Module 3 : Electric Propulsion

EV considerations: Electric propulsion systems are at the heart of EVs and HEVs which
consists of electric motors, power converters, and electronic controllers as shown below :

Electric Motor : It converts the electric energy into mechanical energy to propel the vehicle, or
vice versa, to enable regenerative braking and/or to generate electricity for charging the on-
board energy storage.
Power Converter: It is used to supply the electric motor with proper voltage and current.
Electronic Controller : It commands the power converter by providing control signals to it, and
then it controls the operation of the electric motor to produce proper torque and speed,
according to the command from the driver. It has following three functional units
a) Sensor,
b) Interface circuitry and
c) Processor.
The sensor is used to translate the measurable quantities, such as current, voltage,
temperature, speed, torque, and flux, into electric signals through the interface circuitry. These
signals are conditioned to the appropriate level before being fed into the processor. The
processor output signals are usually amplified via the interface circuitry to drive power
semiconductor devices of the power converter.
Types of Motors for Electric Propulsion in EV / HEVs : The motors used in
EVs and HEVs requires frequent starts and stops, high rates of acceleration/deceleration, high torque
and low-speed hill climbing, low torque and high-speed cruising, and a very wide speed range of
operation. Several motors employed in EVs and HEVs is shown below :

Advantages of commutator less motors are higher efficiency, higher power density, and lower
operating cost. They are also more reliable and maintenance-free compared to commutator DC
motors; thus, commutator less electric motors have now become more attractive.

DC Motor Drives for EV/HEVs and its Speed Control ,

DC Motors employed for EV / HEVs should have adjustable speed, good speed regulation, and
frequent starting, braking, and reversing capabilities.
DC Motor Principle of Operation and its Performance : When a current carrying
conductor is placed into a magnetic field, it
experiences a force which is perpendicular to the
conductor and magnetic field as shown below :
The magnetic force is proportional to the wire
length (L), magnitude of the electric current (I),
and the density of the magnetic field (B), that is,

Torque produced

The armature consists of several coils and to obtain continuous and maximum torque, slip
rings and brushes are used to conduct each coil at a position of α = 0. The performance of DC
motors can be described by the armature voltage, back electromotive force (EMF), and field
flux. There are four types of wound-field DC motors, depending on the mutual interconnection
between the field and armature windings as shown below :

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The steady-state equivalent circuit of the armature of a DC motor is shown below :

In series motors, the flux is a function of armature current, φ can be assumed to be

proportional to Ia.

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Combined Armature Voltage and Field Control : At the base speed, the armature voltage
reaches its rated value (equal to the source voltage) and cannot be increased further. To
further increase the speed, the field must be weakened with the increase in speed, then the
back EMF E and armature current constant maintained. The torque produces drops
parabolically with the increase in speed, and the output power remains constant, as shown
below :

Chopper Control of DC Motors : Choppers are used for the control of DC motors because
of their several advantages such as high efficiency, flexibility in control, light weight, small size,
quick response, and regeneration down to very low speeds. Normally separately excited DC
motors with open loop or closed loop
configurations are preferred in traction due to
the control flexibility of armature voltage and
field.
Class A Chopper for the control of DC motors :
This chopper provides only a positive voltage
and a positive current and hence it is called a
single or first quadrant chopper or Step down
chopper or DC-DC Buck Converter.

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The variable δ = ton/T is called the duty ratio or duty cycle of a chopper
ON period of the switch 0 ≤ t ≤ δT ………called duty interval

The switch is opened at t = δT

OFF period of the switch δT ≤ t ≤ 1 …… Freewheeling interval

Average value of the load voltage Va is given by

By controlling δ between 0 and 1, the load voltage can be varied from 0 to V.

The control technologies employed for control of switch S operation is into two categories as
follows :
1. Time ratio control (TRC) : It is also known as pulse width control, the ratio of on time to
chopper period is controlled by following methods :
a) Constant-frequency TRC: The chopper period T is kept fixed, and the ON period of
the switch is varied to control the duty ratio δ.
b) Variable-frequency TRC: Here, δ is varied either by keeping ton constant and
varying T or by varying both ton and T.
2. Current Limit Control (CLC) : It is also known as point-by-point control, δ is controlled
indirectly by controlling the load current between certain specified maximum and
minimum values. When the load current reaches a specified maximum value, the switch
disconnects the load from the source and reconnects it when the current reaches a
specified minimum value.
Class B Chopper or Step Up Chopper or DC-DC Boost Converter :

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During the on period, iS increases from iS1 to iS2, thus increasing the magnitude of energy stored
in inductance L. When the switch is opened, current flows through the parallel combination of
the load and capacitor C.

The main advantage of a step-up chopper is the low ripple in the source current, hence it is
suitable for low-power, battery-driven vehicles. Also step-up choppers are used in
regenerative braking of DC motor drives.

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Multiquadrants Control of Chopper-Fed DC Motor Drives
The separately excited DC
motor used in an electric or
hybrid electric vehicle has two
separate DC/DC converters
supplying the armature and
field windings from the same
energy source as shown in
figure.
The application of DC motors on EVs and HEVs
requires the motors to operate in multiquadrants as
shown below :
Two-Quadrant Control using Class C Chopper for
Forward Motoring and Regenerative Braking of
EV/HEV DC Drive : In Class C Chopper the self-
commutated semiconductor switch S1 and diode D1
constitute one chopper, and the self-commutator
switch S2, and diode D2 form
another chopper as shown below.

Both the choppers are controlled

simultaneously, both for motoring
and regenerative braking.
The operation of Class C Chopper is
described in below mentioned 4
steps:
1) In this circuit, freewheeling
occurs when S1 is off and the
current is flowing through
D1. This happens in interval

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δT ≤ t ≤ T, which is also the interval for which S2 receives the control signal. If ia falls to
zero in the freewheeling interval, the back EMF immediately drives a current through
S2 in the reverse direction, preventing the armature current from remaining zero for a
finite interval of time. Similarly, the energy transfer is present when S2 is off and D2 is
conducting—that is, during the interval 0 ≤ t ≤ δT. If the current falls to zero during this
interval, S1 conducts immediately because ic is present and V > E. The armature current
flows, preventing discontinuous conduction.
2) During the interval 0 ≤ t ≤ δT, the motor armature is connected through either S1 or D2.
Consequently, the motor terminal voltage is V, and the rate of change of ia is positive
because V > E. Similarly, during the interval δT ≤ t ≤ T, the motor armature is shorted
through either D1 or S2. Consequently, the motor voltage is zero, and the rate of change
of ia is negative
3) During the interval 0 ≤ t ≤ δT, the positive armature current is carried by S1 and the
negative armature current is carried by D2. The source current flows only during this
interval, and it is equal to ia. During the interval δT ≤ t ≤ T, the positive current is
carried by D1 and the negative current by S2.
4) From the motor terminal voltage waveform, Va = δV. Hence

When
δ V > E …….. Motoring Operation
δ V < E …….. Regenerative Braking Operation
δ V = E …….. Noload Operation takes place.
Class E chopper for Four-Quadrant Operation of DC Drive for EV / HEV
Four-quadrant operation is obtained by combining two class C choppers as shown below, and
it is called as a Class E chopper:

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In this chopper, if S2 is kept closed continuously, and S1 and S4 are controlled, a two-
quadrant chopper is obtained, which provides positive terminal voltage (positive speed) and
the armature current in either direction (positive or negative torque), giving a motor control in
quadrants I and IV (Forward Motoring and Reverse Braking).
Now if S3 is kept closed continuously and S1 and S4 are controlled, another a two-
quadrant chopper is obtained, which can supply a variable negative terminal voltage (negative
speed), and the armature current can be in either direction (positive or negative torque), giving
a motor control in quadrants II and III (Reverse Motoring and Forward Braking)..
This control method has the following features: the utilization factor of the switches is
low due to the asymmetry in the circuit operation. Switches S3 and S2 should remain on for a
long period. This may create commutation problems and restricts the maximum permissible
frequency of operation.

Induction Motor Drives for EV / HEV

Compared with DC motor drives, the AC induction motor drive has additional advantages such
as its lightweight, small volume, low cost, and high efficiency makes it suitable for for EV and
HEV applications. There are two types of induction motors, wound-rotor and squirrel-cage
motors. Because of the high cost, frequent maintenance and lack of sturdiness, wound-rotor
induction motors are less preferred for electric propulsion in EVs and HEVs.
Principle of operation of Induction Motor :

Each phase is fed with a sinusoidal AC current, which has a frequency of ω and a 1200
phase difference between each other. Thee phase supply currents ias, ibs, and ics in the three
stator coils a–a′, b–b′, and c–c′ produce alternative magnetic motive forces (mmfs) Fas, Fbs, and
Fcs, which are space vectors.

The resultant stator mmf vector

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The reaction between the rotating stator mmf and the rotor conductors induces a voltage in the
rotor and electric current in the rotor. In turn, the rotating mmf produces a torque on the rotor,
and hence the rotor rotates at a speed less than the synchronous speed.
ω = Angular velocity of the rotating stator mmf
mechanical angular velocity of stator

the relative speed between the stator rotating field and the rotor called slip speed is given by

NS  N
or S
Ns
The frequency of the induced rotor voltage

Steady State Performance of Induction Motor :

The per-phase equivalent circuit of an induction motor is shown below

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 10

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 11
Torque–slip characteristics of an induction motor :

Sm = Rated slip of induction motor

In the region of 0 <s < sm, the torque increases approximately linearly with the increase
of slip until reaching its maximum at s = sm, then it decreases with the further increase of the
slip.
At s = 1, the rotor speed is zero, and the corresponding torque is the starting torque,
which is less than its torque at s = sm.
The region of 0<s<1 is the forward motoring region.
In the region of s>1, the rotor torque is positive and decreases further with the increase
of slip, and the rotor speed is negative, which is the is reverse braking.
In the region of s < 0, that is, when the rotor speed is greater than the synchronous
speed, the motor produces a negative torque.
The speed–torque characteristic of a fixed-voltage and fixed-frequency induction motor is not
appropriate to vehicle traction applications. This is due to the low starting torque, limited
speed range, and unstable operation in the range of s>sm. Thus, for traction applications, an
induction motor must be controlled to provide proper speed–torque characteristics,

Constant Volt / Hertz or (V/f) Control of Induction Motor Drive of EV / HEV : In this
method the torque–speed characteristic of an induction motor is varied by simultaneously
controlling the voltage and frequency. The configuration of V/f control is shown below:

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Rated motor field current is given by

Xm and Lm are magnetizing reactance and inductance

To maintain a constant flux, the E/ω should be kept constant and equal to Erated/ωr

Equation 1 indicates that with constant E/ω, the maximum torque is constant with varying
frequency. Equation 2 indicates that sm ω is constant, resulting in constant slip speed, ωsl.

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 13

Power Electronic Control of Induction Motor for EV / HEV
A DC/AC inverter for obtaining a variable frequency and variable-voltage to feed the induction
motor is shown below :

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the frequency of the motor voltage can be changed by changing the frequency of the reference
voltage. Modulation index m is given by

A is the Amplitude of the reference sinusoidal voltage, Va, Vb, or Vc,

Am is the Amplitude of carrier voltage.

The fundamental voltage ‘Vf’ increases linearly with m until m = 1

For m >1, the number of pulses in Vao, Vbo, or Vco becomes less, and the modulation ceases to
be sinusoidal.
Field Oriented Control (FOC) or Vector Control of Induction Motor : The constant
volt/hertz control of the induction motor is suitable to motors with slow speed regulation. But
for fast speed varying and high performance FOC is preferred for EV and HEV propulsions. In
FOC independent control of the field and armature currents is carried out to achieve optimal
control of induction motor as shown below:

FOC of induction motor is similar to a separately excited DC motor in two aspects:

1. Both the magnetic field and the torque developed in the motor can be controlled
independently.
2. Optimal conditions for torque production, resulting in the maximum torque per unit ampere,
occur in the motor both in the steady state and in transient conditions of operation.

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Hall effect : The Hall-effect principle is named for physicist Edwin Hall. In 1879 he discovered
that when a conductor or semiconductor with current flowing in one direction was introduced
perpendicular to a magnetic field a voltage could be measured at right angles to the current
path.

Permanent Magnetic BLDC Motor Drives for EV / HEVs

Principle of operation : A BLDC motor drive consists mainly of the BLDC machine, the digital
signal processor (DSP)-based controller, and the power-electronics-based power converter, as
shown below.

The rotor position sensed by position sensors H1, H2, and H3 (hall effect sensors) information
is fed to the DSP-based controller, which in turn supplies gating signals to the power converter
by turning on and off the proper stator pole windings of the machine to control the torque and
speed of BLDC motor.
Classification of BLDC Motors :
a) Based on construction

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 16

b) Based on shape of back EMFs
i. Trapezoidal-shaped back EMF BLDC motor : its ideal characteristics are
 Rectangular distribution of magnet flux in the air gap
 Rectangular current waveform
 Concentrated stator windings
ii. Sinusoidal shaped back EMF BLDC motor : ideal characteristics are
 Sinusoidal distribution of magnet flux in the air gap,
 Sinusoidal current waveforms,
 Sinusoidal distribution of stator conductors.

Permanent Magnet Materials used in BLDC Motors : There are three classes of PMs
currently used for electric motors:
1. Alnicos (Al, Ni, Co, Fe).
2. Ceramics (ferrites), for example, barium ferrite (BaO × 6Fe2O3) and strontium ferrite
(SrO × 6Fe2O3).
3. Rare-earth materials, that is, samarium–cobalt (SmCo) and neodymium–iron–boron
(NdFeB).
Sensorless Techniques: Normally position sensors are expensive and have less reliability in
military applications etc., To overcome these drawbacks following sensorless techniques are
employed :
1. Methods using measurement of currents, voltages, fundamental machine equations and
algebraic equations
2. Methods using observers.
3. Methods using back EMF sensing
4. Unique sensorless techniques using Artificial Neural Networks (ANN), Fuzzy logics etc.,

Advantages of a BLDC motor : Several advantages of BLDC motor are as follows:

a) High efficiency due to the use of PMs for excitation, which consume no power, absence
of commutators and brushes and its losses.
b) Compactness due to small size and light in weight
c) Ease of control as control variables are easily accessible and constant throughout the
operation of the motor.
d) Ease of cooling due to absence of current circulation in the rotor and hence BLDC motor
does not heat up.
e) Low maintenance, great longevity, and reliability
f) Low noise emissions due to electronic control

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 17

Disadvantages of a BLDC motor :
BLDC motor drives also suffer from some disadvantages as follows:
a) Cost of PM is more
b) Limited constant power range
c) Safety as Large rare-earth PMs are dangerous which may attract metallic objects.
d) Magnet demagnetization due to large opposing mmf’s and high temperatures
e) High-speed capability is limited due to less mechanical strength of the assembly
between the rotor yoke and the PMs.
f) Inverter failures in BLDC motor drives due to short circuits in control.
Performance Analysis of BLDC Motor : A simplified equivalent circuit of one phase is shown
below :

The speed–torque performance with constant voltage supply is shown below. At low speed,
while starting, very high torque is produced, which results in very high current due to the low
back EMF and it may damage stator windings.

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With a variable voltage supply, the winding current can be restricted to its maximum by
actively controlling the voltage; thus, a maximum constant torque can be produced, as shown
below :

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 19

Switched Reluctance Motor (SRM) Drives for EV / HEVs
SRM is a doubly salient and singly excited machine and is suitable for variable-speed motor
drives due to its low cost, rugged structure, reliable converter topology, high efficiency over a
wide speed range, and simplicity in control. The SRM is capable of high-speed operation
without the concern of mechanical failures that result from high-level centrifugal force.

SRM drive system for EV / HEV consists of the SRM, power inverter, sensors such as
voltage, current, and position sensors, and control circuitry such as the DSP controller and its
peripherals, as shown below:

The SRM has salient poles on both the stator and the rotor. It has concentrated windings on

stator and no winding or PM on the rotor. The configuration of SRM depends on the number

and size of rotor and stator poles. Ex:8/6 SRM, 6/4 SRM etc.

Working Principle : 6/4 SRM and its supply arrangement is shown below:

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 20

The switching pulses applied to switches S1 to S6 is shown below:

When a stator phase is magnetized, a closed magnetic field is generated between the stator, the
air gap and the rotor. This magnetic field tends to
minimize the reluctance by reducing the air gap which
creates a rotor movement. When a stator pole is aligned
with a rotor pole, it is said that they are in the position
of minimum reluctance, and when they are completely
unaligned, it is said they are in the position of maximum
reluctance. This characteristic of the motor makes it
possible to create a rotational movement of the rotor by
magnetizing and demagnetizing each phase in the right
position of the rotor. A three phase 6/4 SRM cross
section is shown below. Phase number in conventional SRMs can be defined as

Ps 6
m  3
Ps  Pr 6  4
where
Ps = Number of Stator Poles
Pr = Number of Rotor Poles
The number of stator poles and rotor poles can be expressed as
Ps = k m
Pr = k (2m ± 2)
where

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 21

k = multiple of the basic combination of stator and rotor poles = 2
ignoring the mutual inductance between phases, the SRM with short-pitched winding (SCSRM)
produces torque based on the rate of change of the self-inductance is expressed as

where
La, Lb and Lc are the self-inductance of each phase
In three phase SRM, two phases are needed to be excited simultaneously in FCSRMs. The overlap
angle of stator poles and rotor poles is constant for various rotor positions, and that means the
self-inductances of phase windings are almost constant. The SRM with full-pitched winding
(FCSRM) produces torque based on the rate of change of the self-inductance is expressed as

where

Mab, Mbc and Mca are the mutual-inductance between two phases
The output torque of an SRM is the summation of the torque in all phases

The relation between the motor torque and the mechanical load is usually given by

where
J, B, and TL are the moment of inertia, viscous friction, and load torque, respectively.
The relation between position and speed is given by

Applications of SRM : Some of the applications of switched reluctance motor are as follows :
a) Used as adjustable-speed drives and general-purpose industrial drives
b) Application-specific drives like Compressors, Fans, Pumps, Centrifuges;
c) Domestic drives like Food Processors, Washing Machines, Vacuum Cleaners;
d) Electric vehicles application;
e) Aircraft applications;
f) Servo-drives.

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SRM Motor Characteristics: Some of the characteristics of SRM motor are listed below,
a) Synchronous operation.
b) Low-cost motor due to simple and magnet-free construction.
c) Rugged structure, reliable converter topology, high efficiency
d) More suitable for high-speed/power density applications.
Advantage of SRM : Several advantages of reluctance motor are listed below,
a) It does not require an external ventilation system as the stator and rotor slots projected.
The airflow maintained between the slots.
b) The rotor does not have winding since therefore no need to keep the carbon brush and
slip ring assembly.
c) Since the absence of a permanent magnet, such motors are available at a cheaper price.
d) A simple three or two-phase pulse generator is enough to drive the motor.
e) The direction of the motor can be reversed by changing the phase sequence.
f) Self-starting and does not require external arrangements.
g) Starting torque can be very high without excessive inrush currents.
h) High Fault Tolerance.
i) Phase losses do not affect motor operations.
j) High torque/inertia ratio.
k) High starting torque can be achieved.
Disadvantage of Switched reluctance motor : Several disadvantages of switched reluctance
motor are listed below,
a) Creates Torque ripple at high-speed operation.
b) The external rotor position sensor is required.
c) Noise level is high.
d) At a higher speed, the motor generates harmonics, to reduce this, we need to install
larger size capacitors.
e) Since the absence of a Permanent Magnet, the motor has to designed to carry a high
input current. It increases the converter KVA requirement.

SRM Drive Converter :

The input to the SRM drive is DC voltage, which is obtained from diode rectifiers or from
batteries. The torque developed by the motor can be controlled by varying the amplitude and
the timing of the current pulses in synchrony with the rotor position using following inverter
topologies or configurations for SRM drives:
a) Classic Half-Bridge Converter
b) R-Dump Converter;

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c) n + 1 Switches Converter Or Miller Converter
d) 1.5 n Switch Converter
e) C-Dump Converter.
a) Classic Half-Bridge Converter :
The configuration of the classic
converter is shown above and its main
advantage is the flexibility in control.
This half-bridge converter uses 2n
switches and 2n diodes for an n-phase
machine. All phases can be controlled
independently, which is essential for very high-speed operation., When the two switches S1
and S2 are turned on the DC bus voltage, Vdc, is applied to the phase-1 winding. By turning off
S1 and holding on S2, when the phase is energized, the current freewheels through the S2 and
D1. In this mode, phase-1 is not getting or giving energy to the power supply. When S1 and S2
are turned off, the phase-1 current flows through D2, the Vdc positive terminal, the Vdc
negative terminal, D1, and phase-1 winding. During this time, the motor phase is subjected to
negative DC bus voltage through the freewheeling diodes. The energy trapped in the magnetic
circuit is returned to the DC link.
b) R-Dump Converter : It’s a converter
configuration use fewer switches compared
to classic converter. This R-dump-type
inverter uses one switch and one diode per
phase. This drive is not efficient; during turn-
off, the stored energy of the phase is charging
capacitor C to the bus voltage and dissipating
in resistor R. Also, a zero voltage mode does
not exist in this configuration.
C) n + 1 Switches Converter or Miller Converter
An (n + 1) switch inverter is shown below. In
this inverter, all phases share a switch and
diode so that an overlapping operation
between phases is not possible, which is
inevitable in high-speed operations of this
motor.

18EE646 : Electrical Vehicle Technologies, Module – 3 Class Notes Page 24

d)1.5 n Switch Converter : This converter
eliminates the problem of overlapping
operation between phases by sharing
switches of each couple nonadjacent phases,
as shown below. Its operation is limited to an
even number of phases of SRM drives.

e) C-dump converter : It’s a popular

inverter configurations and is shown
below. It has the advantage of fewer
switches and allows independent phase
current control. In this configuration,
during the turn-off time, the stored
magnetic energy charges capacitor C, and
if the voltage of the capacitor reaches a
certain value ‘Vc’, it is transferred to the
supply through switch Sc. The main disadvantage of this configuration is that the negative
voltage across the phase coil is limited to the difference between the voltage across the
capacitor Vc and the system power supply voltage.
Modes of Operation : e = Back emf voltage, Vs = DC bus voltage
The speed at which e = Vs, is called as base speed.
a) Low Speed Operation:
If speed < base speed, e < Vs,
SRM operation at is low speed.
Maximum torque is available in
this case when the phase is turned
on at an unaligned position and
turned off at the aligned position,
and the phase current is regulated
at the rated value by hysteresis or
PWM control. The typical
waveforms of the phase current,
voltage and flux linkage of the
SRM below the base speed are
shown below.

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b) High Speed Operation : If speed > base speed, e > Vs, SRM operation is at high speed.
The back EMF increases with the rotor speed and it leads to a decrease in the phase current,
and hence, the torque drops as shown below. The phase current is limited by the back EMF
and maximum power of the SRM drive is almost constant. The typical waveforms at high-
speed operation are shown below. The advancing of the phase turn-on position is limited to
the position at which the phase inductance has a negative slope with respect to the rotor
position. If the speed of the rotor further increases, no phase advancing is available for
building a higher current in the phase and the torque of the SRM drops significantly. This
mode of operation is referred to as the natural mode operation.

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Generating Mode of Operation (Regenerative Braking)
The switched reluctance generator (SRG) is a singly excited machine, so to get power from it, it
should be excited near the rotor aligned position and then turned off before the unaligned
region

The driving circuit for Switched Reluctance Generator (SRG) is similar to that of SRMs as
shown below.

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When the switches are turned on, the phase gets energy from the supply and the capacitor.
During the turn-off period, the freewheeling current from the motor charges up the capacitor
and delivers energy to the load. Since there is no PM in this motor, during the start-up and
initial condition, it needs an external source such as a battery to deliver energy to the phase.
Depending on the output voltage during phase-on time, both the capacitor /external source
provide the current to the load and the phase coil.
In the generating region, the back EMF is negative, so it helps the phase to be charged very
fast; then during turn-off, the back EMF opposes the negative supply voltage, and it decreases
slowly:

In certain conditions such as high speed and high loads, the back EMF voltage is greater than
the bus supply voltage, so the current increases even after turning off the phase which needs a
oversized converter to limit uncontrollable torque. It results in additional cost and overall
sized system. Hence, the power electronic converter should be designed for the worst possible
case.

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Sensorless Control of SRM: The Excitation of the SRM phases needs to be properly
synchronized with the rotor position using shaft position sensor for effective control of speed,
torque, and torque pulsation. But position sensors complex and reduce the reliability of the
drive system. This drawback restricts SRM application in some specific environments, such as
military vehicles. To overcome these drawbacks the following sensorless control methods are
used.
1. Phase flux linkage-based method.
2. Phase inductance-based method.
3. Modulated signal injected methods.
4. Mutually induced voltage-based method.
5. Observer-based methods.

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