Switched Reluctance Motors For Electric Vehicle Applications Introduction.pdf
1. Answer: Topic: Switched Reluctance Motors For Electric Vehicle
Applications Introduction
Answer:
Topic: Switched Reluctance Motors For Electric Vehicle Applications
Introduction
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
2. 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.
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
3. 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;
To develop mathematical models for SRM control techniques
To determine the optimal control parameters for SRM
To evaluate the type of the inverter and controller to be used for maximum performance.
Develop MATLAB/Simulink model for SRM control and its application in electric vehicle.
Literature Review: Introduction
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
Related Work
The development of electric drives started back on 18th 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
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
4. 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.
Basic Characteristics Of SRM
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.
5. 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.
6. Control Of SRM
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;
The converter must be able excite the phase before entering to demagnetizing and
generating region.
Each motor phase has atleast 1 switch enabling it to conduct independently.
The demagnetization power from outgoing phase needs to be feedback to dc-link capacitor
or dc source or use in the incoming phase.
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.
The converter needs to be a single power source rail to reduce the voltage across the
switches.
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.
Mode 1- magnetization mode
Mode 0 – Freewheeling mode
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 3rd 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.
7. Summary
This chapter involves the description of literature and includes the introduction, related
work, basic features of SRM, control techniques and summary.
Methodology: Introduction
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 vx 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;
8. 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
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
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
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.
9. 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.
Results Analysis
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
14. 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;
Mass of the vehicle (m) =42000kg
Horizontal distance from axis of CG (a) =1.4m
Height of the CG from ground (h) = 0.5 m
Horizontal distance of CG from rear axis (b) =1.6
Drag coefficient (Cd) =0.4
Frontal area (A) =1.2 m2
Longitudinal vehicle velocity (Vx)
Mass density of air (2 kg/m3
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
15. 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.
16. 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;
The simulation results can be applied in analysing the suitability performance of EV using
Simulink environment
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.
17. 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.
