effective method of electric braking for traction application_v5_may8_draft
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EFFECTIVE METHOD OF ELECTRIC BRAKING FOR
TRACTION APPLICATION
A PROJECT REPORT
Submitted in partial fulfillment of the Requirement for the award of the
Degree of
MASTER OF TECHNOLOGYin
Power Electronics and Drives
By
Jayakrishnan V K
12MPE0001
Under the Guidance of
Prof. Thirumalaivasan R Assistant Professor, SELECT, VIT University
&
Mr. Shunmugavel MadasamySenior System Engineer, HTS
Mr. Muthukumar MurthySystem Engineer, HTS
SCHOOL OF ELECTRICAL ENGINEERING
VELLORE INSTITUTE OF TECHNOLOGY (University)
VELLORE. (TN) 632014
(May 2014)
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EFFECTIVE METHOD OF ELECTRIC BRAKING FOR
TRACTION APPLICATION
A PROJECT REPORT
Submitted in partial fulfillment of the Requirement for the award of the
Degree of
MASTER OF TECHNOLOGYin
Power Electronics and Drives
By
Jayakrishnan V K
12MPE0001
Under the Guidance of
Prof. Thirumalaivasan R Assistant Professor, SELECT, VIT University
&
Mr. Shunmugavel MadasamySenior System Engineer, HTS
Mr. Muthukumar MurthySystem Engineer, HTS
SCHOOL OF ELECTRICAL ENGINEERING
VELLORE INSTITUTE OF TECHNOLOGY (University)
VELLORE. (TN) 632014
(May 2014)
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CERTIFICATE
This is to certify that the Project work titled “Ef fective Method of E lectri c
Braking for Traction Appli cation ” that is being submitted by Jayakrishnan V K is in partial
fulfillment of the requirements for the award of Master of Technology, is a record of
bonafide work done under my guidance. The contents of this Project work, in full or in
parts, have neither been taken from any other source nor have been submitted to any other
Institute or University for award of any degree or diploma and the same is certified.
Mr. Shunmugavel Madasamy Prof. Thirumalaivasan R
Senior System Engineer Assistant Professor
Honeywell Technology Solutions Lab Vellore Institute of Technology
The thesis is satisfactory / unsatisfactory
Internal Examiner External Examiner
Approved by
Director
(SCHOOL OF ELECTRICAL ENGINEERING)
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ACKNOWLEDGEMENTS
My work and life for the past 8 months has been exciting and memorable. I am thankful to
numerous people in my life for their continuous support, encouragement, help and assistance
that helped me in the completion of this thesis.
First of all I would like to thank VIT University and Honeywell Technology Solutions Labs
Pvt. Ltd for the opportunities that they provided for the completion of my dissertation.
I would like to express my gratitude to Dr. G Vishwanathan, Chancellor, VIT University for
the excellent infrastructure and academic facilities. I express my sincere thanks to Dr. Partha
Sharathi Mallick, Dean, School of Electrical Engineering for his support during the entire
course. I would like to express my gratitude to Dr. Rajasekar N, Division Chair, Power
Electronics and Drives for his continuous support, encouragement and advice.
I would like to specially thank Prof. Thirumalaivasan R for his guidance, encouragement and
timely advice during the entire course of my project work. I would like to thank all the
professors of VIT University for their valuable feedback and advice they gave during project
reviews and discussions.
I express my sincere gratitude to Mr. Shunmugavel Madasamy and Mr. Muthukumar Murthy
for their continuous support, help, patience and understanding throughout the period of my
internship in Honeywell. I cannot express in words how thankful I am for the precious time
that they spent with me and helped motivate me.
I would like to thank my family for supporting and encouraging me in pursuing my degree.
Without their support I would not have been able to complete my degree.
Finally I would like to thank all my friends for providing a good atmosphere, their advices,encouragement and help for which I shall be ever grateful.
Jayakrishnan V K
Reg. No. 12MPE0001
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ABSTRACT
Electric drives for traction applications are one of the most promising technologies
that can lead to significant improvements in vehicle performance. Recent advancement of
smart power switching element leads to control electric drives actively for all the four
quadrants operations.
The traditional method of mechanical braking causes a lot of energy wastage as
unwanted heat, wear and tear etc. In case of electric braking is achieved by dissipating/
storing energy by conversion into electrical energy. Electric braking provides us with an
efficient way of braking which can aid the mechanical brake.
This proposal is to study the effective method for electric braking mode of a PMSM
drive with minimum or no changes in control hardware topology. The energy dissipation will
be controlled by using inverter, stator windings and cable impedance. This method
maximizes the system braking efficiency without additional braking unit.
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TABLE OF CONTENTS
LIST OF FIGURES ................................................................................................................... 4
LIST OF TABLES ..................................................................................................................... 5
LIST OF SYMBOLS ................................................................................................................. 6
1. INTRODUCTION................................................................................................................ 7
1.1 Electric Machines .............................................................................................................7
1.2 Electric Traction ...............................................................................................................8
1.3 Braking .............................................................................................................................9
1.3.1 Regenerative Braking ..............................................................................................10
1.3.2 Dynamic Braking .....................................................................................................11
1.4 Necessity of the System .................................................................................................11
1.5 Literature Review ...........................................................................................................11
1.6 Objective ........................................................................................................................14
1.7 Overview of Thesis ........................................................................................................14
2. THE PMSM DRIVE SYSTEM......................................................................................... 16
2.1 Permanent Magnet Synchronous Motor .........................................................................16
2.1.1 Permanent Magnet Materials ...................................................................................16
2.1.2 Classification of Permanent Magnet Motors ...........................................................17
2.1.2.1 Direction of Field Flux ...................................................................................... 17
2.1.2.2 Flux Density Distribution .................................................................................. 18
2.1.2.3 Permanent Magnet Radial Field Motors ........................................................... 18
2.1.3 Position Sensors .......................................................................................................19
2.1.3.1 Optical Encoders ............................................................................................... 19
2.1.3.1.1 Incremental Encoders ................................................................................. 20
2.1.3.1.2 Absolute Encoders ...................................................................................... 20
2.1.3.2 Position Resolvers ............................................................................................. 21
2.1.3.3 Hall Sensors ...................................................................................................... 21
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LIST OF FIGURES
Figure 1.1: Traction System ....................................................................................................... 8
Figure 1.2: Four Quadrant Operation......................................................................................... 9
Figure 1.3: Block Diagram for Regenerative Braking ............................................................. 10
Figure 1.4: Block Diagram for Dynamic Braking ................................................................... 11
Figure 2.1: PMSM Drive System ............................................................................................ 16
Figure 2.2: B-H Curve for Different Permanent Magnet Materials ......................................... 17
Figure 2.3: Surface Mounted Permanent Magnet Motor ......................................................... 18
Figure 2.4: Interior Permanent Magnet Motor ......................................................................... 19
Figure 2.5: Optical Encoder ..................................................................................................... 19
Figure 2.6: Incremental Encoder .............................................................................................. 20
Figure 2.7: Absolute Encoder .................................................................................................. 20
Figure 2.8: Position Resolver ................................................................................................... 21
Figure 2.9: Hall Sensors ........................................................................................................... 22
Figure 2.10: Signals from Hall Sensor for One Cycle ............................................................. 22
Figure 2.11: Single Phase Inverter ........................................................................................... 23
Figure 2.12: Three Phase Inverter ............................................................................................ 23
Figure 3.1: Hall Sensor Timing Diagram ................................................................................ 28
Figure 3.2: Operation of Six-Step Commutation ..................................................................... 29
Figure 3.3: Open Loop Block Diagram ................................................................................... 30
Figure 3.4: Closed Loop Block Diagram ................................................................................. 30
Figure 3.5: Current Flow Path for Third Winding ................................................................... 32
Figure 4.1: Phase Currents for Six-Step Commutation with Pulses ........................................ 33
Figure 4.2: Diode Voltage and Currents with Pulses ............................................................... 34
Figure 4.3: MOSFET Voltages and Currents with Pulses ....................................................... 35
Figure 4.4: Diode and MOSFET Currents with Pulses............................................................ 36
Figure 4.5: Switching Pulses to Inverter for One Cycle .......................................................... 37
Figure 4.6: Pulses for Third Winding Method Applied to Phase A ......................................... 38
Figure 4.7: Phase Currents with Third Winding Method Applied to Only Phase A ............... 39
Figure 4.8: Speed Waveform for Third Winding Method Applied to Phase A ....................... 39
Figure 4.9: Switching Pulses for PMSM Machine with Third Winding ................................. 40
Figure 4.10: Speed for Different Duty Ratio Provided to Third Winding ............................... 41
Figure 4.11: Torque Waveform for Different Duty Ratios ...................................................... 41
http://c/Users/e838870/Documents/Braking/Docs/Report/Effective%20Method%20of%20Electric%20Braking%20for%20Electric%20Traction%20Application_v5_may8_draft.docx%23_Toc387313839http://c/Users/e838870/Documents/Braking/Docs/Report/Effective%20Method%20of%20Electric%20Braking%20for%20Electric%20Traction%20Application_v5_may8_draft.docx%23_Toc387313839http://c/Users/e838870/Documents/Braking/Docs/Report/Effective%20Method%20of%20Electric%20Braking%20for%20Electric%20Traction%20Application_v5_may8_draft.docx%23_Toc387313839
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Figure 4.12: Phase Currents with Third Winding Method Applied to All the Phases ............ 42
LIST OF TABLES
Table 2.1: Device Switching and Power Capability ................................................................ 24
Table 2.2: Switching Scheme for VSI ..................................................................................... 24
Table 3.1: Hall Position Information and Switching Sequence ............................................... 28
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LIST OF SYMBOLS
A – Phase A
B – Phase B
C – Phase C
α – Alpha Axis
β – Beta Axis
q – Quadrature Axis
d – Direct Axis
VDC – DC Link Voltage
Ld – Direct Axis Inductance
Lq – Quadrature Axis Inductance
N – Rotor Speed
R s – Phase Resistance
P – Pole Pairs
Te – Electromagnetic Torque
θ – Rotor Angle
ωr – Angular Speed
λ m – Permanent Magnet Flux
B – Viscous Friction Coefficient
J – Moment of Inertia
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1. INTRODUCTION
1.1 Electric Machines
In our everyday lives we use lots of electric machines (EM) even without noticing.CD and DVD players, hard disk drives, fans, air conditioning systems, vacuum cleaners,
washing machines, refrigerators, mixer and grinders, vibration system on cell phones and
electric windows in cars are some examples of EM in our day to day life. Smaller EM can be
found in electric wristwatches, while bigger EM can be found in power generation plants,
wind turbines, industrial processes or different types of transportation systems such a trams,
trains, or electric cars.
The popularity of electric machines is due to its advantages: low maintenance
requirements, low weight, compact size, clean installation, quiet operation and high
efficiency (up to 98%, vs. the internal combustion engines that give up to 40%), zero
emissions, wide range of operation (from a few watts to hundreds of MW), high speed range,
high power and torque density, full torque availability even at low speeds and good control
characteristics.
Traditionally, electric machines were designed to be used in industry, mostly on
steady state, i.e., they would always work in the same operating point. However, EMs
designed for certain specific applications, like traction, need to change its behaviour
according to the demand of speed and power. An electric machine cannot be operated at any
speed-torque combination we want. The operating range is limited by the thermal, electrical
and mechanical characteristics of the machine.
Thermal aspects limit the maximum current that can pass through the motor windings
due to the heat generated in the windings by the resistive losses. On the other hand, DC link
voltage and mechanical considerations limit the maximum speed of the machine. The voltage
induced in the stator windings is proportional to the time derivative of the flux that links
them, higher the rotor speed, faster the variation of the stator windings’ flux linkage and
hence higher the induced voltage. The maximum voltage that can be modulated in the
inverter is limited by the DC-link voltage. So in order to control the motor, it has to be
ensured that inverter modulated voltage is less than the DC link voltage all the time, either by
limiting the maximum rotational speed or using field weakening methods. Bearing losses
depend on the speed of rotation, so higher the speed, higher the losses are. Besides, there is
the possibility for the occurrence of mechanical resonance at high rotational frequencies,
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which may lead to instabilities that can cause damage to the machine. In order to operate the
machine safely, its operational limits must be known.
1.2 Electric Traction
Last decade has seen an unprecedented growth of adjustable speed drives that
provides us with a variety of advantages – from process performance improvement to power
savings. All these can be attributed to the developments in power electronics and micro-
electronics which have enabled us in achieving higher efficiency and better power savings.
An electromechanical system that converts electrical energy to mechanical energy of
the load being driven is called an electric drive. The load can be a conveyor belt, traction
motors etc. The functional block diagram for a traction system incorporating electric drive is
as show in figure 1.1. It comprises of a motor M that drives the traction system such as
vehicle, train etc. through a mechanical transmission (gear, gearbox). It includes a power
converter and a control system that helps in achieving the desired performance of the traction
system. The power converter transforms the grid electrical energy to the motor supply energy
in response to the set point speed or path command. Motor is an electromechanical converter
that converts supply energy to the electromagnetic energy of the air gap and then to the
mechanical work on the motor shaft. The gear system transforms the mechanical energy to
Figure 1.1: Traction System
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the load work. The controller compares and regulates the actual output of the system to the
set point and also takes into account the disturbances while achieving regulation.
1.3 Braking
A brake is a machine element and its principle object is to absorb energy during
deceleration. Vehicles use brakes to absorb kinetic energy whereas hoists and elevators use it
to absorb potential energy. When braking is achieved by connecting the moving member to a
stationary frame kinetic energy is converted to heat energy. This is wastage of energy and
also causes wear and tear of frictional lining material.
Conventional braking system employs braking by absorbing kinetic energy by
friction, by making the contact of the moving body with brake liner which causes the
absorption of kinetic energy and this is dissipated in form of heat in surroundings. Each
braking action is associated with the momentum gained by the vehicle being absorbed and to
re-accelerate, we have to redevelop that momentum by consuming more power from the
engine. Hence it results in huge energy wastage, heat generation and wear and tear of the
brake liner as well as the wheel.
Braking can be achieved more efficiently with the use of electric machines. Figure 1.2
shows the four quadrant operation of an electric machine. It can be inferred from the figurethat an electric machine acts as a brake when the torque generated opposes the motion.
Forward
Motoring
ω
Te
Reverse
Braking
Reverse
Motoring
Forward
Braking
Figure 1.2: Four Quadrant Operation
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There are different methods of braking by the use of an electric machine. The most
common methods are regenerative braking and dynamic braking. We have the flexibility to
dissipate the energy during braking operation as heat (dynamic braking) or re-generate it back
to the source depending on the need and the application concerned. Braking by electric
machine is very smooth, noiseless and un-wanted wear and tear can be avoided.
1.3.1 Regenerative Braking
Regenerative brake is an energy recovery mechanism which slows a vehicle by
converting its kinetic energy into another form (electric), which can be fed back to the source
or stored until needed.
Regenerative braking can be applied to a machine that is driven by an electric source.
During the motoring interval energy is fed from the electric source to machine and during the
braking interval, energy is regenerated from the electric machine and is fed back to the AC
source by using an inverter or stored in battery by using a converter. For regeneration, the
back emf generated by the machine should be greater than the supply voltage. Regeneration
is achieved only when this condition is satisfied and braking is achieved by Lenz’s law. The
basic block diagram for achieving regenerative braking is shown in figure 1.3. This is more
advantageous method as we supply energy back to source. This is the concept of re-
generating energy.
Rectifier Inverter Motor
Inverter
DC
Link
A
B
C
Figure 1.3: Block Diagram for Regenerative Braking
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1.3.2 Dynamic Braking
Dynamic braking of an electric machine involves dissipating the energy generated by
the electric machine during the braking interval in order to achieve braking. One of the
common ways to achieve dynamic braking is by using a resistor network connected to thearmature or the DC link through a control switch to dissipate the energy. By controlling the
duty ratio of the pulses to the control switch the time taken for braking can be controlled.
Varying the duty ratio of the pulses changes the effective resistance of the dissipation element
from zero to the maximum value. The basic block diagram describing dynamic braking is as
shown in figure 1.4.
Rectifier Inverter Motor DC
Link
A
B
C
Dissipating
Element
Figure 1.4: Block Diagram for Dynamic Braking
1.4 Necessity of the System
A system having electric braking comes with a package of advantages over the
traditional mechanical braking system. Electric braking is smooth compared to the frictional
braking system, gives a higher braking efficiency or more as compared with frictional brakes
with less heating, wear and tear. For traction applications, the primary need is to have the
system working at a better efficiency with lesser losses – be it while motoring or braking.
Each element in a traction system is required to run for a long duration and it is difficult to
have the traction system elements replaced often. The brake liner of the mechanical braking
system has to be replaced periodically. By using a method of electric braking, this can be
avoided.
1.5 Literature Review
For the last 100 years electric braking has been used for different applications – in
automobiles, rail cars, trams etc. There has been a lot of research in the area of electric
braking. Different topologies have been introduced, discussed and modified for the purpose
of efficient electric braking, some for general use and some for use in specific applications.
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For the past two decades and more, Permanent Magnet Synchronous Machines, PMSM, has
been one of the most widely discussed electric machine and of great interest to researchers.
T Sebastian et.al, in 1986 [13] presented equivalent circuit models after reviewing
advancements in permanent magnet synchronous motors. The paper gave the comparison of
measured and computed parameters. In the same year, T M Jahns et.al, [14] discussed the
special features that distinguished permanent magnet machines from other class of ac
machines with respect to adjustable speed operation. The paper describes permanent magnet
machines as robust and with high power density, capable of operating over wide speed ranges
at high motor and inverter efficiencies. Smooth responsive torque control was achieved by
controlling phase current magnitude and phase angle with respect to the rotor orientation.
K T Chau et.al, [12] presented an overview of the permanent magnet machine drive
systems for electric vehicle (EV) and hybrid electric vehicle (HEV) with emphasis on
machine topologies, drive operations and control strategies. It also discusses about the
different control strategies with simulation results. M Rakesh et.al, [10] described the
different braking techniques that can be used for a permanent magnet machine drive used in
locomotive application. The methods like dynamic braking, plugging and regenerative
braking for a permanent magnet machine was analysed and simulated. The results were
discussed with illustrations of waveforms.
J Cody et.al, [11] discussed the application of brushless DC motor (BLDC)
technology in electric vehicles with emphasis on regenerative braking. The control required
for reversal of energy flow has been explained. By the use of independent switching scheme
regenerative braking was achieved. In this scheme only the lower switches are turned on
during braking and control is achieved in conjunction with pulse width modulation (PWM)
techniques. During the on time of switch, voltage is boosted and during the off time energy is
recycled to the source. The scheme was illustrated with switching tables, current paths and a
prototype was developed.
In another publication by B Tan et.al, [9] speed performance comparison for different
braking methods were analysed. A method for detecting the different phase currents for the
different braking methods were proposed along with a phase current control method based on
current cut off feedback. The two methods of braking analysed were dynamic and plug
braking. Plug braking had the inverter switched such that the back EMF generated was
opposing to the source voltage. In dynamic braking all the lower switches were turned on
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simultaneously resulting in shorting of the machine windings and energy dissipation. The
drawbacks were also explained along with the emphatic study of change of current and
energy flow path. A Frechowicz [8] discusses about a method of PM motor drive operation
with the use of electronic commutation and two zone speed control. Smooth transition from
constant torque operation to constant power operation is claimed. It is achieved by partially
shunting the machine windings by the use of electronic switching elements. By controlling
the windings energized and the inverter switches, different braking methods were elaborated
with waveforms.
M K Yoong et.al [7] explains about regenerative braking for electric vehicle
application. The working principle and the braking controller were studied to promote
efficiency and realisation of energy savings. The control used is six-step commutation inaccordance with the hall position information and control is achieved by PWM logic. During
braking, the pulse sequence for the inverter is changed so as to achieve regeneration. Zhang
et.al in [6] analysed the performance of permanent magnet machine for plug and regenerative
braking. Analysis of both six-step commutation and field oriented control (FOC) has been
presented. A permanent magnet machine model was simulated and both the control strategies
have been discussed. Plug and regenerative braking methods were analysed using both six-
step commutation and FOC and the results were presented and discussed. The advantages and
disadvantages of both FOC and six-step commutation has been pointed out.
Cheng-Hu Chen et.al, in [2] discusses about the design and implementation of a cost
effective single stage bidirectional DC-AC converter without the use of any additional power
components and passive elements. Three switching strategies named according to the number
of switches conducting were derived from six-step commutation to suit the different
performance indices during braking. The strategies were theoretically and experimentally
analysed and suggested the use of a variable braking control strategy. Single-switch and
three-switch strategies were considered suitable for high speed situations and two-switch
strategy for low speed and emergency stoppage conditions.
Jing et.al in [1] discusses the use of a permanent magnet machine for battery electric
vehicle. The hall sensor resolution along with driving and braking control has been analysed.
The paper also discusses about the methods of position estimation to compensate for the
positioning error due to misalignment of hall sensors. The driving and braking scenarios were
analysed using two methods – six-step commutation and FOC. Two variants of six-stepcommutation for braking were proposed and a method for plugging by six-step commutation
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has also been discussed. Using FOC plugging and regenerative braking were analysed. The
braking methods FOC and six-step commutation were compared and evaluated by simulation.
The patents by B Anuradha et.al [3], C Hanlon et.al [4] and B L Beifus [5] discusses
about different methods for the braking of a permanent magnet machine. In [5] a method of
braking was presented wherein the machine windings are short circuited after the motor
comes down to a pre-determined set speed. The shorting causes energy to be dissipated
within the inverter switches and the machine windings. The patent [4] describes a method
wherein the windings are shorted for achieving braking and control is achieved by PWM and
by determining the instant of on and off of the power switches with respect to the back emf.
The patent [3] describes a similar method of braking with the control related to the DC bus
voltage. During braking when the DC bus voltage increases beyond a set limit, the thirdlower switch is turned on during the instant when the other two lower switches are on so that
the energy to the DC link is limited. When the DC bus voltage comes down below the limit,
the third lower switch is turned off.
All these methods have various advantages and disadvantages. This thesis aims to
develop a scheme of electric braking which is simpler than FOC and reduces the need of an
external dissipative element and achieve faster braking.
1.6 Objective
The objective of this thesis is to develop a system of electric braking for traction
applications wherein braking has to be achieved by using the machine windings. This thesis
aims to indentify a suitable topology and modification of the topology for using the machine
windings to brake, reducing the utilization of braking chopping elements, have a minimal
impact on inverter switches and lesser changes in the drive circuits.
1.7 Overview of Thesis
This thesis is organised into the following chapters
Chapter 1 gives the introduction to electric machines, braking and the necessity of the
system. It also briefly describes literature review on the different methods of braking
available for electric braking of a permanent magnet motor.
Chapter 2 gives background theory on the PMSM drive system explaining in brief
about the PMSM machine, the control of PMSM machine, DC bus and the inverter.
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Chapter 3 describes about the selection of braking topology and the modification
carried out in that topology to achieve braking by the use of a non-conducting
winding of the machine. It also describes about the open loop and closed loop block
diagrams in brief.
Chapter 4 gives the simulation results with analysis and justifications.
Chapter 5 deals with the conclusion and future work.
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2. THE PMSM DRIVE SYSTEM
This chapter describes the drive system, the different components of the system such
as the permanent magnet synchronous motor, inverter, controllers, the DC bus system etc.
Also, a brief description of the permanent magnet machine, its classification and review of
the permanent magnet materials used is presented. The drive system comprises of 4 main
components, a permanent magnet motor, inverter, control unit, DC bus and DC source. The
basic model of a PMSM drive system is shown in Figure 2.1.
V-DC
Inverter
Permanent Magnet
Synchronous
Machine
Load
Position Sensor Control UnitControl Input
Ia, Ib, Ic
Va, Vb, Vc
Pulses I-abc
Figure 2.1: PMSM Drive System
2.1 Permanent Magnet Synchronous Motor
A permanent magnet synchronous motor (PMSM) uses permanent magnets in the
rotor to produce the air gap magnetic flux instead of an electromagnet. Compared to
induction machines, it has a higher efficiency, reliability and greater torque to size ratio.
PMSM machines have become more competitive nowadays due to the developments in high
density magnetic materials at cheaper costs. This has made PMSM an ideal choice for
traction applications where motor size and efficiency are the primary constraints.
2.1.1 Permanent Magnet Materials
Motor performance is directly affected by the property of the permanent magnet used
in rotor and proper knowledge is required for the selection of the materials and in
understanding PM motors.
Earlier magnetic materials were manufactured from hardened steel. They get
magnetized easily but, could not hold enough magnetic energy and are easily demagnetized.
The development of other magnetic materials like Aluminium Nickel and Cobalt alloys
(ALNICO), Strontium Ferrite or Barium Ferrite (Ferrite), Samarium Cobalt (SmCo) and
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Neodymium Iron-Boron (NdFeB) has advanced the performance of magnetic materials in
terms of flux density. The latter two categories are called first generation and second
generation rare earth magnets respectively. SmCo has a higher flux density but come at a
very huge price. NdFeB are the most commonly used rare earth magnets in PM motors
nowadays. Figure 2.2 displays the flux density versus magnetizing field (B-H curve) mapping
of these magnets. From the figure we can identify that Neodymium magnets and Samarium
Cobalt are the best suited PM materials for rotor. The two main qualities required in a
permanent magnet for use in rotor are
High remenance
High coercive force
Figure 2.2: B-H Curve for Different Permanent Magnet Materials
2.1.2 Classification of Permanent Magnet Motors
2.1.2.1 Direction of Field Flux
By the direction of the field of flux, PM motors are broadly classified into radial field
motor and axial field motor. In radial field motor, the flux is along the radius of the motor
whereas in axial field motor, the flux is perpendicular to the radius of the motor. The most
commonly used type is the radial field motor.
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2.1.2.2 Flux Density Distribution
Based on flux density distribution, motors are classified into PMSM and BLDC. The
major distinguishing factor is the shape of back EMF; PMSM has sinusoidal back emf
whereas BLDC has trapezoidal back emf. This is in turn due to the distribution of flux in theair gap. PMSM has sinusoidal flux distribution and hence sinusoidal distribution of
conductors whereas BLDC has rectangular flux density distribution and concentrated stator
conductors.
2.1.2.3 Permanent Magnet Radial Field Motors
There are two different ways to place a permanent magnet on the rotor of a PM
machine. The magnet can be mounted on the surface of the rotor resulting in surface mounted
PM motors or it can be mounted interior to the rotor.
Surface mounted PM motors shown in Figure 2.3 are easy to build, specially skewed
poles that gets easily magnetized in order to minimize the cogging torque. This configuration
is only suitable for low speed applications because of the mechanical instabilities in the rotor
at high speeds. They have practically equal inductances in both axes due to small saliency.
The rotor core is made of punched laminations with the permanent magnets mounted on the
surface using adhesives. Magnets of opposite magnetization are kept in an alternating fashion
to produce radially directed flux which reacts with the winding currents to produce
electromechanical torque.
Figure 2.3: Surface Mounted Permanent Magnet Motor
In interior PM motors as shown in Figure 2.4 the magnets are mounted inside the
rotor. This is best suited for high speed applications. There is inductance variation in this type
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2.1.3.1.1 Incremental Encoders
These types of encoders have good precision and are relatively easy to implement but
lack information when the motor is at rest position and for precise position, the motor must
stop at the starting point.
Thy type which is commonly available has a two channel output. These two code
tracks are positioned 90 degrees out of phase and with the help of this we can identify both
the position and the direction of rotation as shown in Figure 2.6. If one channel leads the
other, then the motor is rotating in a particular direction and if it lags, then the direction is
reversed. Monitoring the relative phase of signals and the number of pulses helps in tracking
the position and direction of rotation.
Figure 2.6: Incremental Encoder
2.1.3.1.2 Absolute Encoders
Absolute encoders as shown in Figure 2.7 capture the rotor position with a precision
that is dependent only on the number of bits of the encoder and can even measure the
standstill position. These types of encoders are used where the device remains inactive for a
long time or moves at a slow speed.
Figure 2.7: Absolute Encoder
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2.1.3.2 Position Resolvers
Also called rotary transformers, position resolvers work on the principle of
transformer operation. The primary winding is placed on the rotor and voltages are induced in
the two secondary windings that are placed on the stator as shown in Figure 2.8 anddepending on the rotor shaft angle, the induced voltages shifted by 90 degree would be
different. The two stator windings are placed in quadrature with one another. One of the
output windings is made to align with the reference winding, generating full voltage on that
winding and zero on the other and vice versa. The rotor theta is extracted by the knowledge
of these two voltages.
Figure 2.8: Position Resolver
2.1.3.3 Hall Sensors
Hall sensor is a transducer that varies its output voltage in response to a magnetic
field. Hall sensor in its simplest form operates as an analog transducer directly returning a
voltage. Using group of sensors, the relative position of the rotor magnet can be deduced. The
signals from the Hall sensor can be used by a microcontroller for controlling the speed of a
machine as in a permanent magnet motor. Figure 2.9 shows a hall sensor connected to the
rotor of a machine. The signals from a hall sensor for one cycle are shown in Figure 2.10.
When a pole of the rotor magnet comes in alignment to the hall sensor, it generates a pulse.
By a combination of multiple hall sensors, the position is identified.
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Figure 2.9: Hall Sensors
Figure 2.10: Signals from Hall Sensor for One Cycle
2.2 Voltage Source Inverter
An inverter is a static power converter device that produces AC output from a DC
power supply. An AC output is required in adjustable speed drives (ASDs), uninterruptable
power supplies (UPS), active filters, flexible AC transmission systems (FACTS) and for
numerous other systems. The magnitude, frequency and phase should be controllable for a
sinusoidal AC output.
A voltage source inverter (VSI) is characterized by a well-defined switched voltage
waveform across the terminals. The AC voltage frequency can be variable or constant
depending on the application. A VSI can be sub categorised as single phase and three phase
inverter. Single phase inverters cover low power ranges and is used in single phase UPS, in
multi cell configurations, power supplies etc. The circuit for a single phase inverter is as
shown in Figure 2.11.
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S1
S2
S3
S4
V-DC
P
N
Figure 2.11: Single Phase Inverter
Three phase inverter consists of six power switches connected as shown in Figure
2.12 to a DC voltage source.
S1
S2
S3
S4
V-DC
A
S5
S6
B
C
Figure 2.12: Three Phase Inverter
The inverter switches are carefully chosen based on the requirements of operation,
ratings and the application. There are several devices available such as thyristors, bipolar
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junction transistors (BJTs), MOS field effect transistors (MOSFETs), insulated gate bipolar
transistors (IGBTs) etc. The device list along with their power switching capabilities is shown
in Table 2.1.
Table 2.1: Device Switching and Power Capability
Device Power Capability Switching Speed
BJT Medium Medium
GTO High Low
IGBT Medium Medium
MOSFET Low High
Thyristor High Low
MOSFETs and IGBTs are preferred by industry because of the MOS gating permits
high gain and control advantages. MOSFET is considered a universal power device for low
power and low voltage applications, IGBT has wide acceptance for motor drives and other
applications in the low and medium power range. The choice between MOSFET and IGBT is
a trade-off between power rating and the switching frequency required. The inverter switches
are given pulses in such a way that no two switches in the same leg conduct at the same time.
Similarly, there cannot be any undefined state in the VSI as it would give an undefined AC
output and hence the switches of any leg of the inverter cannot be turned off simultaneously.
The switching scheme for a normal VSI is as shown in Table 2.2.
Table 2.2: Switching Scheme for VSI
Switches
State
Output
S1 S2 S3 S4 S5 S6 Vab Vbc Vca
1 0 0 1 0 1 1 Vi 0 -Vi
1 0 1 0 0 1 2 0 Vi -Vi
0 1 1 0 0 1 3 -Vi Vi 0
0 1 1 0 1 0 4 -Vi 0 Vi
0 1 0 1 1 0 5 0 -Vi Vi
1 0 0 1 1 0 6 Vi -Vi 0
1 0 1 0 1 0 7 0 0 0
0 1 0 1 0 1 8 0 0 0
The output voltage control of a VSI can be achieved by controlling the pulses to theswitches or by controlling the DC link voltage. The control of switching is achieved
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commonly by the use of pulse width modulation (PWM) techniques. A lot of PWM
techniques have been developed for the control of inverter and reducing the harmonics. The
other method of controlling the output voltage consists of controlling the magnitude of the
DC link voltage. For this thesis, we have used the method of controlling the DC link voltage
to control the VSI output. The pulses to the inverter switches have been given based on six-
step commutation logic which is described at a later part in this thesis.
2.3 DC Bus
The DC bus acts as the medium for power transfer from source to the voltage source
inverter. It can be considered as a complex system consisting of capacitors, protective
elements and filters. The main purpose of the DC bus is to provide constant ripple free
voltage for the VSI. The unregulated DC voltage from the rectifier is given to the DC bus
which removes the ripples from the voltage and feeds to the VSI. Apart from this, a braking
chopping element is present that protects the DC bus from over voltage. When the voltage
exceeds the specified limit, the protective circuit turns on and dissipates the extra energy in a
resistive load. The source for the DC voltage can be from AC-DC rectifier or DC voltage
source. In this thesis, the DC voltage has been obtained as the output from the PI controller.
2.4 Control UnitThe function of the control unit is to provide switching pulses to the inverter so as to
drive the PMSM machine. The position sensors sense the rotor position and provide it to the
control unit. Pulses are generated based on this position feedback from the PMSM machine.
Position feedback is an essential for the proper operation of a PMSM machine.
Based on the position feedback from the PMSM, the phases to be excited are
identified and given pulses. The working happens in such a way that when one pair of
winding is excited, the rotor moves to lock in with the excited windings. As soon as the rotor
catches up, the next pair of winding is energised and the rotor follows. This maintains the
synchronism.
2.5 Operation of PMSM
The operation of the machine is by the interaction of the magnetic fields generated by
the rotor PM and the magnetic field induced by the excitation of the stator winding by the
voltage source inverter. The three phase rotating magnetic flux generated by the statorwinding interacts with the rotor magnetic flux. The rotor locks in with the rotating magnetic
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field. A PM synchronous motor is driven by a sine wave voltage coupled with the given rotor
position. The generated stator flux together with the rotor flux from the PM defines the torque
and hence the speed of the motor. The sine wave voltage output has to be applied to the three
phase winding system in a way that the angle between the stator flux and the rotor flux is kept
close to 90 degrees for maximum torque generation. This necessitates the need for continuous
position information and hence electronic control.
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3. THE BRAKING SYSTEM
This chapter deals with the braking topology used and how the braking system used in
this thesis was developed.
3.1 Selection of Topology
There are a lot of topologies available for PMSM machine. The most used one being
field oriented control (FOC). The other topologies include six step commutation and
sinusoidal pulse width modulation techniques. The best method is FOC, but the aim of this
thesis is to achieve braking by the use of machine windings. When using FOC or sinusoidal
PWM methods, we do not have control over the windings conducting at an instant. The
conduction is continuous and use of the machine windings for braking is not possible because
we cannot obtain an instant where one of the machine winding is not conducting and can be
used for baking. The only topology with which we can use the machine windings was six-
step commutation. Six-step commutation is a discrete method of control of the machine
windings and each phase winding can be controlled individually. Each electric cycle is
divided into discrete intervals and phases are controlled during that interval. Additionally six-
step commutation is simpler to implement than the other methods.
3.2 Six-Step Commutation
Six-step commutation also commonly known as 120 degree commutation or
trapezoidal commutation is the most commonly used method of providing pulses to a PM
rotor machine. The position information is obtained by the use of Hall-effect sensors. Figure
3.1 shows the typical Hall sensor timing diagram with 120 degree angle of separation. For
clockwise rotation the sequence of Hall signals obtained will be H-A, H-B and H-C, where A,
B and C represent the machine phases. For counter clockwise operation the Hall sequence isH-B, H-C and H-A. The six-step commutation sequence turns on two switching power
devices in a predetermined sequence for each motor phase. A six-step sequence was
generated using the Hall position information.
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Figure 3.1: Hall Sensor Timing Diagram
By this method only two motor windings are turned on at any time. The Table 3.1
shows the Hall position information and the phases that are turned on during each Hall states.
The operational modes in six-step commutation are shown in Figure 3.2. Each section has a
60 degree interval to conduct two motor phases at the same time. The switching sequence is
determined in such a way that the motor phase windings that gets excited ensures the
continuity of the rotation of the rotor.
Table 3.1: Hall Position Information and Switching Sequence
Phases Back-EMF Polarity Switches
A B C Ea Eb Ec S1 S2 S3 S4 S5 S6
1 0 1 +1 -1 0 ON OFF OFF ON OFF OFF
1 0 0 +1 0 -1 ON OFF OFF OFF OFF ON
1 1 0 0 +1 -1 OFF OFF ON OFF OFF ON
0 1 0 -1 +1 0 OFF ON ON OFF OFF OFF
0 1 1 -1 0 +1 OFF ON OFF OFF ON OFF
0 0 1 0 -1 +1 OFF OFF OFF ON ON OFF
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Figure 3.2: Operation of Six-Step Commutation
3.3 The System Model
The open loop block diagram of the drive system used in this thesis is as shown in the
Figure 3.3. The inverter used is an H bridge voltage source inverter with MOSFET as the
switching devices. The pulses to the inverter are generated from the control unit that
generates the six-step commutation logic based on the Hall position feedback. The six-step
commutation logic pulses are generated by implementing the logic described in Table 3.1.
The inverter supplies voltage to the PMSM machine. The DC voltage source is connected to
the DC bus which comprises of a filter, a braking chopping element and DC link capacitor.
The open loop system was used in analysing the six-step commutation logic and identifying
the instant when the non conducting winding can be turned on.
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Inverter
DCSource
EMIFilter
Prot.
Circuit(Chop
per)
DC
Link
3 phaseFilter
Machine
PWM
Control
Figure 3.3: Open Loop Block Diagram
In the closed loop system as shown in Figure 3.4 the torque generated from the
machine is taken as feedback and compared with the reference value. The error is fed to a PI
controller that generates the DC voltage which is fed to the DC bus. The PI controller values
were obtained by using Ziegler Nicholas method. The values were further fine tuned by trial
and error method. The six-step commutation logic was modified to include the proposed
topology and implemented in closed loop.
In open loop, the braking system was analysed by using a PMSM machine block aswell as by modelling the machine as an R-L load connected to a sinusoidal voltage source
that acts as the back-emf.
Inverter
PI
Controller
EMI
Filter
Prot.
Circuit
(Chop
per)
DC
Link
3 phase
Filter Machine
Six-Step
Pulse
Position Sensor
&
Pulse Generator
Te
Error Reference +- V
- D C
Figure 3.4: Closed Loop Block Diagram
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3.4 Method of Third Winding
From the logic for six-step commutation we can understand that at each instant only
two phases are conducting and one phase is open. The method proposed in this thesis takes
use of this open or non conducting third winding to achieve faster braking. For achieving this,
the non-conducting winding is turned on during the braking interval by the use of pulse width
modulated signals.
From the simulation waveforms shown above, we can identify that the free-wheeling
diodes are conducting during the off time of that particular phase. Theoretically this
conduction should not take place. This is happening because the diodes are getting forward
biased by the differences in the instantaneous voltages of back-emf and supply voltage. In
this thesis, we make use of this particular region of dead time for introducing the non
conducting winding to achieve faster braking. The non conducting winding, also referred to
as the third winding, is turned on in such a way that it aides the braking process by passing
greater current and hence more dissipation of energy as losses. Say for example that the diode
1 is conducting during the interval when phases B and C are active. We turn on the non
conducting phase, i.e. phase A in a manner that aides braking. This is done by turning on the
lower switch of the A phase inverter leg by the use of pulse width modulation schemes.
Similarly, the non-conducting instances for each phase is identified and turned on for faster
braking.
Figure 3.5 shows the energy flow path in the PMSM machine and inverter switches
when the third winding method is used for braking. The figure has been drawn taking into
consideration only the ON instant for half the dead time (30 degree of the 60 degree dead
time). The instance when phases B and C are conducting by normal six-step commutation has
been taken for analysis.
The current path for ordinary six step commutation has been highlighted in blue. For
the instance when phases B and C are conducting, corresponding switches T3 and T6 are on.
The back-emf polarity during this instant will be opposing the supply voltage. In this thesis
the case of regeneration has not been taken into consideration. The magnitude of this current
is within the normal operating range.
In the third winding method the six-step commutation pulses are modified to turn on
the non-conducting winding during its particular dead time. In the case considered here,
phase A is the non-conducting phase and for this particular interval, switch T2 is turned on
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for half the dead time (for first 30 degrees) and T1 is turned on for last half (last 30 degrees).
The current through inverter and machine windings change when T2 is turned on as part of
third winding method. It is highlighted in red colour. It can be observed that current through
phase A increases and acts as the return path for the current from the phase B. The current
through phase C reduces. In effect, the direction of current flow reverses, creating an
opposing torque and this causes the braking to achieve in a faster rate. Similarly, this happens
for the other phases during their corresponding dead times. This effect is more pronounced
when we provide the third winding method to all the phases during their corresponding dead
times.
Figure 3.5: Current Flow Path for Third Winding
The control unit in the model for normal six-step commutation was modified to
incorporate the third winding method of braking. The instant when braking mode starts was
identified and the third winding or the non conducting winding was turned on by giving
PWM pulses. This method of braking was analysed by providing PWM pulses for different
duty ratios.
The simulations for electric braking of PMSM using the third winding method were
done. The simulations were carried out with different duty ratios given to the third winding.
The speed and torque waveforms for 25%, 50% and 80% duty ratio along with the case of no
third winding (normal six-step commutation) are presented. The phase current waveform for
80% duty ratio is also presented.
The simulations were carried out by giving torque reference. Speed was controlled byvarying the reference torque.
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4. SIMULATION AND RESULTS
4.1 Six Step Commutation
The simulation of a PMSM machine using six-step commutation logic was carried outfor understanding the working, current flow paths and identifying the instant for using the
non-conducting winding. The waveforms are presented below. Figure 4.1 shows the three
phase currents of the PMSM machine during six-step commutation control. It can be
observed that the conduction of the freewheeling diodes during the dead time is causing the
phase currents to have ripples. These ripples are reflected in the voltages across the phases
and in the voltages across the MOSFET and diodes and can be seen in the figures yet to
come.
Figure 4.1: Phase Currents for Six-Step Commutation with Pulses
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Figure 4.2 and Figure 4.3 shows the diode and MOSFET voltage and current
waveforms super imposed with the six-step commutation pulses. It can be observed that the
freewheeling diodes are conducting during the dead time. This conduction is attributed to the
instantaneous difference in the supply and back-emf voltages forward biasing the diode.
Figure 4.4 shows the diode and MOSFET currents superimposed with the six-step
pulses for one electrical cycle. The pulses given to the machine is captured in Figure 4.9.
Figure 4.2: Diode Voltage and Currents with Pulses
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Figure 4.3: MOSFET Voltages and Currents with Pulses
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Figure 4.4: Diode and MOSFET Currents with Pulses
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Figure 4.5: Switching Pulses to Inverter for One Cycle
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4.2 Method of Third Winding
4.2.1 Method of Third Winding Applied to only One Phase
Analysis was also done by giving the third winding method of braking to only one
phase of the PMSM machine, in this case to phase A. The pulses, current waveform and
speed waveform for this particular test case is as shown in Figure 4.6, Figure 4.7 and Figure
4.8 respectively. It was observed that there was no significant change in the time taken for the
speed to come to zero. The effect due to the turn of only one winding for braking was absent.
Figure 4.6: Pulses for Third Winding Method Applied to Phase A
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Figure 4.7: Phase Currents with Third Winding Method Applied to Only Phase A
Figure 4.8: Speed Waveform for Third Winding Method Applied to Phase A
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4.2.2 Method of Third Winding Applied to only All Phases
The simulation waveforms for electric braking with the third winding method applied
to all the machine phases are presented in this section. Figure 4.9 depicts the switching pulses
provided to the H-bridge inverter. Along with the six-step commutation logic, the pulses foreach switch and thereby each phase were modified to incorporate the third winding logic. The
switching was done at 20 kHz frequency. The presented pulses are with a duty ratio of 80%
during the instant when third winding is on.
Figure 4.9: Switching Pulses for PMSM Machine with Third Winding
The speed waveform of PMSM machine with third winding method is shown in
Figure 4.10. The simulation was carried out for duration of 20 seconds and for PWM pulses
having different duty ratios – 25%, 50% and 80% along with the case of without the third
winding method i.e. normal six-step commutation. It can be observed that as the duty ratio
increases, the speed of the machine comes down at a faster rate. The torque waveform of the
PMSM machine braking with the third winding method is shown in Figure 4.11.
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Figure 4.10: Speed for Different Duty Ratio Provided to Third Winding
Figure 4.11: Torque Waveform for Different Duty Ratios
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The phase current of the PMSM machine with third winding method used for braking
is shown in Figure 4.12. It can be observed that the current rises initially. This rise is
attributed to two main reasons, one the torque commanded at that instant is high and
secondly, the machine at start up draws more current.
Figure 4.12: Phase Currents with Third Winding Method Applied to All the Phases
This thesis has not taken soft starting techniques into consideration. The braking
mode starts at 5 second and the third winding method is introduced at the same time. It causes
the machine to come to halt at a faster rate. The higher stop time is attributed to the higher
inertia of the machine.
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5. CONCLUSION
5.1 Summary
A method of electric braking for a PMSM machine used in traction application wasdeveloped and verified by simulation using Matlab/Simulink
The working of a permanent magnet synchronous machine was studied for both
motoring as well as braking mode. The effects of the phase currents during braking and the
current flow path for each phase through the machine and the inverter was analysed and
studied.
An interval was identified during an electrical cycle of braking by the use of six- step
commutation logic. During this interval a non conducting third phase was turned on. This
resulted in faster braking of the PMSM drive system.
5.2 Future Scope
The impact on the DC bus due to the turn on of the non conducting winding is an
open area for research. In this thesis, the DC bus voltage was the controlled parameter. The
drive system can be modified to keep the DC bus voltage constant and achieve performance
by giving PWM pulses to the inverter instead on square pulses and control by varying the
duty ratio.
Removing the external braking chopping element and regenerating the energy back
with very less harmonics are two additional areas where further research can be carried out.
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