Design And Control Of Switched Reluctance Motor For EV Applications

ME5507 Electrical Services and Lighting Design

Literature Review

Topic: Switched Reluctance Motors For Electric Vehicle Applications

Background

The electric drives are very vital parts of electric vehicle and its requirement relies on the load and available main characteristics. Due to robust and simple structure, high thermal capability, high torque-inertia ratio, reliability, high speed potentiality brushless drive switched reluctance motor has become common compared to other drives as an economical alternative for PM brushless motor for several applications (Gan, 2018). However, this type of motors suffers from acoustic noise and torque ripples that inhibits its high performances. The SRM operational principle are straightforward and simple, but appropriate control for the motor is required. The inherent nonlinearity of the motors makes the torque generation to rely upon the poles geometry characterised by dependence on rotor position and stator current. The structure of SRM is as depicted in figure 1.1 below;

 

Figure 1. 1 Schematic diagram for SR motor (Huang, 2015) .

SRM rotates by a reluctance torque that originates from the variation of magnetic circuit resistance. The rotor and stator have salient poles made from laminated electrical steel. The winding coil concentration are only installed in the stator. Thus, making the structure of SRM to be simpler in the design compared to synchronous motors or induction motors, due to presence of permanent magnet or winding coils in its rotor (Dos Santos, 2013). The possibility of SRM withstanding the operations at high temperature and high-speed rotations under inferior surfaces of the road are high through absorbing the vibrations and impacts. Besides, this motors have some demerits which include high noise and torque pulsation. These challenges can only be solved through development of power electronics and improved techniques. The basic enhancement of SRM drives has backed its extensive application in different fields and has resulted to evaluation of SRMs in EVs. Reports from research institutes on design of SRMs for electric vehicle and its performance have depicted a positive outcome enhancing the control strategy for its application (Cheng, 2017).

For decades now, several researchers have focused on SRM for industrial applications facilitating its rapid development and implementation. The SRM are considered highly reliable in EV drives application due to their superior fault tolerance characteristics (Guerrero, 2016). Low switching and rotor losses in SRM creates a high system efficiency over a wide range.  Thus the SRM motors are not modelled and optimized for fixed speed. However, due to minimum losses and high efficiency of SRM it produces high torque to weight ration compared to dc or ac motors (Chiba, 2012). The high starting torque of the motor due to low rotor losses realizable allows a prolonged operation of the motor under stall conditions. The demerits of SRM i.e. acoustic noise, torque ripple and electromagnetic interferences need to be matched carefully to a specific motor to enhance maximum performance. The SRM needs a more conductor coupling. However, the non-linear nature of the motor operation makes analytical design extremely difficult (Rajpurohit, 2018). Therefore, this dissertation proposes a simplified method of control of SRM that applies the mathematical functionalities in describing the flux linkage saturation depending on rotor position and winding current.

Basic Features of SRM

Scope 

The SRM represent a gifted candidate for EV application because of its robust and simple structure, high fault-tolerance, low cost of manufacturing and wide range of operating speed. Despite its characteristics, the chief blocking factors for the motor drive are vibration, torque ripple and acoustic noise. The torque ripple reduction can be achieved through use of latest control technique and improving the mechanical design of the motor. The control and analysis of SRM is quite complicated due to doubly-salient structure and non-linear magnetic features. For electric vehicle application, the generation of maximum torque is needed over a wide speed range to encounter the friction and provide a capability of climbing. Moreover, the motor drive efficiency need to be improved to prolong the vehicle range and reduce the torque ripple to prevent the fluctuations of the speed. Besides, getting the best values of average torque and ripple in this type of the motor is impossible. However, the proper picking of switch-off and switch-on angles with current controller in the system can improve the energy efficiency and torque production of the drive. Therefore, this dissertation proposes a technique of controlling SRM operation to encounter the challenges the drive experiences in EV application.

Problem Statement

EVs are considered as s green transportation mode because of low maintenance, no emissions, safety drive and reduced noise pollution. The EV propulsion system comprises of controller, power converter and motor. The electrical propulsion is required to deliver high performance, torque capability and high-efficiency operations. For the EV motor, the key features include providing high efficiency, flexible drive regulation, low acoustic noise and fault-tolerance (Prajapati, 2021). The motor drive is also required to have the ability of handling the voltage fluctuation from the source. For this consideration the SRM is appropriate candidate for the propulsion. These motors provides a large robustness in the EV design. Besides the lack of rotor winding or permanent magnets reduces its cost and provides increased high-speed operation ability (Chuantian, 2019). The motors have a reliable converter topology. The series connected stator winding have a switch that prevents shoot-through faults that is associated to AC field. Furthermore, the low rotor inertia permits high torque and fast response (Xiang, 2018). The robust rotor design results maximum speed and permissible temperature for the rotor. The SRM inherent 4-operational quadrant which achieves the propulsion demands for EVs. However, the salient structure of SRM results high non-linearity in its magnetic features results to complicate in designing and control of the motors. They also produces acoustic noise and torque ripples. Therefore this work proposes a design for controlling of SRM to weaken the torque ripple and noise.

Aim and objectives

The main aim of this dissertation is to design an efficient, robust and simple SRM drive for EV applications. To achieve this aim the following objectives were highlighted. They include;

1. To develop mathematical models for SRM control techniques
2. To determine the optimal control parameters for SRM
3. To evaluate the type of the inverter and controller to be used for maximum performance.
4. Develop MATLAB/Simulink model for SRM control and its application in electric vehicle.

This section entails the discussion of the literature review and comprises the introduction, related work, basic features of SRM, control techniques and summary of the chapter

The development of electric drives started back on 18^th century when principle of electromagnetic induction was demonstrated by Faraday. Following the invention the electric motor drives were invented and classified to 2 i.e. DC and AC as depicted in figure 2.1 below

Control Techniques

 

Figure 2.1 Types of electric motors

In EV and HEV PMSMs are common for the application, however these motors depends on permanent magnet from rare-earth material. Limited reserves, high cost and environmental effects associated with the material extraction and refining limits its application in mass electric vehicle markets. Besides the sensitivity of Pm at high temperature compromise the performance of the motor at harsh environment. Therefore, there is a need of developing alternative motors to enhance high–performance on the electric vehicle. P. Mattavelli et al. (2005) researched on the application of PMSM. The motor was driven through sinusoidal signal to attain the lower torque ripple. The stator windings creates a sinusoidal flux density within the air gap. This motor possessed a feature of brushless and induction motor. To achieve high torque specification at low speed, high efficiency and high density, variable frequency drive were applied. Besides, the VFD control strategy increased the system complexity and thus much attention for appropriate speed control was required. Reid B et al. (1987), designed a steeper motor that comprised of concentrated winding coils. The motor torque is generated when current switches from stator coils to another generating the magnetic attraction between the stator and rotor rotating the rotor to a stable position.

Among the competing motor strategies, SRMs have received a great attention for both research and industrial application. SRMs have several merits including absence of collector, brush and magnets resulting to less maintenance cost and increased reliability. Makwana et al. (2011) analysed the performance of SRMs. He discovered that have low fault tolerance operation and weight with a high efficiency of about 0.95 .Due to absence of permanent magnets, the motors operates at high speed and copper losses is negligible  because of no rotor windings. Thus, lower rotor temperature compared to other motors and cooling of the motor can be achieved easily. Low inertial in SRM plays a vital role of VFD application for fast response. The motor rotor also has a lower inertia compared to other motors due to lightweight rotor structure. The motor phases do not have an impact on each other i.e. once its phase fails then the motor continues to run. Some of the EV requires a power train to work in constant energy. Besides, this is not a vital issue as a revised motor design and constant energy region can further be extended. Gao Y et al. (2012) investigated the of potential SRMs application in HEV and EV propulsion and concluded that SRMs are the most precise motor type for the application.

The SRM is synchronous type of the machine with the torque generated by tendency of its locomotive parts which moves to a position with maximum inductance from excited windings. The motor stator shelters the set of windings or coils per salient pole typically coupled in series within the opposing poles. The set coils are concentrically wound with no phase overlapping resulting to very little mutual inductance and enhancing a greater copper portion utilized in winding to act as active length. The motor rotor also is constructed similarly to that of laminated Pm, therefore needs no slip rings or brushes and permits a higher temperature of operation increasing its durability. Thus the motor is doubly salient and singly excited with the rotor and stator having salient structure of the poles as depicted in figure 2.2 below.

Performance of SRM in EV Application

 

Figure 2.2 Rotor and stator of SRM.

The operation basics of DC current is applied to phase to create a magnetic flux which flows through the rotor. The motor rotor is positioned in a way which minimizes the flux reluctance route thus maximizing the excited winding inductance and creates a torque aligning to salient poles of the stator and the rotor. As a result of simplicity inherent, the motor is high reliable and uses a low-cost variable-speed drive.

The power conversion in SRM relies on the stator and rotor magnetic interaction that changes with the change in its angular position. When the pair of poles aligns exactly to stator phases then aligned position is attained. And when axis has equal distance between interpolar axes is aligned exactly with stator poles for a specific phase and it is termed as unaligned position. When the poles of the rotor are symmetrically misaligned with poles of the stator of a phase, the location is termed as unaligned position and have a minimum inductance. The SRM inductance profile can be categorised to 3 regions which includes; constant, increasing and decreasing periods. In case of the constant current flows via the phased winding, the positive torque is created and when the motor is operated at inductance increasing region and inductance decreasing regions, a negative torque I produced. For the constant excitation case, no torque is created due to positive and negative torque that cancels each out making the shaft torque to be zero. This led to attainment of an effective rotating energy switching excitation that needs to be synchronized by the inductance profile as illustrated below.

 

Figure 2.3 Inductance profile

The SRM torque is produced to minimize the reluctance and its magnitude in each phase can be described as proportional to inductance slope and square of current and current squared and can be produced regardless the current direction. As the torque polarity is changed because of the inductance slope, a negative zone of inductance is produced in accordance to rotor position. The switching excitation is required to be synchronized for the motoring torque with the rotor’s position angle. The minimum losses of the low switching and rotor in the controller generates a high overall efficiency for the system over a wide range of control. Thus, SRM are not modelled and optimized to fixed synchronous speed. Because of its minimum losses and high efficiency of the SRM it has an overall rule, generating a higher torque (power) to weigh ratio as DC or AC motor standards. Because of the low losses from the rotor with extremely high torque which is realizable allowed for prolonged operation of the motor in stall condition. The simplicity of SMR i.e. brushless and magnet-free enables the integration easily with a driven machine compared to other conventional motors.

The main purpose of SRM drive is to utilize the current in each phase coordinated with the position of the rotor to attain the desired torque output and operating mode. The main advantage of the SR motor is that it utilizes unidirectional current for its operation in the 4-quadrantsthat entails the application of few semiconductor switches for the converter design and also opens for a wide range of circuit options as compared to other types of motors that needs sinusoidal and bi-directional current. Due to inductive nature of the winding phase, the switches need to be protected from transient because of the induced voltages produced after commutation and the system current provides the conduction path and freewheeling diodes or other clamping mechanism is needed. The choosing of converter topology for a given application is a vital issue. The SRM converter basically requires the following;

1. The converter must be able excite the phase before entering to demagnetizing and generating region.
2. Each motor phase has atleast 1 switch enabling it to conduct independently.
3. The demagnetization power from outgoing phase needs to be feedback to dc-link capacitor or dc source or use in the incoming phase.
4. The converter power can be delivered to one phase whilst extracting it concurrently from other phases. The converter therefore allows the control of phase overlap.
5. The converter needs to be a single power source rail to reduce the voltage across the switches.

Proposed Method of SRM Control

However, the asymmetrical converter are the most applied type of motors in SRM drives. These motors have 2 main switches and 2 flywheel diodes per circuit phase. During chopping period, 1 switch is turned on and the other turned off, the current flows via the on switch and freewheel diode. And during commutation period, the switches are turned off and the magnetic energy in the motor is discharged with flywheel diode through continuing current. This occurs in 3 modes i.e.

1. Mode 1- magnetization mode
2. Mode 0 – Freewheeling mode
3. Mode -1 –Demagnetization mode.

During energisation or magnetization mode, the 2 switches are on and the phase winding current rises. During the freewheeling state which is the second mode, only 1 diode and 1 switch are on. No voltage applied across the winding phase and current flows via one diode and switch, although it decays gradually.  Therefore, no power transfer from or to supply. The 3^rd mode, demagnetization both the main switches are off and the power in phase winding is directed towards the supply through the freewheeling diode. Thus the voltage is reversed along the winding phase forcing the current to decay rapidly to zero.

Summary 

This chapter involves the description of literature and includes the introduction, related work, basic features of SRM, control techniques and summary.

This chapter illustrated the method of the study and entails the introduction, design models, mathematical model, Simulink model and summary of the chapter

Design Models for SMR-driven EV  

The SRM requires to be operated with a power converter because it is an electronic commutated motor. The symmetric converter is also applied for this case and DC supply is applied in exciting the phase winding.  For the design two models are used which includes the mathematical model and MATLAB/Simulink model as described below;

Mathematical Model 

The voltage drop in the SRM can be expressed as;  

The electromagnetic torque for SMR can be expressed as; 

Then the mathematical block diagram for SRM can be demonstrated as shown.

 

Figure 3.1 SRM mathematical block diagram

In modelling the vehicle output, the longitudinal velocity of the EV with input longitudinal force used to rear and front wheels. Assuming the drive for rear wheels needs longitudinal force and the vehicle accelerates with a velocity v_x  can be expressed as; 

 

MATLAB/Simulink model 

Using the mathematical equation derived above the Simulink model was developed consisting of position sensor, current control, converter block, vehicle model and speed controller block. Detailed SRM-driven EV implementation of different subsystem blocks is as illustrated below;

1. Position Sensor – This block involves working out the rotor position angle in relative to zero angle reference in electrical cycle. For this case 6/4 SRM is used and each phase inductance is periodicity by 90 degrees. Thus, it is precise to transform the position angle of the rotor from the mechanical equation. The Simulink model for position sensor is as shown.

 

Figure 3.2 Simulink model for position sensor block

1. Converter Block – Asymmetric bridge converterwas adopted for this design and its function was implemented in Simulink model. The model had 2 power diodes and 2 switches. The step motion for SRM was realized through switching off and on the phase windings. The block arrangement in the converter is as illustrated below;

 

Figure 3.3 Asymmetric-Bridge Converter  

1. Controllers –in cascade control technique speed error do exist between the rotor velocity and its command which can be controlled by use of the speed controller to produce the current command. The command control is as shown.

 

Figure 3.4 Speed controller Simulink model

The current controller controls the current feedback errors that originates from the control voltage and for this case all types of controller are used to simulate the proposed model which includes the P, PI and PID controller. The hysteresis is also adopted here

1. Electric Vehicle model – The electric vehicle model is applied to realize the driver motor and the position sensor applied in detecting the rotor position for the vehicle. The Simulink model for EV is as shown;

 

Figure 3.5 Electric vehicle Simulink model

The complete Simulink models for SRM-driven electric vehicle was carried out without the controller and also with the 3 types of controllers (P, PI and PID) as illustrated below.

Conclusion

 

Figure 3.6 SRM driven Electric Vehicle Simulink model

 

Figure 3.7 P control of SRM driven Electric Vehicle Simulink model

 

Figure 3.8 PI control of SRM driven Electric Vehicle Simulink model

 

Figure 3.9 PID control SRM driven Electric vehicle Simulink model  

Summary 

This section elaborates the method of research and design of the model and involved introduction, design models, mathematical model, Simulink model and summary of the chapter.

This chapter entails the analysis of the simulation results from the designed model of the SRM-driven electric vehicle. It includes introduction, simulation results and summary of the chapter.

Simulation Results 

The SRM applied in simulation was taken from Simulink model with block parameters and the specification was as depicted in table 1 below. The block input was the mechanical load torque that is negative for generating and positive for motoring operation. The simulation was run in a continuous mode and turn off and turn on angles were kept constant at 45 and 40 degrees respectively. The simulation parameter is as shown.

Table 1. Simulation parameter

Parameter	Value	Units
Inertia	0.05	Kg.m2
Stator resistance	0.05
Initial speed	0	rad/s
Initial position	0	rad
Friction	0.02	N-m.s
Maximum current	450	A
Aligned inductance	23.6e-3	H
Unaligned inductance	0.67e-3	H
Maximum flux linkage	0.486	Vs
Model (generic )	6/4
Proportional controller	10
Integral controller	20
Derivative	50
Gravitation  force	9.81	m/s2

For the EV modelling the 2 equal sized wheels that moves backward or forward along the longitudinal axis development. The vehicle parameters are as illustrated below assuming that the vehicle is in vertical stable state thus its axis is perpendicular to horizontal plane. The parameter includes;

1. Mass of the vehicle (m) =42000kg
2. Horizontal distance from axis of CG (a) =1.4m
3. Height of the CG from ground  (h) = 0.5 m
4. Horizontal distance of CG from rear axis (b) =1.6
5. Drag coefficient (Cd) =0.4
6. Frontal area (A) =1.2 m²
7. Longitudinal vehicle velocity (Vx)
8. Mass density of air (2 kg/m³
9. Radius of the wheels = 0.3 m

The simulation results for SRM model without the controller is as explained below. The results shows the motor speed and that of the vehicle is almost similar at instant time therefore validating the proposed strategy for the simulation of the SMR driven EV system. The flux variations for the SR motor is as illustrated below. 

 

Figure 4.1 Flux variations for the SR motor  

 

Figure 4.2 Phase current

 

Figure 4.3 Total torque generated by SR motor

 

Figure 4.4 Angular velocity of the SR motor

The performance of the electrical vehicle shows that the output current waveform is constant while the rotor position turns off and on the angle. The speed and distance covered by the vehicle increases continuously with time but later the speed starts to decrease as shown below.

 

Figure 4.5 Speed waveform for SRM driven electric vehicle

From the figure above it can be depicted that at the start the speed increases rapidly and after 10 seconds the brake is applied therefore the speed decreases.

 

Figure 4.6 Distance covered by the vehicle

The distance waveform above is very close to linear graph which is much appropriate for the electric vehicle. The waveform depicts the superior characteristics of the EV at its initial and running torque and the acceleration.

However to manipulate the control of the system to bring the parameters to a set point. However for this motor type, the performance of P, PI and PID controllers almost delivers the same value of maximum speed. As the proportional gain is increased, it gives a smaller phase margin and amplitude, faster dynamics and large sensitivity of the noise. Applying P controller also decrease the rise time and steady state error and also causes oscillations of adequate aggressive dead time. The performance of P and PI is as illustrated below

 

Figure 4.7 Simulation results for P control.

 

Figure 4.7 Simulation results for PI control.

PID controller have an optimum dynamics of control which includes; 0 steady error, higher stability, faster response and no oscillations. Application of the derivative component in addition to PI involves eliminating the oscillations and overshoot in the system’s output response .Therefore the PID can be applied with higher order including more energy storages. The PID simulation results for EV motor is as shown.

 

Figure 4.8 PID control results

Summary 

This chapter entails the analysis of the simulation results and includes the following; the introduction, simulation results and summary.

Conclusion

Using the designed model it can be depicted that is quite equivalent to operating the SRM-energised electric vehicle. The model running on the Simulink environment benefits greatly the application of technology. The SRM drives can sometime be extremely fault lenient. This is due to the motor winding can be only found on the stator and are easily cooled. To encounter stationary inertia of the motor then large starting torque is needed by the EV. The fewer switching devices of the system due to torque output does not rely on the current polarity. The SR motor drive is well suited for high-speed applications due to its wide range of working speed. The drive also operates in either motoring or generation mode.

The proposed model concept was to compare the response of SRM drive with the 3 controllers. The prime reason of considering 4-phase 6 stator and 4 rotor poles motor was to minimize the generated torque ripple. Overall the ripple constraints were less through increasing the number of poles for the designed motor drive. Therefore the SMR-driven EV dynamic performance was predicted through use of Simulink platform for the simulation. The results shows that application of a controller achieves the required output and provides excellent tracking reference for the EV speed thus, enhancing its control.  Therefore it can be concluded that;

1. The simulation results can be applied in analysing the suitability performance of EV using Simulink environment
2. SRM drives are the most suitable motors for EV applications.

References

Cheng, K., 2017. Design of a new enhanced torque in-wheel switched reluctance motor with divided teeth for electric vehicle. IEEE Transactions on Magnetics, 11(53), pp. 1-4.

Chiba, A., 2012. Design of switched reluctance motor competitive to 60-kW IPMSM in third-generation hybrid electric vehicle. IEEE Transactions on Industry Applications, 6(48), pp. 2303-2309.

Chuantian, Y., 2019. Design and optimisation of an In-wheel switched reluctance motor for electric vehicles. IET Intelligent Transport Systems, 1(13), pp. 175-182.

Dos Santos, F., 2013. Multiphysics NVH modeling: Simulation of a switched reluctance motor for an electric vehicle. Transactions on Industrial Electronics, 1(61), pp. 469-476.

Gan, C., 2018. A review on machine topologies and control techniques for low-noise switched reluctance motors in electric vehicle applications. IEEE Access, Volume 6, pp. 31430-31443.

Guerrero, J., 2016. New integrated multilevel converter for switched reluctance motor drives in plug-in hybrid electric vehicles with flexible energy conversion. IEEE Transactions on Power Electronics, 5(32), pp. 3754-3766.

Huang, J., 2015. Vibration effect and control of in-wheel switched reluctance motor for electric vehicle. Journal of Sound and Vibration, Volume 338, pp. 105-120.

Prajapati, P., 2021. Design and optimisation of slotted stator tooth switched reluctance motor for torque enhancement for electric vehicle applications. International Journal of Ambient Energy, pp. 1-6.

Rajpurohit, B., 2018. Comparative analysis of permanent magnet motors and switched reluctance motors capabilities for electric and hybrid electric vehicles. IEEMA Engineer Infinite Conference (eTechNxT) , pp. 1-5.

Xiang, C., 2018. Vibration mitigation for in-wheel switched reluctance motor driven electric vehicle with dynamic vibration absorbing structures. Journal of Sound and Vibration, Volume 419, pp. 249-267.