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Zeszyty Problemowe Maszyny Elektryczne Nr 81/2009 113

Jarrad Cody, zdemir Gl, Zorica Nedic, Andrew Nafalski, Aaron MohtarUniversity of South Australia

REGENERATIVE BRAKING IN AN ELECTRIC VEHICLE

ODZYSKIWANIE HAMOWANIA W POJAZDACH ELEKTRYCZNYCH

Abstract: Electric vehicles have been attracting unprecedented attention in light of the volatile market pricesand prospect of diminishing supplies of fuel. Advances in battery technology and significant improvements inelectrical motor efficiency have made electric vehicles an attractive alternative, especially for short distancecommuting. This paper describes the application of Brushless DC (BLDC) motor technology in an electricvehicle with special emphasis on regenerative braking. BLDC motors are being encountered more frequentlyin electric vehicles due to their high efficiency and robustness; however a BLDC motor requires a rather com-

plex control to cope with the reversal of energy flow during the transition from motoring regime to regenera-tive braking. In an electric vehicle, regenerative breaking helps to conserve energy by charging the battery,thus extending the driving range of the vehicle. There is a number of different ways to implement regenerative

braking in a BLDC motor. This paper describes the Independent Switching scheme for regenerative braking[1] as applied to a developmental electric vehicle at the University of South Australia.

1. Electric vehicles and regenerativebraking

In recent times, electric vehicles (EVs) have re-ceived much attention as an alternative to tradi-tional vehicles powered by internal combustionengines running on non-renewable fossil fuels.This unprecedented.focus is mainly attributableto environmental and economic concerns linked

to the consumption of fossil-based oil as fuel ininternal combustion engine (ICE) powered ve-hicles.With recent advances in battery technology andmotor efficiency, EVs have become a promis-ing solution for commuting over greater dis-tances. Plug-in EVs utilise a battery systemwhich can be recharged from standard poweroutlets. Since performance characteristics ofelectric vehicles have become comparable to, ifnot better than those of traditional InternalCombustion Engine (ICE) vehicles, EVs pre-

sent a realistic alternative.Regenerative braking can be used in an EV as away of recouping energy during braking, whichis not possible to do in conventional ICE vehi-cles. Regenerative braking is the process offeeding energy from the drive motor back intothe battery during the braking process, when thevehicles inertia forces the motor into generatormode. In this mode, the battery is seen as a load

by the machine, thus providing a braking forceon the vehicle.It has been shown that an EV, which uses re-generative braking can have an increased driv

ing range of up to 15% compared with an EV,which only uses mechanical braking [2].A rare case when regenerative braking can notoccur is when the battery is already fullycharged [3]. In such a case, braking needs to beeffected by dissipating the energy in a resistive

load.Mechanical braking is still required in EVs for anumber of reasons. At low speeds regenerative

braking is not effective and may fail to stop thevehicle in the required time, especially in anemergency. A mechanical braking system isalso important in the event of an electrical fail-ure. For example, if the battery or the systemcontrolling the regenerative braking failed, thenmechanical braking becomes critical.It is common in electric vehicles to combine

both mechanical braking and regenerative

braking functions into a single foot pedal: thefirst part of the foot pedal controls regenerative

braking and the final part controls mechanicalbraking. This is a seamless transition from re-generative braking to mechanical braking, akinto the practice of putting the brakes on in aconventional ICE vehicle.

2. BLDC motor

Principally, a brushless DC (BLDC) motor is aninside-out permanent magnet DC motor, inwhich the conventional multi-segment commu-

tator, which acts as a mechanical rectifier, is re-placed with an electronic circuit to do the com-

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Zeszyty Problemowe Maszyny Elektryczne Nr 81/2009114

mutation. [6]. Consequently, a BLDC motor re-quires less maintenance and is quite robust [7].A BLDC motor has a higher efficiency than aconventional DC motor with brushes [6]. How-ever, a BLDC motor requires relatively com-

plex electronics for control.

Fig. 1. Permanent Magnet BLDC Construction[4]

In a BLDC motor permanent magnets aremounted on the rotor with the armature wind-ings being hosed on the stator with a laminatedsteel core, as illustrated in Figure 1. Rotation isinitiated and maintained by sequentially ener-gising opposite pairs of pole windings, whichare said to formphases. Knowledge of rotor po-sition is critical to correctly energising thewindings to sustain motion. The rotor positioninformation is obtained either from Hall Effectsensors or from coil EMF measurements.

3. BLDC motor control

Two separate modules (stages) are required inorder to control a BLDC motor: a power mod-ule and a control module.A BLDC motor requires a DC source voltage to

be applied to the its stator windings in a se-quence so as to sustain rotation. This is done byelectronic switching using an inverter as shownin Figure 2. The inverter circuit employs a halfH-Bridge for each stator winding [8].

A_High B_High C_High

A_Low B_Low C_low

A

B

CBAT

BLDC

MOTOR

Fig. 2. Power Inverter Circuit (Adapted from[10])

In the case of a BLDC motor with three pairs ofstator windings, a pair of switches must beturned on sequentially in the correct order toenergise a pair of windings. This commutationsequence is shown in Table 1, with NC (NotConnected) designating the pairs of statorwindings (phases) which are not energisedduring this commutation step. Table 2 showsthe corresponding switching sequence.

Table 1. Forward Commutation Sequence

Step Hall A Hall B Hall C Phase A Phase B Phase C

1 1 0 0 -V +V NC

2 1 0 1 NC +V -V

3 0 0 1 +V NC -V

4 0 1 1 +V -V NC

5 0 1 0 NC -V +V

6 1 1 0 -V NC +V

Forward/Clockwise Motoring Commutation Sequence

Table 2. Forward Switching Sequence

Step PWM Switch ON Switch OFF Switch

1 B_High A_Low Remaining

2 B_High C_Low Remaining

3 A_High C_Low Remaining

4 A_High B_Low Remaining

5 C_High B_Low Remaining

6 C_High A_Low Remaining

Forward/Clockwise Motoring Inverter Operation

Figure 3 illustrates the current flow from theinverter circuit at the first commutation step to

energise the winding pairs of phases A and B.


A_Low B_Low C_low

A

B

CBAT

BLDC

MOTOR

Fig. 3. Motoring Current Flow for a Commuta-tion Sequence (Adapted from [10])

A similar strategy can be applied to achieve re-versal of the sense of rotation, as shown in Ta-

bles 3 and 4.

Table 3. Reverse Commutation Sequence

Step Hall A Hall B Hall C Phase A Phase B Phase C

1 1 0 0 +V -V NC

2 1 0 1 +V NC -V

3 0 0 1 NC +V -V

4 0 1 1 -V +V NC

5 0 1 0 -V NC +V

6 1 1 0 NC -V +V

Reverse/Anticlockwise Motoring Commutation Sequence

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Table 4. Reverse Switching Sequence


1 A_High B_Low Remaining

2 A_High C_Low Remaining

3 B_High C_Low Remaining

4 B_High A_Low Remaining

5 C_High A_Low Remaining

6 C_High B_Low Remaining

Reverse/Anticlockwise Motoring Inverter Operation

A number of switching devices can be used inthe inverter circuit; however MOSFET andIGBT devices are the most common in high

power applications due to their low output im-pedance [6].A microcontroller is commonly used to readrotor position information from the Hall Effectsensors and determine which phase to energise,switching the appropriate device as depicted inTable 1. Alternatively, phase EMFs can bemonitored to determine the rotor position insensorless applications.

4. BLDC regenerative braking

Regenerative braking can be achieved by thereversal of current in the motor-battery circuitduring deceleration, taking advantage of themotor acting as a generator, redirecting the cur-rent flow into the supply battery. The same

power circuit of Figure 2 can be used with anappropriate switching strategy. One simple andefficient method is independent switching inconjunction with pulse-width modulation(PWM) to implement an effective braking con-trol [9].In independent switching, all electronicswitching devices are offwhile applying regen-erative braking. The bottom switching devicesare on for the 120 degree portion of the cycle,corresponding to the flat top part of the phaseEMF, as illustrated in Figure 4. All top switches

are kept turned off.

Fig. 4. BLDC EMF with Corresponding SwitchSequence [9]

PWM is used to control the level of regenera-tive braking by varying the duty cycle of thePWM.Figure 5 shows the current flow path duringcoasting, during which there is no current ex-change between the BLDC and the battery [9].


A_Low B_Low C_low

A

B

CBAT

BLDC

MOTOR

Fig. 5. Coasting Current Flow for First Com-mutation (Adapted from [10])

In this mode, the energised windings allow thecurrent to flow through the low-side of thePWM switch and through the freewheeling di-ode of the low-side high phase switch. Thus nocurrent flows from the BLDC machine to thesupply battery.During regenerative braking, current in thewinding is reversed and supplied back into the

battery. In this mode, all switches are turned offand the current can flow back through the free-wheeling diodes. Figure 6 shows an example ofthe current flow when the winding pairs of theA and B phases are energied. In this example,the current can flow through the freewheelingdiode of the high-phase high-side switch,A_High, through the battery and through thelow-phase low-side switch, B_Low.To control the level of braking the PWM dutycycle is varied, which essentially toggles thecurrent flow between regeneration and coasting.The maximum level of regeneration occurs

when the low-side switches are all turned off.Consequently, the duty cycle is varied fromhigh to low. Therefore, by simply disconnectingthe inverter circuit (power module) from thecontrol source controlling the invertersswitching sequence (control circuit), regenera-tive braking will occur to its maximum poten-tial.Table 5 below shows the switching sequenceapplied during regenerative braking with inde-

pendent switching. It is noted that the low-sideswitches are switched with PWM and all other

switches remain off.

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A_Low B_Low C_low

A

B

CBAT

BLDCMOTOR

Fig. 6. Regenerative Current Flow for Step 1 ofCommutation (Adapted from [10])

The switching steps are controlled by thecontrol stage measuring the Hall Effect sensorreadings, similar to the motoring process.

Table 5. Forward Regenerative Switch Se-

quence


1 A_Low NIL Remaining

2 C_Low NIL Remaining


4 B_Low NIL Remaining



Forward/Clockwise Regenerative Inverter Operation

Regenerative braking can also be applied whilstthe vehicle is in reverse as shown in Table 6.The same method is used as in the forward

mode; however the phases are energised differ-ently and thus require a different switching se-quence as shown in Table 6. This switching se-quence is determined by the control stage and

by the use of a forward/reverse switch interface.

Table 6. Reverse Regenerative Switch Sequence






5 A_Low NIL Remaining6 B_Low NIL Remaining

Reverse/Anticlockwise Regenerative Inverter Operation

5. Application

A commercially available BLDC motor hasbeen selected to replace the currently used con-ventional DC motor in the developmental elec-tric vehicle of the University of South Australia,known by the acronym TREV, from Two-per-son Renewable Energy Vehicle. TREV is shownin Figure 7.

Fig. 7. TREV Electric Vehicle

Based on the motor controller presented in [8],a simple control stage, that uses a Microchip

dsPIC30F4012 microcontroller [11] has beenbuilt and tested.The control stage controls the motoring in for-ward and reverse modes, with the PWM controllinked to a 5k potentiometer to control themotor speed. The control can select betweenforwardand reverse by the use of a three wayswitch with the sequence of reverse, neutral andforward.The control stage employs independentswitch-ing for regenerative braking control. The stagecontrols the PWM of the low-side switches withanother 5k potentiometer. The potentiometersets the level of braking for regenerative brak-ing. The extent of battery recharging was meas-ured on the input terminals of the power moduleduring tests (Figure 8).A commercial power stage was chosen and pur-chased for TREVs drive system, after havingverified the control concept with test benchmeasurements, using a simplified version of the

power stage constructed in-house.

Fig. 8. TREV Control Stage

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The final drive system implemented in TREVincludes a brake pedal, which combines regen-erative braking action with mechanical brakingaction. Using a 5k potentiometer, the first

portion (1/3 of the brake pedal) controls thelevel of regenerative braking; the remaindercontrols mechanical braking. Acceleration iscontrolled by another 5k potentiometer con-nected to the acceleration pedal.

6. Conclusion

The regenerative braking system described inthe paper hss been successfully type-tested. Thesystem employs the Independent Switchingstrategy to control the flow of current duringvarious stages of the cruise profile. The work is

in progress to fit TREV with the commercialBLDC motor and a commercial power supplytogether with the controller developed in-house.

7. Acknowledgements

The authors gratefully acknowledge the invalu-able contributions to the TREV project of DrPeter Pudney, Senior Research Fellow, Centrefor Industrial & Applied Mathematics Univer-sity of South Australia.

8. References

[1]. Cody J, 2008, Regenerative Braking Controlfor a BLDC Motor in Electric Vehicle Applica-tions, Honours Paper in Bachelor of Engineeringdegree, University of South Australia, School ofElectrical and Information Engineering.

[2]. Ford R. 2007, Regenerative Braking BoostsGreen Credentials, Railwaygazette,http://www.railwaygazette.com/ fea-tures_view/article/2007/07/7577/regenerative_braking_boosts_green_credentials.htm, viewed: 23rd June2008.

[3]. Regen Braking Q4D Sales Information WebPage, http://www.4qd.co.uk/fea/regen.html, viewed:

15th Aug 2008[4]. 2002 Brown W. , AN857 Brushless DC MotorControl Made Easy. Microchip Technology Inc,available fromhttp://www.microchip.com/stellent/idcplg?IdcSer-vice=SS_GET_PAGE&nodeId=1824&appnote=en012037, viewed: 15 August 2008.

[5].Rashid M. H., 2004, Power Electronics: Cir-cuits, Devices and Applications Prentice Hall, 3rdEdition

[6]. Emadi, A., 2005, Handbook of AutomotivePower Electronics and Motor Drives, CRC Taylor

& Francis.

[7]. 3-Phase BLDC Motor Control with Hall Sen-sors Using MC56F8013: Targeting User Guide.Freescale Semiconductors

[8]. Mohtar A., Nedic Z., Machotka J., 2008. Acompact and affordable BLDC motor controller for a

microelectronics remote laboratory, InternationalConference on Embedded Systems and Applica-tions, ESA 2008, Las Vegas, Nevada, USA, pp.75-80.

[9]. Su, G J., McKeever J.W. Samons K.S., Designof a PM Brushless Motor Drive for Hybrid ElectricalVehicle Application.,http://www.ornl.gov/~webworks/cpr/pres/107923

_.pdf, viewed: 15th August 2008

[10]. Chen, J-X., Jiang, J-Z.and Wang, X-J., 2003,Research of Energy Regeneration Technology inElectric Vehicle. Shanghai University Press, Vol-

ume 7, Number 2.[11]. Microchip Technology Inc., 2007,dsPIC30F4011/4012 Data Sheet,http://ww1.microchip.com/downloads/en/ Device-Doc/70135.pdf

Authors

The authors are with the School of Electrical andInformation Engineering, University of South Aus-tralia, Adelaide, South Australia, Australia.

Jarrad Cody received hisBEng degree in Electrical

and Mechatronic Engi-neering in 2009 from theUniversity of South Aus-tralia (UniSA), Adelaide,Australia. During his timein study he worked on theUniversitys Two Seat Re-newable Energy Vehicle(TREV) project. Sincecompleting his studies late

2008 he has worked as a Hardware Design Engineerat BAE Systems Australia on the Airborne EarlyWarning and Control (AEW&C) project.

E-mail: [email protected]

zdemir Gl has hadextensive experience as anengineering educator inaddition to his substantialindustrial experience. Hisacademic career has in-cluded teaching and re-search in electrical engi-neering at universities inTurkey, Australia, France,Belgium Switzerland,

Finland and Greece. He holds the degrees of MESc,ME and PhD, all in electrical engineering. He is cur-

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rently Associate Professor at the University of SouthAustralia. His research interests have been focussedon electrical machines and drives, and include mod-elling and simulation of electrical machines usingnumerical methods, design optimisation of electro-

magnetic devices and condition monitoring of elec-trical machines. He is the recipient of several na-tional and international awards for excellence in re-search, teaching, engineering education and com-munity service. He is the author and co-author ofsome 300 publications.E-mail: [email protected]

Zorica Nedic received herMESc degree in electricalengineering, specializingin electronics, from theUniversity of Belgrade,

former Yugoslavia. Sheobtained her ME in electri-cal engineering (control) in1997 from the Universityof South Australia(UniSA), Adelaide, Aus-tralia. She worked for six

years as a design engineer at the Institute MihajloPupin in Belgrade. Since 1991, she has beenworking as a lecturer in electrical engineering at theUniSA. She is currently studying for her PhD degreeat the UniSA in the field of modelling biologicalvision.

E-mail: [email protected] Nafalski's careerincludes assignnments inacademic and research in-stitutions in Poland, Aus-tria, Wales, Germany,France, Japan, USA, Can-ada, and Australia. Heholds BEng(Hons), GradDipEd, MEng, PhD andDSc degrees.He is Chartered Profes-

sional Engineer and Fellow of the Institution of En-

gineers, Australia, Fellow of the Institution of Engi-neering and Technology (UK), Senior Member ofthe Institute of Electrical and Electronic Engineers(USA) and Honorary Member of the Golden KeyInternational Honour Society. He is currently a Pro-fessor of Electrical Engineering at the University ofSouth Australia in Adelaide. His major research in-terests are related to electromagnetic devices, mag-netic materials and technologies as well as innova-tive methods in engineering education. His teachingareas cover analysis and design of electrical circuitsand devices, electromagnetic compatibility and in-formation technology. He has published over 300

scholarly works in the above fields He has beenleader of some 40 technical and educational research

and development projects. He has received numer-ous national and international awards for excellencein research, teaching, engineering education andcommunity service.E-mail: [email protected]

Aaron Mohtar received hisBEng degree in electronicsand microengineering in2006 from the University ofSouth Australia (UniSA),Adelaide, Australia. He iscurrently studying for hisPhD degree at UniSA with afocus on remote laboratoriesin microelectronics fabrica-

tion. Since 2004, he has been involved in teachingelectronics, microprocessor and microcontroller courses atthe School of Electrical Information Engineering withinUniSA. His interests include energy efficient commuting,robotics and biomedical devices.E-mail: [email protected]

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