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Traction Motor Drive of Electrical Vehicle: Types, Performances and Control

Conference Paper · June 2022

DOI: 10.1109/EEAE53789.2022.9831309

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)

Traction Motor Drive of Electrical Vehicle:

Types, Performances and Control
Nikolay Djagarov
Electrical Department
Nikola Vaptsarov Naval Academy
Varna, Bulgaria
jagarov@ieee.bg

Abstract - The development, improvement and use of electric Electric vehicle can receive energy from various sources,
vehicles is one of the fastest growing areas of electrical engineer- and they must have a high energy density and power. Another
ing. This is due to their undoubted qualities and environmental requirement for the energy source is fast charging, long oper-
requirements. One of the most important elements of electric ation, less costs and maintenance. Batteries are the main
cars is the traction motor drive. The article reviews types, per-
formances and control drives. The main parameters, character-
source of energy for electric vehicles, largely determining the
istics of electrical drives are considered, indicating their ad- main characteristics of EV. The main efforts of researchers
vantages and disadvantages, their use in different types of cars. and manufacturers are aimed at developing high-performance
Particular attention is paid to the methods and means of control batteries of different physical and chemical basis and produc-
of electrical drives, which largely determines the control char- tion technology.
acteristics of the electric vehicle, their efficiency and their mile-
age. Flywheels are used as an energy accumulator. The fly-
wheel acts as an engine during storage. The recovery of the
Keywords — electrical vehicle, traction motor drive, traction kinetic energy of the flywheel is used to drive a generator to
motor types, mechanical and electrical gears, power electronic produce electricity. Modern flywheels have their own rotors
converter, motor drive control made of complex materials (carbon composites) located in a
vacuum chamber suspended by magnetic bearings.
I. INTRODUCTION
The development, improvement and use of electric cars is Supercapacitors (SC) use an ion-enriched liquid dielec-
one of the fastest growing areas of recently, electric vehicles tric. Charges are stored physically, providing high power den-
(EV) have become an increasing part of used cars. This is sity. Because there are no chemical reactions, supercapacitors
due to their advantages: reduced greenhouse gas emissions; have a long service life, but the lack of a chemical reaction
easy and effective control; high motor torque; energy recov- causes a low energy density.
ery in braking modes. All this led to the development of this Fuel cells (FC) use chemical reactions to produce electric-
type of transport. ity. Hydrogen is the fuel of this reaction, and the other ingre-
EVs consist of various subsystems that participate in the dient in the process of generating energy is oxygen. The elec-
process of power supply, propulsion and control of the vehi- tricity generated by the fuel cells goes to the traction electric
cle [7]. There are different types of EV, differing in: type of motor, and the excess energy accumulates in batteries or su-
power supply (batteries, capacitors, flywheels, fuel cells); percapacitors. The advantage of using FC is that it produces
used traction motors (electric and internal combustion motors its own electricity that does not emit carbon. Another great
for hybrids); number of motors and mechanical gear; power advantage is that recharging these vehicles takes the same
electronic converters; methods and means for control of trac- time it takes to refuel a conventional vehicle with a petrol
tion motors; methods and means for charging the batteries. pump.

Electric vehicles can be classified as: battery electric ve- The issues and problems of EV have been studied and
hicle (BEV); hybrid electric vehicle (HEV); plug-in hybrid published in numerous publications: articles, dissertations,
electric vehicle (PHEV); fuel cell electric vehicle (FCEV). monographs, which discuss various aspects of electric vehi-
Each of these types of EV has advantages and disadvantages, cles, their main subsystems and management [1-6].
there are problems that are being worked on and for which a Figure 1 shows the electric vehicle configuration and sub-
solution is expected. system. The system consists of two main subsystems: energy
Only BEV configurations will be analyzed in the article propulsion subsystem and energy source subsystem. The fig-
(Fig.1) [7]. In this type of electric vehicle there are no com- ure shows the main parts of the subsystems.
plex mechanical devices, only the motor is moving, and it is The purpose of the article is to review of traction motor
controlled by various methods and means. drive of electrical vehicle.
EV uses different schemes of mechanical gear between II. MECHANICAL GEARS
motors and wheels, using different numbers of electric mo-
tors and different mechanical connections. This variety The most popular options available for electric motors
causes different modes of operation and loads of traction mo- [8,9,10]:
tors and requires appropriate control.

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
• the electric motor is connected to the primary shaft of and allows different wheel speeds. When cornering, the inner
the gearbox (Fig.2.a); wheels move with a smaller radius, so the wheels rotate at
• the electric motor is connected to the axial differential different speeds.
of the drive axle (Fig.2.b);
Advantages of the electronic differential: simple without

Wheel
Brake
Energy Propulsion Subsystem

Power Electric Mechanical

Controller
Converter Motor Transmission

Accelerometer
Wheel

Fig.1. Electric vehicle configuration

Energy Energy and subsystem
Management Source
1 4 1 2
4
2 5 3
6 3

a b
Charger
Energy Source 1
2
Subsystem

Energy c
Fig.2.Tractionsystem
Fig.2.Traction
Fig.2.Traction systemlocation
system locationin
location in wheel:111–––wheel;
inwheel:
wheel: wheel;
Wheel;
Source
2 - electric
22 -- Electric motor; 3 - axle half;
Axlehalf;
electric motor; 3 - axle 4 - differential;
Differential;555---
half;44--differential;
• the electric motor is connected directly to the drive gearbox;
gearbox; 6 - gearbox
Gearbox;66--gearbox
Gearbox
wheels (Fig.2.c).
mechanical parts and adaptive control for specific driving
Each option has its advantages and disadvantages. The conditions; energy recovery in braking mode simultaneously
variant of Fig.2.a significantly increases the weight of the ve- by the four motors. But there are also problems with the sta-
hicle due to the transmission itself, the gearbox, the mechan- bility of movement associated with the need for symmetrical
ical differential and other parts. The advantage is that this distribution of torque on both sides. This is not enough when
traction system allows a wide range of choices between elec- changing the traction of the tires and the road, as the wheels
tric machines with series excitation. rotate at different speeds, which requires control of traction.
The variant of Fig.2.b significantly reduces the weight of The forces that the traction motor must overcome are the
the electric vehicle due to the lack of mechanical gear, gear- forces due to gravity (fg), wind (fwind), rolling resistance (frr)
box and other details. However, this method does not com-
pletely solve the problem with the power reserve for a quick
start. The advantage, as in the first option, is that this method
allows a wide range of choices among serial electric vehicles.

ref

_ ref.left
K1
 Fig.4. Diagram of the forces acting on the vehicle
=f(,V)
and inertial effect (fI) (Fg.4). The figure also shows: motor
_ ref.right traction (ft); normal force (fn).
K2
Traction force is described by the following two equations
left [11]:
V =(left+right)/2 right

Fig.3. Block diagram of an electronic differential

When the motors are placed in the wheels (Fig.2.c), there
is no gear, which causes the need for an electronic differential
(Fig.3). It provides the required torque for each drive wheel (1)

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
rotors, directly mounted on the outer rim of the wheel. Trac-
(2) tion has the highest transmission efficiency, but the charac-
teristics of the traction motor deteriorate.
where: Mcar - mass of the car; vcar – car speed; air - air density;
Cdrag - aerodynamic drag coefficient; Afront - front area. III. MOTORS
The traction motor converts electrical energy from the
The forces acting on the EV depend on the current road
battery into mechanical energy driving the vehicle. In braking
conditions, especially the coefficient of friction of the road
and the slope, and different methods of driving control are modes, the motor switches to generator mode and returns ki-
used depending on the road conditions [13]. netic energy to the battery. Different EVs have different num-
bers of motors. The requirements for them are: high power
The classic mechanical gear (Fig.5.a) has some important and torque, wide speed range, high efficiency, reliability,
disadvantages [12]: physical contact with friction and heat strength, low cost, low noise and small size.
loss; necessary lubrication; brushing teeth; fatigue of the ma-
The traction motor plays an extremely important role in
terial. The magnetic gear (MG) (Fig.5.b) eliminates these
shortcomings. The mechanical transmission has more than the development of EV. Different types of AC and DC trac-
two wheels, which reduces power density and efficiency. In tion motors are used for the vehicle propulsion (Fig.7) [31].
With the help of power electronic converters, a third class of
magnetic transmission, the main losses are in the magnetic
motors appeared – electronically switched machines (ECM).
circuit, and the mechanical losses (friction with air and bear-
ings) are the same as in the gears. The following types of propulsion motors are used in EV:
direct current t; permanent magnet brushless direct current;
induction; synchronous; permanent magnet synchronous;
brushless altering current; brushless direct current; switched
reluctance; double feed induction [20].

a b
Fig.5. The mechanical and magnetic gears
The principle of operation of the MG is based on the
modulation of the magnetic field created by the rotating mag-
netic poles of the high-speed rotor and the stator poles (fig.6).
The field of the fixed core will interact with the field of the
outer low-corrosion rotor and force it to rotate in the opposite
direction.
Outer rotor PM with
alternating
Fixed part polarity Fig.7. Different AC and DC traction motors for EVs
DC motors are low efficiency, high volume, low reliabil-
Inner rotor ity due to the commutator and brushes.
Brushed DC motors have permanent magnets (PM) in the
stator, and the rotors have brushes to supply the stator. These
motors provide maximum torque at low speeds. Disad-
vantages are their large volume, low efficiency, heating from
the brushes and the associated reduction in efficiency. These
shortcomings have led to the cessation of their use.
Steel The permanent magnet brushless DC motor (BLDC) has
teeth a permanent magnet in the rotor (most often NdFeB), and the
stator is powered by alternating current provided by an in-
verter. The lack of losses in the rotor makes it more efficient
Fig.6. Cross section of the main elements of than induction motors. This motor is lighter, smaller, with
an efficient magnetic gears better cooling, more reliable, has higher torque density and
There are different MG configurations: radial; in the form
specific power. But due to its limited ability to weaken the
of spokes; with surface mounting. Surface mounting has the
field, the range of operation with constant power is small.
best power density. The MG is usually attached to the inside
of the motor rotor. Integrated configurations are used to re- The torque also decreases with increasing speed due to the
duce passives elements, with the motor having an internal sta- back e.m.f.. Hybrid BLDCs with an additional excitation coil
tor and an external rotor. To the outer rotor is attached the are also used. Another hybrid motor is used, which is a com-
inner rotor of MG. With a distributed drive system (Fig.2.c), bination of reluctance motor and PM motor. By controlling
the thrust is created by several electric motors with external the power converter, the efficiency of PM BLDCs as well as

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
the speed range (up to 4bas) can be improved, although the reduce their efficiency and require cooling. These motors
efficiency decreases at very high speeds due to PM demag- have a lower power density as well as a limited constant
netization. PM provides a higher flow density in the air gap. power range.
The characteristics of PM BLDCs are shown in fig.8, where
the torque remains constant at the maximum, but begins to Constant Constant High
decrease exponentially for speeds above the base speed. torque power speed
Permanent magnet synchronous motors (PMSM) can op-
Torque
Stator
voltage
Stator
Motor torque [pu]

current Slip

base speed speed

Fig.9. Induction motor drive characteristics
Switched reluctance motor (SRM) (doubly salient motor)
Motor speed [pu] is synchronous motors driven by unipolar current inverter.
Fig.8. Characteristics of a permanent magnet These motors have robust construction, low cost, high speed,
brushless DC motor high reliability, high power density, wide range of constant
erate in a wide range of speeds without a gearbox. This fea- power, which is why they are used as traction motors in EV.
ture makes these motors more efficient and compact. These Disadvantages are: high noise due to variable torque; low ef-
motors are suitable for wheel placement as they provide high ficiency; large size and weight. Despite their simple construc-
torque at very low speeds. For PMSM with an external rotor, tion, their control is not easy due to the grooves and teeth
no rotor bearings are required. But their disadvantage is that (saturated) in their construction. Despite their shortcomings,
during operations in the wheel there are large losses in the interest in them is growing and modifications have appeared
magnetic circuit at high speeds, which reduces the stability. with the use of permanent magnets in them. Reduction of
PMSM has a high energy density due to the use of a perma- noise and torque ripples is achieved by a double stator wind-
nent NdFeB magnet. The flux in the air gap is sinusoidal, and ing, which provides superior torque density and increased
the motors are controlled by vector method. PMSM is the speed range compared to conventional SRM.
most used motor in battery electric vehicle (BEV). Synchronous reluctance motor (SynRM) rotates at syn-
chronous speed and combines the advantages of PM and
d asynchronous motors. SynRMs are robust and fault tolerant
like an induction machine, efficient and small like a PM mo-
P tor, and do not have the drawbacks of PM systems. Their con-
M trol is similar to the control of PM motors. Their disad-
q vantages are: complex control; low power factor; complex
production (rotor design). Improving the power factor can be
improved by increasing the polarity by axially or transversely
laminated rotor structures (fig.16).
PM assisted synchronous reluctance motor has a higher
Fig.10. SynRM with axially Fig.11. Permanent magnet power factor than SynRM by introducing PM into the rotor.
laminated rotor assisted SynRM Additional PMs in the rotor magneto-conductor increase ef-
Initially, EV used induction motors due to their proper- ficiency. Thus, in case of overload and high temperature,
ties: simple construction; low price; high reliability. With the there is no demagnetization as in IPM. Figure 17 shows a per-
advent of vector control and direct torque control, induction manent magnet assisted SynRM with permanent magnets em-
motors have become even more suitable for EV. By attenuat- bedded in the rotor.
ing the field, the speed range can be extended above the basic Axial flux ironless permanent magnet motor is the most
speed, while maintaining a constant power, and in vector con- modern for use in electric vehicles. There is an external rotor
trol the speed range can reach 3-5 pu. Figure 9 shows the me- without channels and without a magnetic conductor and the
chanical characteristics of an induction motor, where the stator also has no magnetic conductor. The stator core is also
maximum torque is kept constant up to the basic speed and missing, which reduces the weight of the machine. The air
then decreases exponentially. gap is of the radial type, which increases the power density.
Induction motors are cost-effective, robust and very easy The advantage of the motor is that the rotors are mounted on
to operate. But their low power factor increases the power of the sides of the wheels, placing the stator windings on the
the control inverter. Losses of eddy currents in the rotors

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
shaft. The lack of channels improves efficiency by minimiz- which is a magnetic circuit without windings or permanent
ing winding losses. magnets. This reduces the inertia of the rotor and increases its
acceleration. Their cooling is facilitated, as heat is released
mainly in the stator. Hybrid reluctance permanent magnet
motors are also used, which combine the advantages of both
types of motors.
Many types of traction motors are used in EV and each of
them has its advantages and disadvantages in terms of com-
pactness, efficiency, speed range and reliability [19]. Three
main types of motors are used in electric vehicles: induction
motors (IM), synchronous excitation motors and permanent
magnet motors.
The synchronous motor with control excitation has a
higher power factor. Due to the losses in the rotor, the motor
has lower efficiency and requires cooling of the rotor. Other
disadvantages are: the need to regulate excitation; presence
of contact rings and brushes that reduce reliability. Brushless
synchronous motors are used to overcome these shortcom-
ings.
Permanent magnet motors have maximum compactness,
power density and efficiency. Mostly open-pole motors with
internal permanent magnets are used, which create additional
torque and have increased power density. Most motors have
a distributed winding that minimizes harmonics of magneto-
driving force and losses. The main disadvantage is the cost of
Fig.12. Efficiency map of the electric machine permanent magnets and their sensitivity to increased temper-
Of all the types of motors so far, the most compact are atures. Increasing the number of pole pairs reduces the vol-
synchronous motors with excitation by permanent magnets. ume of the motor, but increases losses and heating.
The efficiency of the electric motor for different torque-speed When choosing a traction motor for EV, not only the qual-
characteristics can be seen in Fg.12 [11,12]. It can be seen ity/price criterion is taken into account, but also the mechan-
that the efficiency is highest at constant torque τs,cont and nom- ical and electromagnetic characteristics. The torque (acceler-
inal speed ns,nom. The figure also shows the angular velocity ator pedal) or rotation speed (cruise control) of the traction
ns,grad, nominal speed ns,nom om, maximum speed ns,max, con- motor is controlled. In both cases, the variables voltage, cur-
stant torque τs,cont, peak torque τs,max and the torque contour rent and frequency are measured and controlled. This is done
τs,limit. through the power electronic converter, ensuring the correct
Figure 13 shows the two types of arrangement of perma- operation of the traction motor.
nent magnets – inside and outside the rotor of PMSM.

Permanent
Stator magnet
Winding
Rotor
N
N

S a b
S
Fig.14. Synchronous motors rotors. (a) 2-pole cylindri-
Inside rotor Outside rotor cal, with field coils distributed in slots, (b) 2-pole sali-
“In-runner” “Out-runner” ent pole with concentrated field winding
Fig.13. Inside and outside rotor motors Synchronous motors with excitation by direct current sup-
The main requirements for EV electric traction motor plied by contact rings and brushes in the rotor or brushless
drive are: high initial torque, high power density and high ef- excitation by excitation machine on the same shaft and recti-
ficiency. Brushless DC (BLDC) and permanent magnet syn- fier in the rotor are used. The rotor can be salient -pole or
chronous (PMSM) motors have high starting torque, high ef- cylindrical (Fig.14).
ficiency and no mechanical commutators [8]. There are two
types of BLDC and PMSM motors: out-runner; in-runner. Brushless permanent magnet synchronous motors
The main advantages of the induction motor with a squirrel (BLPMs) have surface-mounted or grooved rotor magnets
cage rotor are that the rotor is not powered, has no permanent (Fig.15, 4-pole surface-mounted, 10-pole buried/internal
magnets and is more affordable. In recent years, reluctance types). The advantage is the lack of excitation system and
motors have appeared, having a simpler design, the rotor of

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
while the insulated DC-DC converters are suitable for EV
with low and medium power.
The three power converters use different conversion
schemes. We will consider only the power electronic convert-
ers used in the electric drive of the traction motor. The topol-
ogies used on these converters depend on the type of traction
motor powered.
Traction power converters can act as voltage sources or as
Fig.15. Permanent magnet synchronous motor rotors; current sources. Voltage converters are used more because
4-pole (left); 10-pole (right) they are more efficient than current converters and there is a
capacitor in the DC link that is smaller than the choke in the
higher strength, and the disadvantage is the inability to regu- current converters. The capacity of this capacitor smoothes
late the excitation and sensitivity to increased temperature. the pulsating currents generated by switching the switches.
Reluctance motor is the simplest synchronous motor of A four-quadrant power transducer is used to control the
all, and the rotor consists of a set of layers that align it with DC traction motor, allowing rotation in both directions with
the stator field. regenerative braking in both directions.
starting cage flux guide
Open-end winding is used for additional reservation
(Fig.18.a). This circuit has two inverters with two levels. This
converter operates at reduced power with an open or short
circuit. The converter has more switches and drivers, respec-
a b c tively, increasing the price but not increasing the power den-
sity. Reduction of current ripple in the DC connection can be
Fig.16. Reluctance motor rotors (4-pole)
achieved not by increasing the capacitance of the capacitor,
The rotor resembles the rotor of a square cage induction but by a segmented inverter of two inverters, shown in red
motor, with parts of its periphery cut off to force the flow and blue (Fig.19). The stator windings of the motor can coin-
from the stator to enter the rotor in other areas where the air cide in phase or be phase shifted.
gap is small (Fig.16.a). Alternatively, flow paths are provided
by removing part of the magnetic conductor in the rotor to A three-phase DC-AC converter or 3-phase inverter is
direct the flow in the desired path (Figures16 b,c). usually used at medium and higher power (> 5kW) [21,22].
Figure 20 shows two main three-phase inverters on three lev-
IV. POWER CONVERTERS els with three half-bridge inverter arms. The power of the
Figure 17 shows typical placements of different convert- converters can be increased by increasing the number of
ers in an EV, which show the directions of power flow and phases, which also improves the control of the inverter. Dif-
the corresponding modes of the traction motor [1]. The con- ferent topologies, control and modulation are used to reduce
version can be DC-DC or DC-AC, where the conversion of losses, electromagnetic interference, harmonics and valve
power and transmission depends on the mode of operation of voltage.
the vehicle and the traction motor.
Bridge converters require an isolated power supply for
The classification of EV converters can be done by topol- each main cell, while a multistage modular converter uses a
ogy, control, modulation and optimization. The configuration common voltage source. The advantage of multicellular to-
of EV converters can be categorized into two groups: non- pologies is their modularity, which simplifies design and
insulated and insulated [16]. The non-insulated converter is maintenance. However, it has a large number of components,
suitable for EV when operating at medium and high power, complex voltage balance of the capacitor complex control.

Acceleration

Motor
Drives Motors
Utility
Plug Braking

Braking Recharging

AC/DC Rectifying DC/AC Energy

Converter Inverting Converter Storage

Acceleration Discharging

Fig.17. Typical placements of different converters in an EV

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)

(a)

(b)

Fig.20. Three-phase three-level inverter topologies:

(a) VSI topology; (b) CSI topology
motor, most often a two-level voltage inverter (VSI) simple
design, robust and easy to operate [19]. Switching losses and
inverters reduce losses and dv/dt.

Fig.18. Three-level inverter topology.

(a) dual inverter with equal voltage ratio.
(b) Three-level NPC inverter

Fig.21. Classification of multilevel inverter

Multistage inverters use switches with lower parameters,

use higher switching frequencies, reduce harmonics. Differ-
ent topologies with different characteristics are used (Fig.21).
Their main drawback is their complexity.
Multicellular inverter topologies use a basic cell with sev-
eral configurations for the required number of output voltage
levels, as shown in Fig.22. Modular and multicellular con-
verters provide redundant power supply, reduce harmonics,
increase the power of the converter with limited component
parameters. The construction of these converters is done by:
parallel and series connection of converters and components;
multi-level converters.
Increasing the maximum amplitude of the output voltage
Fig.19. Changing from a standard two level at fundamental frequency is done by overmodulation, which
VSI drive to the segmented drive system creates low-order harmonics and is therefore rarely used.
Such an increase in amplitude can be obtained by modified
modulation based on the injection of zero-sequence signals
A four-quadrant power transducer is used to control the
DC traction motor regenerative braking in both directions.
Different topologies are used to control the AC traction

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)

b
a

Fig.22. Multicell multilevel inverter topologies

(a) cascaded H-Bridge, (b) flying capacitor,
(c) modular multilevel converter
modulation with level shift. This allows to obtain a multi-
stage form of the output voltage of the inverter.
Control systems are crucial for the proper operation of
EVs. They provide good ride quality, sufficient power re-
quired, assessment of available energy from on-board sources
and their proper use to cover the maximum distance, short
charging without interference in the charging network.
V. TRACTION DRIVE CONTROL
A. EV control systems
The main requirements for the traction electric drives of
EV are [32]: 1. High density and instantaneous power; 2. high
torque at low speeds and high power at high speeds; 3. Very
wide speed range at constant torque and constant power; 4.
Fast torque response; 5. High efficiency in a wide range of
speed and torque; 6. High efficiency of regenerative braking;
7. High reliability and strength; 8. Low price.
Driving control such as traction control, cruise control
and different driving modes in EV is more efficient, as EV
driving forces are easier to control and require less refor-
mation between mechanical and electrical areas.

c
a
Conventional PWM methods for DC/AC converters: a
square shape modulation method that maximizes DC voltage,
generating a curve shape with a maximum modulation index
(ratio between output voltage peak and DC voltage value) for
account of many low-order harmonics. This method of mod-
ulation is used very rarely. PWM-based methods are used to
improve the shape of the output voltage.
Converter/drive control methods can be classified into the
following groups: hysteresis (current control, direct torque
control, direct power control); linear control (PI-based con-
trol, field-oriented control, voltage-oriented control); sliding b
mode (current control, voltage control); predictive control
(deadbeat, model predictive control, generalized predictive
control; artificial intelligence (fuzzy, neural networks, neuro-
fuzzy.
PWM methods are used to modulate multi-stage VSI
through multiple carrier frequencies (Fig.23). Depending on Fig.23. (a) Level-shifted PWM scheme for a seven-level con-
the order of the carrier frequencies, these methods are divided verter. (b) Harmonic spectrum of the obtained output voltage
into two categories: modulation with phase shift and

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
When the car is moving, it is affected by different forces,
in different directions, with different intensity of change de-
pending on the driving and driving conditions (Fig.24) [32].

Fig.25. Limited performance of the vehicle force

and moment
Figure 25 shows the limiting characteristics of the forces
and torques acting on EV [33].
Traction control quickly changes torque to prevent the tire
from slipping due to an unexpected drop in traction. To this
Fig.24. Potential benefits via system integration end, the controller must have the following characteristics
The dynamics of EV movement is in three directions, lat- [38]: 1. The motor torque must respond to the acceleration
eral, longitudinal and vertical, and the corresponding con- command quickly and accurately; 2. When the wheel slips,
trols: steering, drive/brake system, suspension. Each of these the torque must be reduced quickly for effective re-engage-
areas has a different impact on EV behavior in terms of ment.
safety, driving, driving dynamics or economy [32]. The over- The complexity of all-wheel drive (AWD) is close to the
all dynamics of the car are improved by integrating these sub- point where the drive torque for each wheel can be inde-
systems. pendently controlled [39]. This improves vehicle handling,
The integrated control of EV dynamics is for: 1. Avoid- similar to those provided by dynamic stability control, but
ance of conflicts and interventions between different subsys- without the inevitable reduction in vehicle acceleration.
tems; 2. Use of each subsystem through communication and Therefore, the independent control of the AWD torque distri-
coordination between them. Integrated control is classified bution is particularly useful at accelerations close to the sta-
into two categories: 1. Bottom up - decentralized control bility limit.
structure; 2. From top to bottom. - centralized control struc- The main goal is to improve traction and handling. This
ture. is most often achieved with passive systems that respond to
In a decentralized control structure, each subsystem is differences in wheel speed or wheel torque responses. Active
somewhat independent, but can interact with other subsys- devices that monitor wheel speeds and throttle position con-
tems to achieve a specific function. This is very limited inte- trol wheel slip and thus improve traction. The most sophisti-
gration as it simply integrates sensors and hardware. cated also respond to vehicle control parameters such as
In a centralized control structure, the global controller has steering angle, deflection speed and lateral acceleration to
control inputs to all subsystems. The centralized structure control dynamic stability.
provides better management characteristics. It requires a
more powerful central controller and a more complex open For active torque differentials, the rear axle torque is di-
control structure, allowing for changes, additions and adapta- rected to the outer wheel to improve cornering stability. Dou-
tions. ble clutches are used, which independently control the torque
A multilayer control structure is used. The coordination to each wheel on one axle. What these differentials have in
controller has two main functions: 1. Depending on the dy- common is a higher degree of freedom in torque distribution.
namics of the EV, the controller determines the required con- Completely independent torque distributions have also been
trol forces or torques, then distributes them to each subsystem developed in which the Dynamic Stability Control (DSC)
(for example, tire slip ratio or side slip angle). tire). The con- system stops the individual wheels to control deflection speed
troller then transmits these control forces and control torques and lateral acceleration.
to the layer controllers; 2. The coordination controller may When cornering EV, an electronic differential is used,
switch the subsystems to operate in different modes (for ex- which controls the torque of the individual motors in the
ample, to switch the suspension control between tire grip or wheels. In this way, depending on the speed and trajectory, the
ride comfort). corresponding reference signals for the required torques of
The integrated control system distributes and coordinates each wheel are calculated. Figure 26 shows the structure of
several drives and harmonizes their joint work. The main sys-
tems for controlling the longitudinal and lateral dynamics are
the braking/traction and steering systems (ABS, ESP and
AFS / 4WS).

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)

Fig.26. Structure of vehicle in curve

vehicle in curve with the corresponding parameters, which are 1. Signal injection;
used to calculate the reference signals [34]. 2. Machine model reference:
• Open loop estimator;
B. Mathematical methods of control • Closed loop estimators:
The proportional-integral law of control is the most o Model reference adaptive system (MRAS):
widely used. The precise setting of the PI controller (K p pro-  Stator current;
portional gain and Ki integral gain) improves the control char-  Rotor flux;
acteristics. The methods used to set the controller parameters  Back e.m.f.;
are [48]: 1. The traditional methods of Ziegler-Nichols and
 Reactive power.
Cohen-Coon; 2. New methods using intelligent techniques
o Artificial intelligence (AI) methods:
such as genetic algorithms, fuzzy logic and neural networks.
 Neural networks;
Ziegler and Nichols offer two heuristic approaches based  Fuzzy logic;
on experiments and simulations to adjust the parameters of P,  Genetic algorithms.
PI and PID controllers. The first method requires recording
the reaction of the step in an open cycle, while the second • Luenberger observer;
requires bringing the system to the limit of stability. • Sliding mode observer;
3. Kalman filter (KF) estimators
Cohen-Coon proposed a tuning method using an empiri- • Extended Kalman filter (EKF);
cal model of the process, to reach a quarter-attenuation ratio
• Unscented Kalman filter (UKF).
to obtain a closed-loop response, similar to the method of
Ziegler and Nichols. Lower to higher order observers are also used, as well as
Control by reference model: a reference model is used, to variable structure or artificial intelligence. All model-based
which the control signals are submitted. The difference be- circuits are sensitive to changes or incorrect parameter set-
tween the output signals of the model and the controlled ob- tings [36].
ject is fed to the actuator. The use of MRAS methods for estimating motor speed is
The control of the sliding mode brings the path of the sys- used in sensorless control due to their simplicity, efficiency
tem state to the sliding surface and its switching with the help and low computational complexity. These circuits include ro-
of appropriate switching logic around it to the point of equi- tor current, reactive power, back-e.m.f. and stator current.
librium, whence comes the phenomenon of sliding. Kalman and Luenberger observers are used in electric drives.
Observer KF evaluates the rotor current using a reduced order
C. Estimation of motor speed model of EKF to estimate the rotor resistance. Sliding mode
control (SMC) has less sensitivity to machine parameters and
The speed of rotation of the motor is estimated by the external interference and provides fast transient response.
power of the electric drive model using the following meth-
ods [35,36]:

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
These AI-based methods reduce controller settings and E. The classification of frequency methods
improve resistance to sensitive motor parameters and achieve
for control of electric drives [20-29]
high dynamic performance with low computing capacity.
Main characteristics of their use in electric drives: 1. Im- 1. Scalar:
proves the characteristics of the conventional controller and • U/f=constant;
prevents the influence of the change of the parameters on the • Frequency, pole number change.
control; 2. The system is easily expanded and modified; 3. 2. Vector control:
Used for diagnosis and evaluation of condition parameters; 4. • Field oriented:
Optimizes its efficiency and design. o Rotor field orientation;
Signal injection (SI) methods have low sensitivity to mo- o Stator field orientation;
tor parameters and good performance at low and zero speeds. o Air gap orientation:
They inject a low-amplitude but high-frequency signal to the  Direct;
stator windings, which does not affect the operating mode.  Indirect.
The injected signals are modulated by the orientation of the • Direct torque control;
motor asymmetries and after processing and demodulation
the necessary parameters are obtained. High frequency SI • Nonlinear control;
methods are very complex and there is no common method • Forecasting model control.
for their design. In addition, they create torque ripples and Scalar control. This control is easy to implement and pro-
acoustic noise. vides stable regulation, but has a relatively low speed. The
principle of control is simple, the amplitude is a function of
D. Speed-torque characteristics the frequency, at which the voltage-frequency ratio is con-
Optimal speed-torque characteristics with operating stant. The specific dependence depends on the load on the
restrictions (acceleration, mileage, maximum speed) and motor shaft, maintaining a constant magnetic flux in the air
minimum power can be achieved when operating at constant gap. The scalar control maintains a constant torque regardless
power. Therefore, the motor, drive and steering must provide of the frequency, but at low speeds it decreases, which can be
constant power. avoided by increasing the U/f ratio, so each motor has its own
operating range.
Vector control. It regulates not only the voltage and fre-
quency, but also the angle of the stator current generated with
respect to the rotor field. Different methods are used for this
control, providing different modes of operation and interfer-
ence effects. The flux (d-axis) and torque (q-axis), which are
perpendicular, are controlled independently. A key point is to
determine the position (angle) of the rotor field, depending on
the method of determining which vector control is direct
(with sensors) or indirect (without sensor).
In static mode, the operation of the motor depends on the
values of the projections of the stator current isd and isq, i.e.
on the flux component and the torque component. Figure 28
[50] graphically shows these dependences in the isd-isq plane,
Fig.27. Speed-torque characteristics of EVs in which the voltage limit is an ellipse and the current limit is
from electric traction motor [36] a circle that does not depend on the operation of the motor,
and the ellipse becomes smaller with increasing angular fre-
Figure 27 shows the characteristics of torque-speed EV, quency. And the curve with constant torque is hyperbola.
which operates in the region with constant torque at low This graph shows the correspondence between the speed re-
speed to nominal speed [45]. There is optimal torque in this gions and the values of the stator current vector (Fig.28) [50].
range when power increases with speed. Beyond the nominal
speed, the power is limited and thus the torque decreases with To satisfy the current and voltage, the current vector must
increasing speed. This is due to back e.m.f., which exceeds be inside the common area of the ellipse and the circle. When
the maximum output voltage of the inverter and requires at- working in region I, the speed is lower than the base speed
tenuation of the field. EVs are required to have high efficien- and the ellipse is larger than the circle. The operating modes
cies over a wide range of operating speeds, with a constant are represented by points on the section O-A. At point A the
operating power range of about 3 to 4 times the base speed. current isd=isq, rated, therefore an increase in torque is ob-
They must also have a high torque density and a high over- tained only by increasing isq along the section A-B, while the
load capacity without compromising high fault tolerance and amplitude of the stator current is equal to I s, max at point B
strength. High torque is essential for starting at low speeds corresponding to the maximum torque moment.
and climbing a hill, while high power is useful for a high- At high speeds, the operating point corresponding to the
speed cruise. maximum torque is moved from B to C while the stator cur-
rent remains constant. Point C corresponds to the maximum
speed without reducing the current. The points of the arc BC

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
correspond to the second area. If the reverse e.m.f. is too high,
maximum current cannot be applied and the motor enters a
third area (section C-O).

Fig.28. Representation of the motor operating conditions

in the plane isd-isq
The main disadvantage of vector control is the presence
Fig.29. Braking force distribution
of nonlinearities in the equation of rotation of the motor when
changing the flux coupling of the rotor. When the motor is position encoder) with those of spatial vector modulation
powered by a current inverter, the model becomes more com- (low current distortion, fixed switching frequency).
plicated. This disadvantage is overcome by using non-linear F. EV braking modes
control (control by feedback linearization, control with I / O
decoupling, control with multi-scalar model). In the case of The controllable hybrid braking system controls the
power supply from a voltage inverter and in the case of a con- braking forces on the front and rear wheels following the
stant rotor current coupling, the nonlinear control is equiva- ideal brake force distribution curve (Fig.29), achieving
lent to the vector control. In many other cases, nonlinear con- optimum brake efficiency [40].
trol simplifies the system and increases performance.
Direct torque control (DTC). This method increases the
speed of vector speed control, in which the pulse width mod-
ulation (PWM) for inverter control is replaced by hist-resist
control of the inverter valves. There are several methods for
direct torque management: switching table; direct self-gov-
ernment; state vector modulation and constant switching fre-
quency.
DTC torque ripples depend on the width of the hysteresis
band. Even with a small frequency band, there are pulsations
of torque due to the discrete nature of the hysteresis control-
ler. Very small bandwidth values increase the inverter switch-
ing frequency and switching losses. The use of space vector
modulation (SVM) maintains a constant switching frequency,
which reduces pulsations and switching losses. Fig.30. EV front and rear force distribution

The principle of SVM is to predict and calculate the volt- The required total braking force is distributed between the
age vector. It is based on every three adjacent vectors in each front and rear wheels. However, a mechanical braking force
sector for a two-stage inverter. The application time for each (point b) is also applied at a higher required force than that
vector can be obtained by vector calculations, and the rest of produced by the electric motor, thus following the I-curve
the period will be spent by applying a zero vector. (Fig.30). At low speeds, it is difficult for the electric motor to
produce braking torque due to the low electric movement of
The DTC using space vector modulation SVM includes a the motor, which is why the mechanical brake is applied.
direct estimated calculation of the voltage required to regulate
the stator current and torque. It combines the best features of
direct torque control schemes (fast torque response, without

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 2022 8th International Conference on Energy Efficiency and Agricultural Engineering (EE&AE)
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