Optimum design and research of switched reluctance motor for electric vehicle based on Motor-Cad

With the development of the economy and the deepening of the energy crisis, electric vehicle has now become the hotspot of the global automotive industry. Therefore, as the key part of electric vehicle, the design and research of drive motor has become the focus of attention of engineers and technicians. Because of its simple structure, easy maintenance, high output power, high output torque, wide speed range, and high efficiency, the switched reluctance motor (SRM) has been one of the first choice of electric vehicle driving motors. According to the performance requirements of electric vehicle driving motors, a 3-phase 24/16 pole with rated power of 30 kW switched reluctance motor (SRM) for electric vehicle has been designed in this paper by using Motor-Cad which can be used to optimize the parameters of motor to make the overall structure of the motor more reasonable. In order to reduce the torque ripple of switched reluctance motor, appropriate stator polar arc and rotor polar arc design are used in the motor. To realize the speed control of switched reluctance motor in a wide range, PWM control mode is adopted in this paper, which has fast motor response speed, high power factor and excellent dynamic and static performance. The simulation results show that the performance indexes of the switched reluctance motor meet the design requirements completely.


Introduction
For electric vehicles, the most important thing is the electric motors that drive the electric vehicles.Since the invention of the switched reluctance motor, there has been large amount of research interest in many countries and regions on this new type of motor drive system.After more than 40 years of development, the design of switched reluctance motors and their control systems are being gradually improved.The simple structure of stator and rotor of switched reluctance motor allows the motor to run at high speed and also facilitates the cooling of the motor body.Switched reluctance motors have high starting torque, low starting current and can have constant power output.The wide range of speed regulation of switched reluctance motors can meet the operation of vehicles in various situations and ensure the efficiency of vehicle driving.Switched reluctance motors have high efficiency and low losses, which can extend the range of the vehicle [1].A three-phase switched reluctance motor with 24pole stator and 16-pole rotor for electric vehicles was designed and optimized using Motor-Cad, which has powerful simulation capabilities, in order to make the final design more feasible and reasonable.

Design requirements of motors
In this paper, according to the performance needs of switched reluctance motors for electric vehicles, the design objectives are determined before design: high starting torque, wide speed range, high output power to meet the use of electric vehicles on various road conditions.According to the above requirements, Table 1 lists the specific performance specifications of switched reluctance motors for electric vehicles.

Design of motor dimensions 3.1. Structure of motor
The switching reluctance motor works differently from a conventional AC motor.Conventional AC motors rely on the torque formed by the interaction of the currents generated by the stator winding and rotor winding to drive the motor, but the rotor of a switched reluctance motor rotates based on the principle that the magnetic flux is preferentially pulled through the magnetic circuit of maximum permeability to rotate the rotor.Based on the above principle, the switched reluctance motor has a unique structure of a convex stator and a convex rotor.Moreover, the numbers of stator and rotor poles are not equal, so it is necessary to choose the right number of stator poles and rotor poles according to different applications.As shown in Figure 1, the main material constituting the stator core and rotor core is silicon steel.A centralized winding is installed at each pole of the stator, while no winding is required on the rotor [2].

Introduction of motor design process
In this paper, the determination of the performance index of the motor is considered as a key step in the motor design, and the performance indicators of the switched reluctance motor determines the operating performance of electric vehicles.The specific structure and dimensions of the motor are designed according to the space that can be accommodated by the electric vehicle and the specifications of the user.Several main dimensions of the motor are first determined, and then other appropriate dimensional parameters such as air gap, stator yoke height, rotor yoke height, stator pole arc, rotor pole arc, etc. are designed according to the specific performance requirements of the motor.Once the dimensions of the motor were initially designed, these dimensions were entered into Motor-Cad for simulation experiments.If the results of the simulation experiment do not satisfy the expected requirements, it is necessary to repeatedly correct the previously designed parameters.After repeated modifications of dimensions and software optimization, a complete switched reluctance motor for electric vehicles can be designed with feasibility and rationality.Figure 2 shows the specific design flow of the switched reluctance motor for electric vehicles in this paper.

Design of main dimensions
The design concept of switched reluctance motors also has output equation similar to that used in the design concept of conventional AC motors.The output equation is used to establish the connection between the parameters of the main motor dimensions, electrical load, magnetic load, speed and electromagnetic torque.Before the rotor outer diameter is calculated by the output equation, the electrical and magnetic loads and the appropriate current coefficients should be selected in advance [3].The rotor outer diameter as one of the important parameters of the main dimensions can be calculated by the following equation.
The stator outer diameter, rotor outer diameter, and core length are three indispensable parameters for determining the overall size of a switched reluctance motor, and these three parameters are collectively referred to as the primary dimensions.The concept of main dimension ratio also exists in the design process of switched reluctance motors, but the main dimension ratio of switched reluctance motors is not exactly the same as the main dimension ratio of conventional motors.The formula for the main dimension ratio is: The running quality and economy of the motor are profoundly related to the value of the main dimension ratio.In this paper, the rotor outside diameter is the first parameter to be determined among the main dimensions.Then, the appropriate stator outside diameter is selected in combination with the motor performance and the space occupied by the motor.Finally, the core length can be calculated by the selected primary dimension ratio.

Design of stator pole arc and rotor pole arc and rotor pole cone angle
The stator pole arc and the rotor pole arc are considered to be two important parameters that affect the torque pulsation of switched reluctance motors.The forward self-start and reverse self-start performance of switched reluctance motors is also closely related to the stator pole arc and rotor pole arc.The stator pole arc and rotor pole arc are also designed with the constraints of reducing magnetic leakage, improving starting performance and increasing output torque during the design process.The installation space of the stator winding will be affected by the stator pole arc.If the stator pole arc is too large, the width of the stator slot will be reduced, and finally the space to place the stator winding will be reduced [4].The values of the stator pole arc and rotor pole arc are referred to the following equations: The stator pole arc and rotor pole arc are the key factors affecting the starting performance and torque ripple, and they should be designed to be larger under the premise of space structure and processing cost of the motor.Generally speaking, the stator pole arc should be less than or equal to the rotor pole arc.However, in this paper, the stator pole arc and rotor pole arc can't be too large because of the large number of stator poles and rotor poles and the limitation of motor size.After going through the above discussion, this paper finally decided to adopt a design where the stator pole arc and rotor pole arc are equal.While doing the above design, the rotor pole cone angle is designed together with the rotor pole arc as a special parameter in the design process.Although increasing the rotor pole cone angle causes the rotor core to become heavier and reduces the torque output to a certain extent, this can improve the structural strength of the rotor and achieve the purpose of suppressing torque ripple and improving the torque waveform.With the existing machining accuracy, the rotor pole cone angle is set at 10° in this paper.

Design of other motor parameters 3.5.1. Design of the number of phases and the number of stator poles and rotor poles
The number of phases is closely related to the construction of the motor and the operating performance of the motor.Most intuitively, the motor's starting performance is affected by the number of phases, and switched reluctance motors with more than two phases have the ability to start in either forward or reverse direction at any position.The determination of the number of stator poles is constrained by the number of phases, and the number of stator poles should be an integer multiple of 2 of the number of phases.The number of rotor poles should be an integer multiple of 2. When switched reluctance motors have a larger number of stator poles and rotor poles, torque ripple can be suppressed to some extent.In summary, the three-phase 24/16 pole structure is chosen in this paper.

Design of air gap
The switched reluctance motor has two air gaps, the first air gap and the second air gap.The air gap between the surface of the stator pole and the surface of the rotor pole when they are opposite each other is called the first air gap.The air gap between the surface of the stator pole and the bottom surface of the rotor slot is called the second air gap.In order to obtain a large electromagnetic torque and reduce torque ripple, the first air gap should be as small as possible.But the first air gap will be limited by the level of machining process [5].After determining the first air gap and the second air gap, the rotor slot depth can be calculated.Moreover, once the first air gap and rotor outer diameter are determined, the stator inner diameter can be calculated.

Design of stator yoke height and rotor yoke height
The stator yoke height is considered to be one of the factors affecting torque ripple and noise.When the stator yoke height is increased, the torque ripple is improved and the noise is reduced, but increasing the stator yoke height increases the manufacturing cost [6].The design of the rotor yoke height will be constrained by the rotor slot depth and shaft diameter.Supersaturation may occur in the stator core and rotor core when the maximum magnetic flux density is maximum.Therefore, the design of these two yoke heights should also be considered to prevent the above-mentioned situations from occurring.Once the stator yoke height is determined, the stator slot depth can be determined by combining the rotor outer diameter, first air gap, and stator outer diameter calculated in the previous section.The shaft diameter as the last parameter of the rotor can be calculated after determining the rotor slot depth, the rotor yoke height and the rotor outer diameter.

Design of stator windings
The placement space of the stator windings will be affected by the stator yoke height, stator slot depth, stator pole arc, and other factors.As far as the stator slot space allows, the more turns of the windings, the smaller the peak current of the windings, which is conducive to the heat dissipation of the motor [7].
The stator windings in this paper uses the centralized winding design that is more common in the design of switched reluctance motors, and the flat bottom slot shape is chosen as the slot shape of the stator.
Figure 3 shows a brief placement of the stator windings on the radial direction of the motor.Figure 4 shows the specific placement of the stator coil in the stator slot.

Results of motor design
After the above discussion and design, the final design results for each dimension and other parameters of this switched reluctance motor for electric vehicles can be presented in Table 2.

Design of motor control system
PWM control method is chosen as the main control method in this paper.The control system adopted in this paper is a double closed-loop control system with an inner loop and an outer loop, where the feedback signal of the inner loop is the current of the windings and the feedback signal of the outer loop is the speed of the motor.The PWM signal output by the DSP processor will be received by the main circuit to control the on/off of each phase of the stator windings of the motor.The changes in the current of each phase windings of the motor at different moments will be detected by the control circuit, and the collected current signal will be processed as the feedback parameter for the current closed-loop control.At the same time, the position of the rotor at a certain moment and the speed at that moment will be detected by the control circuit, which will use the speed at that moment as a feedback parameter for the closed-loop speed control [8].The control system diagram of the switched reluctance motor for electric vehicles in this paper is shown in Figure 5.

Torque
Because of the design of suitable stator pole arc and rotor pole arc as well as the incorporation of rotor pole cone angle, the simulated torque waveform of the motor has no significant torque ripple as can be seen in Figure 6.

Magnetic chain
The variation of the magnetic chain with rotor angle is shown in Figure 7.The saturation of the magnetic circuit during motor operation results in a waveform of the magnetic chain that is not ideally linear and symmetrical.The waveform of the magnetic chain of the motor does not show significant ripples after the dimensional parameters of the motor have been optimally designed.Moreover, the motor is able to obtain a large maximum magnetic chain.

Inductance
The variation of inductance with rotor angle is shown in Figure 8, where the motor is able to obtain a small minimum inductance due to the design of each dimension of the stator and rotor in a suitable way.

Output Performance
As can be seen in Figure 9, the motor can output more than 80Nm of torque at start-up, which can achieve the power requirements to the electric vehicle start-up.The phenomenon that the motor torque drops and then rises in the speed range of 0~500rpm is caused by the sudden addition of load during motor start-up.The speed regulation range of the motor is 0~6500rpm, which can be adjusted in a wide range.

Conclusion
According to the performance indicators of electric vehicles proposed in advance and the unique performance advantages of switched reluctance motors, a switched reluctance motor is designed in this paper in a targeted manner.In this paper, Motor-Cad is used to optimize the dimensional data of this motor because of its powerful simulation capabilities.The torque ripple of the motor is improved after designing the appropriate stator pole arc and rotor pole arc.Moreover, the ripple of the magnetic chain is effectively suppressed after optimizing each dimensional parameter by Motor-Cad.The output performance of the motor can be improved by a larger maximum magnetic chain and a smaller minimum inductance.The motor designed in this paper not only has a large starting torque but also has a wide speed range.As a result, this motor is able to provide powerful starting performance for electric vehicles and is also able to cope with a wide range of driving conditions of electric vehicles.Although the performance of the motor designed in this paper has achieved the expected performance indicators, the suppression of torque ripple can be further optimized.Therefore, the next stage of research focuses on optimizing the slant slot structure of the stator using finite element analysis to ultimately reduce torque ripple.

Figure 2 .
Figure 2. Design flow chart of switched reluctance motor for electric vehicles.

Figure 3 .
Figure 3. Stator windings placement on the radial direction of the motor.

Figure 4 .
Figure 4. Stator coil placement in the stator slot.

Figure 5 .
Figure 5.Control system diagram of switched reluctance motor for electric vehicles.

Table 1 .
Specific performance indicators of the SRM for electric vehicles.

Table 2 .
Dimensions and other parameters of the SRM for electric vehicles.