Claim:
1. A novel enhanced torque in-wheel Switched Reluctance Motor, comprising an inner stator, the outer rotor structure wherein the stator has two-teeth per stator pole and the number of rotor poles are more than the stator teeth.
2. A novel enhanced torque motor as claimed in claim 1 wherein the two-teeth per stator structure improves the torque.
3. A novel enhanced torque motor as claimed in any of the preceding claims wherein the increase in the number of rotor poles results in a larger slot area for winding.
4. A novel enhanced torque motor as claimed in any of the preceding claims wherein
the increase in the number of rotor poles allows larger excitation current.
5. A novel enhanced torque motor as claimed in any of the preceding claims wherein
the shape of the stator is convenient for automatic winding.