Effective Method of Electric Braking for Traction Application_v5_may8_draft

8/18/2019 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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