International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
Modelling of Switched Reluctance Motor Drive
for Electric Vehicle Application
Santhan Kumar Ch*1, Aishwarya Verma*2, A. Shanmukha Sai*3, G. Shirisha*4, J. Bharath Raj*5 , T. Dileep Kumar*6
1,2,3,4,5,6
Bharat Institute of Engineering and Technology, Hyderabad
motors and switched reluctance motors (SRMs) due to the
presence of rotor winding and rotor copper losses.
Abstract— The environmental impact of the transportation
sector is significant due to the combustion of fuels. This creates air
pollution and is a significant contributor to global warming through
the emission of harmful gases. Due to the problems caused by the
internal combustion engine (ICE), the automotive industry has
turned to the electrical powered vehicle. The electric vehicle (EV)
model consists of one or more electric motors powered by a battery
pack that can be charged using an on-board generator and the
regenerative braking technology to power the transmission. Among
the motors like Brushed DC motor, Induction motor, Brushless DC
motor, Permanent Magnet Synchronous motor and Switched
reluctance motor (SRM) which can be used in EVs, the SRMs have
become one of the best choices for electric vehicle drive because it
exhibits prominent advantages over other kinds of the electric drive
system. In this paper, the modelling of SRM drive using
MATLAB/Simulink will be done for EV application.
Brushless DC (BLDC) Motors are specifically known for
their high efficiency and high power density. Using
permanent magnet there is no need for production of energy
for stator supply like induction motors and SRMs. BLDC
motor drives have drawbacks like the expensive magnet,
reduced torque in the motor due to the mechanical strength
of the magnet and they have no brush to limit speed which
restricts the maximum speed if the motors are of an innerrotor type [3].
Permanent Magnet Synchronous Motors (PMSMs) are
available for higher power ratings and high efficiency.
PMSM is the best choice for high-performance applications
like cars, buses. Despite the high cost, PMSM is providing
stiff competition to induction motors due to increased
efficiency than the latter. Most of the automotive
manufacturers use PMSMs for their hybrid EVs [4].
Keywords— Electric Vehicles, Switched reluctance motors,
Zero Pollution, PI Controller.
I. INTRODUCTION
Switched reluctance motor (SRM) drives are gaining
much interest and are recognized to have a potential for EV
applications. These motor drives have definite advantages
such as simple and rugged construction, fault-tolerant
operation, simple control, and outstanding torque-speed
characteristics. The rotor structure is extremely simple
without any windings, magnets, commutators or brushes.
The SRM drive has high-speed operation capability with a
wide constant power region and has high starting torque and
high torque-inertia ratio. The disadvantages of SRM drives
are that they have to suffer from torque ripple and acoustic
noise. However, these are not potential problems that
prohibit its use in EV application.
Electric vehicles (EVs), those that use electric motors
instead of Internal Combustion Engine(ICE), have become
very popular. Those who strive to protect the environment
and go green opt electric vehicles. The rapid development in
the field of power electronics and control mechanisms has
created a space for the usage of various types of electric
motors in EVs. The electric motors used for automotive
applications should have characteristics like high starting
torque, high power density, good efficiency, reasonable cost
and high fault tolerance, etc [1]. As there are various types of
Electric motors. The Automotive industry is still seeking for
more relevant electrical drives for EVs as a challenging issue
[2].
The comparative investigation in the efficiency, weight,
cost, cooling, maximum speed, and fault-tolerance, safety,
and reliability for above-discussed motor drives have
resulted in favour of SRMs. SRM drives are considered
superior to other types of motor drives for EVs. Therefore,
SRM drives are ideally suitable for EV applications
nowadays [3].
Brushed DC motors are well known for their ability to
achieve high torque at low speed and their torque-speed
characteristics suitable for the traction requirement and they
have been used on EVs. However, brushed DC motor drives
have a bulky construction, low efficiency, low reliability, and
higher need for maintenance, which makes them more heavy
and expensive.
The paper presents a simulation model of SRM drive. In
section II the details of SRM is discussed. In section III
modelling of SRM drive is presented for which is practical
simulation in MATLAB is presented in section IV. Section V
presents the simulation results for the behaviour of the
model. Finally, section VI gives concluding remarks.
Induction motors (IMs) are simple in construction,
reliability, ruggedness, low maintenance, low cost, and
ability to operate in hostile environments. The absence of
brush friction allows the motors to raise the limit for
maximum speed, and the higher rating of speed enable these
motors to develop high output. However, the controllers of
IMs are at a higher cost than the ones of DC motors.
Furthermore, the presence of a breakdown torque limits its
extended constant-power operation and IMs efficiency is
inherently lower than that of permanent magnetic (PM)
II. SWITCHED RELUCTANCE MOTOR
The switched reluctance motor (SRM) is a type of
stepper motor that runs by reluctance torque. The mechanical
model is simplified to a great extent as it doesn’t have any
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International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
winding on rotor but adds on electronic devices which make
the operation complex. Electronic devices can precisely time
switch, facilitating SRM configurations. Its main drawback is
torque ripple [5].
(a)
Fig. 1 Desired output characteristics of electric motor drives in EVs
(b) 12/10 poles
Fig. 3 Switched reluctance motor configurations. (a) One tooth per pole.
(b) Two teeth per pole (12/10 poles).
B. Principle of Operation
The physical principle behind the SRM is the reluctance
principle. The first is that the magnetic analogue of current,
called flux, needs to travel the path of least magnetic
resistance, called reluctance. The second is that low
reluctance materials like iron and its, nickel, cobalt, etc.; tend
to strongly align to an incident magnetic field. Thus a
reluctance motor merely has a rotor with alternating regions
of high and low reluctance on it, and a stator with several
electromagnets that when energized in sequence (and
regardless of polarity) will pull the low reluctance regions or
poles, along [6].
Fig. 2 Conventional characteristics of an SRM
The torque-speed characteristics of the SRM drive match
very well with the EV load characteristics. The SRM drive
has high-speed operation capability with a wide constant
power region. The motor has a high starting torque and high
torque-inertia ratio [4].
A. Structure of SRM
The origin of this motor can be traced back to 1842, but
the “reinvention” has been possible due to the advent of
inexpensive, high-power switching devices. It has wound
field coils of a dc motor for its stator and has no coils or
magnets on its rotor. Both the stator and rotor have salient
poles, hence the machine is referred to as a doubly salient
machine [1].
C. Equivalent Circuit of SRM
An elementary equivalent circuit for the SRM can be
derived neglecting the mutual inductance between the phases
as follows. The applied voltage to a phase is equal to the sum
of the resistive voltage drop and the rate of the flux linkages
and is given as:
Because of its simple construction, low rotor inertia and
wide speed range operation, SRM is particularly suitable for
gearless operation in EV propulsion. In addition, the absence
of magnetic sources (i.e., windings or permanent magnets)
on the rotor makes SRM relatively easy to cool and
insensitive to high temperatures. The latter is of prime
interest in automotive applications, which demand operation
under harsh ambient conditions [3].
V
Rs .i
d ( , i)
dt
(1)
where Rs is the resistance per phase, and is the flux linkage
per phase given by:
L( , i ).i
(2)
where L is the inductance dependent on the rotor position
and phase current. The phase voltage equation, then, is
The SRM can be designed in 3-phase i.e., 6/4 pole
(which has six stator poles and four rotor poles), 4-phase i.e.,
8/6 pole, with one tooth per pole. And as 12/10 poles with
two teeth per pole, as shown in the figure:
V
d ( L( , i).i)
dt
di
d dL( , i )
Rs .i L( , i).
i. .
dt
dt
d
di
dL( , i )
Rs .i L( , i ).
i. m .
dt
d
Rs .i
(3)
In this equation, the three terms on the right-hand side
represent the resistive voltage drop, inductive voltage drop,
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International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
The air gap power is the product of the electromagnetic
torque and rotor speed given by:
and induced emf, respectively, and the result is similar to the
series excited dc motor voltage equation. The induced emf,
e, is obtained as:
dL( , i )
.
d
k b . m .i
e
e
m
Pa
.i
dL( , i )
d
Te
Rs .i 2
i2.
dL( , i)
dt
L( , i ).i.
di
dt
(12)
A. Block Diagram of SRM Drive
Fig. 7 shows a block diagram of SRM drive. The motor is
excited by a sequence of DC pulses applied at each phase
using the converter circuit. The individual phases of an SRM
are consequently excited forcing the motor to rotate. The
current pulses need to be applied to the respective phase at
the exact position of the rotor relative to the exciting phase.
So, the exact position of the rotor is needed. This can be
achieved with the help of a rotor position sensor via angle
control.
Fig. 5 Single-phase equivalent circuit of SRM
V .i
1 2 dL( , i )
.i .
2
d
III. MODELLING OF SWITCHED RELUCTANCE MOTOR DRIVE
Substituting for the flux linkages in the voltage equation
and multiplying with the current results in instantaneous
input power given by:
Pi
(11)
This completes development of the equivalent circuit and
equations for evaluating electromagnetic torque, air gap
power, and input power to the SRM both for dynamic and
steady-state operations [7].
(5)
Note that the emf constant is dependent on the operating
point and is obtained with a constant current at the point.
From the voltage equation and the induced emf expression,
the equivalent circuit for one phase of the SRM is derived
and shown in Fig. 5.
.Te
From which the torque is obtained by equating these two
equations as:
(4)
where kb may be construed as an emf constant similar to that
of the dc series exciting machine and is given here as:
kb
m
(6)
Here, the last term is physically uninterpretable; to draw
a meaningful inference, it may be cast in terms of known
variables as in the following:
d 1
.L( , i ).i 2
dt 2
L( , i).i.
di
dt
1 2 dL( , i )
.i .
(7)
2
dt
So now we have,
Pi
d 1
.L( , i).i 2
dt 2
R s .i 2
1 2 dL( , i )
.i .
2
dt
(8)
Fig. 6 SRM Drive System
The controller collects the information and also the
reference speed signal and suitably turns on and off the
concerned power semiconductor devices of the switching
circuit so that the desired phase winding is connected to DC
supply. The current signal is also feedback to the controller
circuit to limit the motor current within permissible limits.
where Pi is the instantaneous input power. This equation is
in the familiar form found in introductory electro-mechanics
texts, implying that the input power is the sum of the
winding resistive losses given by Rs*i2, the rate of change of
2
the field energy given by P*[1/2.L(
], and the air gap
power, Pa, which is identified by the term [i2.
i)]/2,
where p is the differential operator, d/dt. Substituting for the
time in terms of the rotor position and speed, with
t
(9)
B. Power Converter
Among various types of converter, configurations are
Asymmetric, Miller, C-dump, R-dump, Bifilar, Buck-Boost,
Resonant, etc. the standard type of converter used for
modelling of 6/4 pole SRM is the Asymmetric bridge
converter [1].
(10)
C. Controller
The current signal is set within limits by using a
hysteresis controller. The speed control of SRM can be done
using controllers like PI (Proportional Integral) controller,
Hysteresis type, Pulse Width Modulation (PWM), SelfTuning Adaptive Control, Fuzzy-PI Control, ANFIS
(Adaptive Neuro-Fuzzy Inference System) Control.
m
in the air gap power results in:
Pa
Pa
1 2 dL( , i) 1 2 dL( , i) d
.i .
.i .
.
2
2
dt
d
dt
1 2 dL( , i )
.i .
. m
2
d
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International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
D. Rotor Position Sensor
To develop positive torque, the currents in the phases of
an SRM must be to the rotor position. The following figure
shows the ideal waveforms (Phase A inductance and current)
in a 6/4 SRM. Turn-on and turn-off angles refer to the rotor
position where the converter’s power switch is turned on and
turned off, respectively [5].
Te (i a , )
2
ia .
dLa (i a , )
d
(15)
The torque of the SRM is a function of the current and
rotor position. For a motor to develop positive torque, the
excitation must be in the positive inductance region. If
excitation persists in the negative inductance region, the
effective torque decreases because of the negative torque
development.
Dynamic torque equation of the motor is given by,
Te
Tl
J.
d
dt
Bm .
(16)
where Tl is the load torque, J is the rotor inertia and Bm is
the viscous friction of the motor. Finite element analysis is
used for the magnetic and torque analysis [8].
IV. MATLAB MODEL OF SRM DRIVE
A. SRM Block Specifications
The simulation of a 6/4 switched reluctance motor based
on MATLAB/Simulink environment is shown in the figure
below. Set the switched reluctance motor block to 6/4 (60
kW preset model), to use a predetermined specific model of a
switched reluctance motor. Due to magnetic saturation, the
inductance profile is generally nonlinear. But if the
nonlinearity is included the computational burden also
increases. So, to just to have a preliminary understanding, the
linearized inductance profile of SRM is used.
Fig. 7 Inductance and current profile
So, the exact position of the rotor is needed. This can be
achieved with the help of a rotor position sensor. A rotor
position sensor is used to generate precise firing command
for the power switches in converters ensuring the drive
circuit stability, direction of rotation and fast dynamic
response.
E. Modelling Equations
The voltage applied across the stator winding is given by,
Va
Ra .i a
La (i a , ).
di a
dt
Eb
In the MATLAB simulation of switched reluctance motor
the following specification are used: Number of stator and
rotor poles = 6/4, Frequency [F] = 50 Hz, Number of phases
= 3, DC supply voltage [Vdc] = 240 volts, Turn on and off
angle = 45° and 75° respectively, Reference current = 200
amps, Hysteresis band = +10, -10.Friction = 0.01 N-M s,
Unaligned inductance = 0.7 m H, Aligned Inductance = 20 m
H, Stator resistance [Rr] = 0.01ohms/phase, Moment of
inertia [J] = 0.0082Kg-m/sec.
(13)
where Va is the applied voltage, ia is stator winding
current, Ra is the winding resistance, La is stator winding
inductance and Eb is the back emf. Flux developed in the
stator winding is given by,
a
i a .La (i a , )
(14)
Torque developed by the motor is given by,
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International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
Fig. 8 Simulation model of SRM with PI controller
B. Asymmetric Converter Block
We assume H-bridge asymmetric converter while
simulating the machine model. In which each machine
phase is connected to an asymmetric half-bridge consisting
of two power switches and two diodes. The power
switches used are IGBTs (Insulated Gate Bipolar
Transistors).
Asymmetric half-bridges permit soft-switching
operations as well, as a result obtaining a zero-voltage
freewheeling state i.e., the phase is energized from the
positive DC voltage and de-energized at zero voltage. No
restriction exists to prevent energizing two phases at the
same time, thus achieving higher torque [9].
The conditions for voltage switching arei.
When, 0° < Rotor angle < Turn-on angle, then
Voltage = 0
ii.
When Turn on angle <= Rotor angle < Turn-off
angle, then Voltage = +V.
iii.
When Turn-off angle <= Rotor angle <
d), then Voltage= -V.
Fig. 10 H-Bridge Asymmetric Configuration
The control takes place applying the voltage source to a
phase coil at turn-on angle, on, until a turn-off angle, off.
After that, the applied voltage is reversed until a certain
demagnetizing angle d, which allows the return of the
magnetic flux toward zero. To apply voltage V in one
phase, the two IGBTs in Figure (b) must be ON. On the
contrary, to apply the negative voltage, -V and assure the
current continuity, the two diodes D and D1 are used [10].
C. Hysteresis current control
Power switches are switched off or on according to the
current is greater than or less than a reference current. The
instantaneous phase current is measured and feedback to
the summing junction. The error is used directly to control
the states of power transistors. It limits the current between
+10A to -10A.
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International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
D. Position Sensor
The control of an SRM is dependent on knowledge of
the relative and absolute rotor position. The current
reference is dependent on the advance turn-on angle and
commutation angle to maximize the air gap torque. Hence,
an incremental rotor angle is required to vary these control
angle variables. But, the control angles are developed with
respect to a constant or
stator
phase (a set of poles). Therefore, the absolute rotor
position information must position the advance turn-on and
commutation angles to generate the
2) Output ‘m’:
The block output m is a vector containing several
signals. We can demultiplex these signals by using the Bus
Selector block from the Simulink library. The signals are
stator voltages, flux linkages, stator currents,
electromagnetic torque, rotor speed, rotor position [11].
V. SIMULATION RESULTS
The simulation results of the SRM model operating at
no load are obtained and are presented.
E. PI Controller Block
The speed of SRM is controlled by a PI controller. The
controller has simplicity, lowest cost, zero steady-state
error, ease of implementation, good speed response and
robustness. In order to provide the desired performance of
SRM, a feedback control system is employed for speed
control of SRM drive. The tuned values of PI controller
constants are dependent on the system.
Fig. 13 Flux generated in three phases of SRM
")* #&( + % ' $ !s the back
emf induced voltage, which will be high for higher speeds
is shown in fig.13. To increase the current growth and
avoid a high back emf opposition, switching operation
takes place at the turn on angles. Using the linear
inductance profile the minimum back emf value will be
zero since L/ = 0.
Fig. 12 PI controller internal block
The parameters Kp and Ki were obtained via a trial and
error format.
u
K p . e Ki
e.dt
Fig. 14 Three-phase currents of SRM
(17)
Fig.14 indicates that no-load torque the numbers of
current chopping increase rapidly for each phase and
switching operation between phases to make the motor
running at a steady-state constant speed [10].
The combination of proportional and integral terms is
used to increase the speed of response and to eliminate the
steady-state error.
e
Setpo int Output
(18)
Where e is the error or deviation of actual measured
value (output) from the set point. The controller attempts
to minimize the error signal e over time by adjusting the
controller output to a new value. Since the PI controller
depends only on the measured variable, it is broadly
applicable.
Stator resistance is 0.05ohms and inertia of the motor
0.05kg/m2. The value of the constants of the controller KP
and Ki is dependent on the system to be controlled, so after
tuning appropriately and testing for best condition, the
values of the constants used for this analysis were obtained
as, KP = 50, Ki = 0.1 [11].
Fig. 15 Torque generated by SRM
From the torque equation, the torque is directly
proportional to the square of the current, therefore
the torque of the switched reluctance motor is independent
of current direction but it depends on dL/d value. Since
the value of dL/d is positive, the torque of switched
reluctance motor is also positive, shown in fig.15. But this
torque contains a lot of noise and harmonics [12-13].
F. Inputs and Outputs to SRM Block
1) Input ‘TL’:
The block input is the mechanical load torque (in N-m).
TL is positive in motor operation and negative in generator
operation.
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International Conference on Green Power Technology in Power Grid : Issues, Challenges & Control (ICGPTPG-2019)
REFERENCES
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Fig. 16 The speed of SRM
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[5]
Conventional PI controller based motor drive systems
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reluctance motor is highly greater, particularly in motion
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[6]
[7]
[8]
[9]
VI. CONCLUSION
For the proposed PI controller strategy, it can improve
both torque ripple and electric efficiency simultaneously,
therefore, the dynamic performance of SRM and EV can
be improved greatly. Thus the 6/4 switched reluctance
motor is driven by asymmetric bridge converter and it has
simple construction and control compared to a
commutation motor. The SRM drive was modelled on
Simulink and simulated for best performance. The PI
controller gave the best result in terms of elimination of
speed overshoot and steady-state error.
[10]
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