Direct Torque Control: A Practical Approach to Electric Vehicle Bhim Singh, Senior Member, IEEE, Pradeep Jain, A.P.Mittal, Member, IEEE, and J.R.P.Gupta
Abstract-- Electric Vehicle (EV) propulsion system using induction motor drive employing Direct Torque Control (DTC) is becoming popular because of quick response and simple configuration. This method consists of the control of the torque and the stator flux directly, based on their instantaneous errors. It allows a precise and a quick control of the induction motor flux and torque. This strategy is extensively used in electric vehicle application. In this paper, the behavior of DTC based induction motor for an EV is studied through simulation using MATLAB. The starting, acceleration, deceleration and braking features of the EV drive are simulated and presented in detail.
1) High instant power and high power density. 2) High torque at low speeds for starting and climbing, as well as high power at high speed for cruising. 3) Very wide speed range including constant-torque and constant-power regions. 4) Fast torque response. 5) High efficiency over wide speed and torque ranges. 6) High efficiency for regenerative braking. 7) High reliability and robustness for various vehicle operating conditions. 8) Reasonable cost.
Index Terms-- DTC, Electric vehicle propulsion, EV, Induction motor.
E
I. INTRODUCTION
LECTRIC vehicle is a road vehicle, which involves with electric propulsion. Electric Vehicles may include battery operated electric vehicles (BEVs), hybrid electric vehicles (HEVs), and fuel-cell electric vehicles (FCEVs). In a world where environmental protection and energy conservation are of growing concerns, the development of EV technology has taken an accelerated pace to fulfill these needs. Concerning the environment, electric Vehicles can provide emission-free urban transportation. The electric propulsion system is the heart of EV [1]. It consists of the motor drive, transmission device, and wheels as shown in Fig. 1. In fact, the motor drive, comprising of the electric motor, power converter, and electronic controller, is the core of the EV propulsion system. The motor drive is configured to respond to a torque demand set by the driver. The accelerator position provides a torque demand as fraction of the maximum available torque. Similarly, the first portion of the brake pedal travel is used to derive a regenerative torque demand; the remaining pedal travel brings in a set of standard mechanical brakes. The major requirements of the EV motor drive are summarized as follows [1].
Bhim Singh is with Department of Electrical Engineering, I.I.T.Delhi, India-110016 (e-mail:
[email protected] ). Pradeep jain, A. P. Mittal, and J. R. P. Gupta are with Department of I & C Engineering, Netaji Subhas Institute of Technology, Dwarka, New Delhi, India-110075 (e-mail:
[email protected],
[email protected],
[email protected] )
0-7803-9525-5/06/$20.00 ©2006 IEEE.
Fig. 1. Electric vehicle composition.
Traditionally, DC motors have been prominent in electric propulsion because their torque-speed characteristics suit traction requirement well and their speed control is simple. Recently, technological developments have pushed commutatorless motors to a new era, leading with the advantages of high efficiency, high power density, low operating cost, enhanced reliability, and low maintenance over DC motors. Induction motors (IMs) are a widely accepted commutatorless motor for EV propulsion because of they are robust, highly reliable and free from maintenance. In order to improve the dynamic performance of induction motor drives for electric vehicle propulsion, vector control technique is preferred. It is well known that vector control needs quite complicated coordinate transformations on line to decouple the interaction between flux control and torque control to provide fast torque control of an induction motor. Hence the algorithm computation is time consuming and its implementation usually requires a high performance DSP chip. In recent years an innovative control method called direct torque control has gained the attraction for electric propulsion system [2]-[4], because it can also produce fast torque control of the induction motor and does not need heavy computation on-line, in contrast to vector control. In this paper the DTC control scheme is used as a basic technique for electric vehicle induction motor drives. The user input (gas pedal) is the rotor speed wm. The control of the induction motor used in an EV is studied over different operating regions using a sensorless DTC technique. II. SYSTEM DESCRIPTION A block diagram of the Direct Torque Control scheme is presented in Fig. 2. DTC comprises three basic functions:
hysteresis control for torque and flux, an optimal switching vector look-up table and a motor model. The motor model estimates the developed torque, stator flux and shaft speed based on the measurements of two stator phase currents and battery voltage (Vdc). Torque and flux references are compared with their estimated values and control signals are produced by using a torque and flux hysteresis control method. The switching vector look-up table (Table I) gives the optimum selection of the switching vectors for all the possible stator flux-linkage space-vector positions. Speed control is achieved using a PI speed controller.
φ sα = Ls i αs + Lm i rβ α
α
s
s
(9)
α
φ r = Lr i r + Lm i s
(10) 3P α α α α Te = (φ ds i qs − φ qs i ds ) (11) 4 The DTC technique is based on the direct torque stator flux and torque control [7]-[10]. A switching table is used for inverter control such that the torque and flux errors are kept within the specified bands. More details of the system are explained as follows. The stator flux can be estimated φˆα = ∫ (vˆ α − iˆα R )dt + φ = (φˆα ) 2 + (φˆα ) 2 (12) s
s
s0
ds
qs
Where vˆαs and iˆsα indicate the measured stator voltage and current, φ s 0 is the initial flux vector, Rs is the stator resistance. The stator phase voltages are estimated using the following equations: −1 − 1 S a vˆa 2 ˆ Vdc 2 (13) − 1 S b vb = 3 − 1 vˆc − 1 2 S c −1
Fig. 2. A DTC based drive induction motor.
III. MOTOR MODELLING AND CONTROL SCHEME In the analysis and simulation of the system, the basic equations of IM are used in instantaneous vector. By Park’s transformation, the equations of the IM in a general reference frame, denoted by the superscript “g” are shown in terms of voltage and flux as follows [5],[6]. A. Electrical Equations: d v sg = i sg Rs + jw g φ sg + φ sg dt 0 = i rg Rr + j ( w g − wr )φ rg +
(1) d g φr dt
(2)
φ sg = Ls i sg + Lm i rg
(3)
φ rg = Lr i rg + Lm i sg
(4)
3P g g g Te = (φ i qs − φ qsg i ds ) 4 ds
(5)
B. Mechanical Equation: d (6) Te − TL = J m wr + Bm w r dt The above equations can be referred to a stationary frame, denoted by the superscript “α” which is with d-axis aligned to the stator winding of phase “a” and be rewritten as follows. C. Electrical Equations: d v αs = i sα R s + φ sα dt d 0 = i rα Rr − wr φ rα + φ rα dt
Where Vdc is the dc link voltage of inverter (battery voltage) and Sa, Sb, Sc are the switching functions which can take either logic “1” or logic “0”. S −1 − 1 a vˆ ds Vdc 2 (14) Sb ˆ = 3 0 − 3 3 v qs S c The estimated developed torque of the motor is 3P ˆ α ˆ α ˆ α ˆ α (15) Tˆe = (φ ds i qs − φ qs i ds ) 4 Noting that the errors of the torque and flux are indicated by, d T and d ϕ , respectively. And defined as − Tˆe
(16)
d ϕ ≡ φ ref − φˆsα
(17)
d T ≡ Te
ref
TABLE I SWITCHING TABLE FOR DTC TECHNIQUE
dϕ = 1
dϕ = 0
N
1
2
3
4
5
6
dT = 1
110
010
011
001
101
100
dT = 0
111
000
111
000
111
000
dT = -1
101
100
110
010
011
001
dT = 1
010
011
001
101
100
110
dT = 0
000
111
000
111
000
111
dT = -1
001
101
100
110
010
011
Where
−(π / 6) + (1 − N )(π / 3) ≤ θ s ( N )〈 (π / 6) − (1 − N )(π / 3) defines
the stator flux position over six regions of the motor controlling (60°), N is sector and d T and d ϕ are the torque (7) (8)
and flux error coefficients calculated as follows. If d T 〉ε T , then d T = 1 If −ε T ≤ d T 〈ε T , then d T = 0
(18) (19)
If d T 〈−ε T , then d T = −1 If d ϕ 〉ε ϕ , then d ϕ = 1
(20) (21)
If d ϕ 〉 − ε ϕ , then d ϕ = 0
(22)
Where ε T and
εϕ
draws more current due to the requirement of the maximum torque with the reduced flux and only way this can be achieved is with the increase in the motor current.
are the acceptable predefined torque and
flux errors, respectively. IV. RESULTS AND DISCUSSION The DTC control scheme has been simulated in MATLAB simulink software. The developed model is shown in Fig. 3. The induction motor is rated three phase, 1 HP, 415 V, 50 Hz, 2 poles and 2.37 N-m base torque. A reference torque of 4.5 N-m is set by the speed controller to accelerate/ decelerate the motor with nominal torque. The simulated waveforms of reference speed, estimated speed, load torque, motor torque, motor currents and estimated flux are shown in Figs. 4-6. The simulation is carried out for following conditions.
Fig. 4. Transient response during starting and field weakening region for step changes of speed.
D. Transient response of vehicle motor during deceleration of vehicle (Regenerative braking) Fig. 4 shows that the three phase motor currents are approximately sinusoidal and one can observe the variation of the frequency of the currents as the drive speed changes from 400 rad/s to 300 rad/s at 0.6 second and also the reversal of a phase current to show the drive behavior during regenerative braking.
Fig. 3. Simulation model of DTC.
A. Transient response of vehicle motor at no load (free acceleration) Fig. 4 shows that the motor drawn inrush current during starting (zero to 300 rad/s) of the motor. This current can be reduced by first establishing the flux with zero speed or torque command for the first few cycles and the giving the speed command to the drive. The three phase motor currents are close to sinusoidal and one can observe the variation of the frequency of the currents as the drive speed changes from zero to the full value nearly rated speed of the motor.
E. Transient response of vehicle motor during reverse rotation Fig. 5 shows that phase sequence reversal of motor currents when the direction of rotation is reversed from (300 rad/s to 300 rad/s at 0.75 second) and (-300 rad/s to 300 rad/s at 1.0 second) and also shows the variation of the frequency of the currents.
B. Transient response of vehicle motor during loading of vehicle Fig. 4 shows that during loading of vehicle (2 N-m), set by the external step source at 0.2 seconds, the electromagnetic torque developed by the motor almost follows the set reference and implying that control is extremely fast. C. Transient response of vehicle motor during acceleration of vehicle (above rated speed) or field the weakening of motor Fig. 4 shows that the stator flux magnitude is reduced while their frequency increased when the speed command input from the step source is above the base speed (for this motor 314 rad/s) at 0.4 second. In field weakening region, the motor
Fig. 5. Transient response during reverse rotation.
F. Transient response of vehicle motor during load variation at constant speed Fig. 6 shows that speed is maintained constant even for change in the load torque from (2 N-m to zero at 1.4 second) and (zero to 2 N-m at 1.6 second).
From Fig. 4-6, it can be observed that torque response in transient state is very rapid especially during the starting stage due to the flux being controlled within its rated value before torque reaching its given value. The result shows that the controller behaves as predicated and their output follows the imposed reference.
[4]
K. Jezernik, “Speed Sensorless Torque Control of Induction Motor for EV’s”, Proc. IEE Intl. Workshop on Advanced Motion Control, 2002, pp. 236-241. [5] P. Vas, Sensorless Vector and Direct Torque Control, New York: Oxford Univ. Press, 1998. [6] Y. S. Lai, “Modeling and Vector Control of Induction Machines- A New Unified Approach”, Proc. of the IEEE Power Engineering Society, Winter Meeting, 1999, pp. 47-52. [7] Hoang Le-Huy, “Modeling and Simulation of Electrical Drives Using MATLAB/Simulink and Power System Blockset”, Proc. IEEE IAS’00, 200, pp. 1603-1611. [8] B. K. Bose, Power Electronics and Variable Frequency Drives, IEEE Press, 1997. [9] D. Casadei, F. Profumo, G. Serra, and A. Tani, “FOC and DTC: Two Viable Schemes for Induction Motors Torque Control”, IEEE Trans. Power Electronics, Vol. 17, No. 5, Sep.2002. [10] I. Takahashi and T. Noguchi, “A New Quick-Response and HighEfficiency Control Strategy of an Induction Motor”, IEEE Trans.Ind Apl, Vol. IA-22, No.5, pp.820-827, Sep.1986.
VII. BIOGRAPHIES
Fig. 6. Transient response during load variation.
V. CONCLUSIONS The DTC based induction motor control for electric vehicle propulsion system discussed in this paper provides quick response, simple configuration and can be a good candidate for electric vehicle propulsion system. The proposed scheme is capable of providing four quadrants operation along with regenerative braking with partial recovery of kinetic energy to charge the battery and thereby improving the overall efficiency of the system. The aspect of torque ripple minimization has not been considered in this paper; however the author’s are working on this aspect which shall be reported elsewhere. VI. REFERENCES [1] [2] [3]
C. Chan, “The State of the Art of Electric and Hybrid Vehicles”, Proc. of the IEEE, Vol.90, No.2, pp. 247--275, Feb. 2002 J. Faiz, M. B. B. Sharifian, Ali Keyhani, and A. B. Proca, “Sensorless Direct Torque Control of Induction Motors Used in Electric Vehicle”, IEEE Trans. Energy Conversion, Vol. 18, No. 1, Mar. 2003. J. Faiz, S. H. Hossieni, M. Ghaneei, A. Keyhani, and A. Proca, “Direct Torque Control of Induction Motor for Electric Propulsion Systems”, Electric Power Systems Research, Vol. 51, pp. 95–101, Aug. 1999.
Bhim Singh graduated from University of Roorkee in 1977 with BE degree, MTech in Power Apparatus and Systems from IIT Delhi in 1979 and PhD from IIT Delhi in 1983. He is currently working as a Professor in IIT Delhi. He is a Fellow of Institution of Engineers (India) and senior member of Institution of Electronics and Communication Engineers. His research interests include active filters, self excited induction generators, FACTS, electric drives. Pradeep Jain graduated from M. M. Engg. College in 2000 with BTech. degree, MTech in Process Control from Netaji Subhas Institute of Technology Delhi in 2002 and is presently pursuing his Ph.D. He is currently working as a Teaching cum Research Fellow in Netaji Subhas Institute of Technology. His research interests include electric vehicle, electric drives and power electronics. A.P.Mittal graduated in 1978 from M.M.M Engg. College, Gorakhpur, M.E. in 1980 from University of Roorkee and PhD in 1991 from IIT Delhi. He has teaching experience of more than twenty years .He is presently Professor and Head of Instrumentation and Control Engineering Division in Netaji Subhas Institute of Technology. He is a Fellow of Institution of Engineers (India) and member of Institution of Electronics and Communication Engineers. His research interests include power electronics, FACTS, active filters J.R.P Gupta graduated from Muzaffarpur Institute of Technology (M.I.T) and received his B.Sc. degree in 1972 and completed his Ph.D. degree from University of Bihar in 1983. He has been in Netaji Subhas Institute of Technology for the last ten years and is presently holding the position of Professor and Head of Instrumentation and Control Engineering Department in Delhi University. His research interests include power electronics, electric drives, power quality.
