An Efficient Implementation of the Space VectorModulation based Three Phase Induction Motor Drive

Muhammad Safian Adeel, Tahir Izhar and Muhammad Asghar SaqibDepartment of Electrical Engineering, University of Engineering and Technology, Lahore 54890, Pakistan

safian.adeel@yahoo.com, tizhar@gmail.com, saqib@uet.edu.pk

Abstract- This paper presents an efficient implementation schemefor the closed-loop speed control of an induction motor withconstant Vlf, slip regulation and incorporating 'space vector pulsewidth modulation' (SVPWM). The system is an embedded systemwhich incorporates an 8-bit reduced instruction set computer(MCU) to implement the speed control and the modulationscheme. Scalar control is inferior to the modern field orientedcontrol in terms of efficiency and response time but it requiresconsiderably lesser hardware resources. Use of space vectormodulation as the modulation technique gives a better harmonicresponse and higher efficiency as compared to the normal regularsampled pulse width modulation techniques. The driveincorporates optimized algorithms and low cost hardware toprovide a better cost-performance relationship. The system alsoincludes features for the digital over-current protection and soft­starting of the motor.

Keywords: Closed-loop speed control of induction motor, slipregulation, space vector modulation, RISC machine, field­oriented control

I. INTRODUCTION

Induction motors are more rugged, light weight, requirelesser maintenance and can be developed to meet any loadrequirement as compared to DC motors. But they exhibithighly coupled, nonlinear and multi variable structures asopposed to much simpler decoupled structures of separatelyexcited DC motors [1]. Hence, the control of induction motorsis somewhat involved, and requires fast-computing andmultiple-resources microprocessor or microcontroller systems.To control the speed and torque of an induction motor appliedvoltage, frequency and current can be varied. The controlstrategy can be implemented by: (1) scalar control, where thecontrol variables are DC quantities and only their magnitudesare controlled, (2) vector control, where both the magnitudeand phase of the control variables are controlled, or (3)adaptive control, where the parameters of the controller arecontinuously varied to adapt to the variations of the outputvariables [1]. The scalar control gives sluggish response but itis the easiest to implement and requires lesser number ofresources. There are multiple variations in the domain of scalarcontrol which include open-loop speed control, closed-loopspeed control with slip regulation, closed-loop speed controlwith torque and flux control etc. The closed-loop control isnormally required to satisfy the steady state and the transientperformance specifications of AC drives [2]. This paperimplements the closed-loop speed control with slip regulation

978-1-4244-4361-1/09/$25.00 ©2009 IEEE

which continuously samples the output speed, compares it withthe given/desired speed, generates an error signal which istranslated into new voltage and speed values such that the ratioof the voltage and the frequency is constant.

Space vector modulation (SYM) is the modulation schemeemployed in the system. It is an inherently digital modulationtechnique, based on the space vector theory [3]. It providessuperior performance compared to regular sampled pulse widthmodulation (PWM) techniques, in terms of reduced harmoniccurrent ripple, optimized switching sequences and increasedvoltage transfer ratios [4] but requires fast processing devicesfor implementation. Usually, Digital Signal Processing boardsor application specific ICs are used to implement the spacevector modulation but these are relatively expensive. Thispaper presents efficient algorithms for the implementation ofscalar control and the space vector modulation on an 8-bitgeneral purpose MCU (Mega32 from Atmel Corporation).Although the MCU restricts the speed feedback samplingfrequency and the PWM frequency to a lower value but still itprovides good efficiency when the cost and performance arecompared. The scheme also implements digital over-currentprotection and soft-starting of the motor.

II. SCALAR SPEED CONTROL OF INDUCTION MOTORWITH SLIP REGULATION

The torque and speed of an induction motor can becontrolled by controlling the applied stator voltage andfrequency. The following equation gives a relationshipbetween the motor torque, input voltage and frequency [1]:

(1)

(2)

According to equation (1), the developed torque of themotor 'Td' is directly proportional to the square of inputterminal voltage 'Ys' and inversely proportional to thesynchronous frequency 'oi,'. According to equation (2), thesynchronous speed 'm,' is directly proportional to the inputsupply frequency 'co'. Hence the speed can be controlled by

-VDC

+VDC

(3)

(4)

(5)

(6)

VOUT X t; =Vk X t; + Vk+1 X Tk+1 + Vz x Tzwhere,

T, =T, X d,

Tk+1 =t, x dk+1

To =r, =t; - (Tk +Tk+1 )

The computation of the duty cycles can be carried out using[9]: where, TS is the sampling time, and Vs/E is themodulation index.The selection of the vector sequence is also important from theperformance point of view. Different sequences give differentharmonic performance. In this paper, the symmetric sequencescheme shown in Fig. 4 is implemented which introduceslesser THD but higher switching losses.

S4

Fig. 2. Three phase voltage source, PWM inverter.

Sl

such a vector can be realized. If the VREF is lying in betweentwo active vectors (Say VI & V2), two vectors (VI & V2 inthe figure) and a zero vector can be used to approximate theoutput voltage vector with the VREF. The duty cycles of thesevectors can be computed using the sampling time, themodulation index and the position of the VREF•

The equations for the computation of the duty cycle aregiven as [1]:

varying the input frequency & keeping the ratio of the voltageand frequency (the torque) constant. The block diagram [5] ofthe drive is given in Fig. 1. Three phase or single phase inputAC is converted to DC which is supplied to the inverter. Thesample of the output speed is fed back and subtracting it fromthe desired speed, an error signal is produced through theproportional plus derivative controller which is limited by themaximum torque capability of the machine. The error signal isthen added into the present value of the speed and a new speedis calculated which is used to calculate the newer value ofvoltage based on constant VIf principle.

The speed is integrated to generate '8', which is required togenerate the SVPWM. The most sensitive part of this schemeis the PI gain control which should be carefully programmedand tested to avoid any stability issues.

III. SPACE VECTOR MODULATION

For high-frequency voltage source inverters as depicted inFig. 2, which can be used as PWM rectifier as well, two widelyapplied PWM methods are the sub-oscillation method [6] andthe space vector modulation [7]. In case of sub-oscillationmethod, the base signals for the transistors in the invertercircuit are produced by comparison of modulation waves withcarrier signals and hence three phase AC waveforms aregenerated. In case of space vector modulation, the inverter istreated as a single unit [1] rather than three separate push­pull driver stages. SVM [7] is based on transformingthree-phase quantities into the a-~ plane in which inverter canhave eight possible states. These states are shown in Fig. 3represented by vectors in a-~ plane. Six out of eight vectorsare active vectors whereas two vectors (Voooand VIII) are zerovectors which are placed at the origin of the a-~ plane [8]. Thewhole region is divided into six sectors. The three vectors ofthe balanced three line voltages which are 120 degree apart,when converted into a-~ plane, constitute a vector denoted inthe figure by VREF, also called the modulating signal. Itrevolves with the electrical frequency and traverses the wholea-~ plane in one time period of the original line voltagewaveform. The objective of the space vector modulation is toproduce a vector using the three phase VSI, which shouldoverlap with the VREF. Using six active and two zero vectors,

Fig. 1. The block diagram of the closed-loop speed control with constant vlf Fig. 3. Output voltage vectors of a three phase VSI in 0 D plane.control and slip regulation.

IV. SYSTEM CONFIGURATION

USt.'T

lntertacc

To Computer

CI.IImrt Measurement &.

Over CWTCnt Protct.'tion

MCU

OperatingSystem ISRMain

Fig. 5. Block diagram of the system.

V. THE MAIN CONTROL ALGORITHM

achieved by gradually increasing the output voltage to theactual value in a time entered by the user.

Three algorithms have been tried for this system. The firstone is given in Fig. 6. In this algorithm, one interrupt is usedfor both the speed control loop and the SVM implementation.It has to be noticed that applying the switching patterns on tothe inverter and then waiting in empty loops until the dutycycle is over, adds considerable delay in the speed sampling asthe processor is idle during these empty wait cycles. Hence toimprove the performance, an algorithm based on two interruptshas been introduced where the implementation of SVM doesnot paralyze the speed measurement and user interface, andhence meeting the real-time constraints. The performance ofthe algorithm would be compared with the algorithm given in[10] which implements Space vector modulation for open loopspeed control on an MCU with PWM channels. The optimized

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The block diagram of the system is given in Fig. 5. Sixpower supplies are used in this prototype. The first one consistsof an uncontrolled rectifier plus an LC filter and it provides DCto the three phase inverter. The remaining 5 consist of a stepdown transformer; an uncontrolled rectifier and a capacitorfilter each. One of these five supplies is used to provide powerto the MCU & the over-current protection. Three are used topower-up the three upper drive circuits for the three upperIGBTs. The remaining one is used to power-up the threedrivers for the three lower IGBTs. Hardware interlock providesprotection against short circuit which may be caused by twotransistors in one arm simultaneously turning on. Sameprotection has also been ensured in software using dead time.The IGBT drivers are push-pull amplifiers with opto-couplersto provide isolation between the MCU and driver circuit. Theuser interface consists of an alphanumeric LCD and a keypad.Although, the current measurement is not needed for the typeof the control implemented but it is included to providesoftware based over-current protection. Current measurementis carried out by adding a current transformer with one of thethree output lines. The output of the CT is connected to aresistor of 0.1 ohm and thus the obtained voltage signal is fedto an OPAMP which amplifies it. The signal is thenconverted to digital format through an 8-bit cheap ADC andthen it is sent to the 2051 MCU which reads it, displays it andoperates a relay if needed. Note that the main microcontrollercan also be used for this purpose as it contains a 10-bit, 8­channel ADC. This feature was initially installed in the mainMCU of the prototype but later on it was shifted to a separatesystem to remove the extra delay introduced by this function.Because to sense the over current, one sample would notsuffice, rather multiple samples over certain duration of timewould be taken to avoid false sensing and it would add aconsiderable amount of delay into the main interrupt's ISR.The prototype is provided with soft-start function and theramp-up time can be entered via keypad. The soft-start is

Fig. 4. Symmetric switching pattern for sector 1. Fig. 6. Single interrupt based algorithm for the closed loop, constant Vlf, SVMbased drive.

TSmin,unoptimized = TISR_MAIN + TSmin,optimized=2 XTISR_MAIN (9)

Minimum sampling time in case of unoptimized algorithm isgiven as:

(7)

(8)TSmin,optimized = TISR_MAIN

where,TcALc=total time consumed by main ISR if no switching ISRoccurs.TD= dead time,TI = total time consumed by the instructions in ISR.

Hence, the optimized algorithm provides double switchingspeed as compared to unoptimized algorithm which is alsointuitive as in the unoptimized algorithm, the total samplingtime consist not only the speed feedback loop time(TISR_MAIN)but also the SVM switching period (Ts). Whereas,in optimized algorithm, speed feedback loop and SVMswitching is carried out in parallel using two interrupts.The sector determination can be carried out using a very simplealgorithm given in Fig. 8.

The algorithm outsmarts the algorithm given in [10] as Itdoes not require an application specific Integrated circuitwhich should have PWM channels. Also, this algorithmimplements closed loop control as compared to the open loopcontrol given in [10]. The algorithm also separates the over­current protection from the main control loop which results inachieving higher switching speeds.

Another algorithm given in Fig. 9 was also tried. This is thesimplest as far as the implementation is concerned. In thisalgorithm, the interrupt service routine for the interrupt ofRTOS only increments the related time variables and the mainloop implements the control loop along with the SVM. Thetime information for integration and other purposes is readfrom the time-keeping variables. This algorithm suffers fromthe same problems as the single-interrupt based algorithm.Hence, the two- interrupts-based algorithm is the optimizedvariation. The gate signals for gI and the line-to-line voltageVab are replicated in Fig. IO(a) and IO(b). The picture of theprototype is given in appendix.

TISR_MAIN should equal Ts. If TISR MAIN is larger than Ts, adelay (TISR_MAIN - Ts) should be .addcd in successive Tsimplementations to avoid the switching loop from runningwithout getting necessary data from TISR MAIN. Hence, theminimum sampling time (TSmin) equals the TISR MAIN and toincrease the sampling time, TISR MAIN would b-e increased.Since TISR_MAIN is constant (unless it is increased by addingdelays), TSmin depends directly upon the MCV throughput.Minimum sampling time in case of optimized algorithm isgiven as:

implementation algorithm is given in Fig. 7 where twointerrupts are used. The main interrupt is the system interruptfor RTOS which normally should have the highest priority buthere it has lower priority to another interrupt which switchesthe sequences on to the inverter. To ensure that the main ISRdoes not stay incomplete in case the interrupt of higher priorityoccurs, few measurements are discussed later. The maininterrupt's ISR scans the keypad several times (as compared tojust one in figure), then it reads the speed, converts that into eusing the elapsed time between the two interrupts, calculatesslip speed, updates the VIf profile, computes modulation index,then finds the sector and the angle with reference to that sector,computes the switching duty cycles based on the TSand storesthem into variables along with respective vectors. The secondinterrupt's ISR first introduces a dead time; the dead time isneeded to avoid the short circuiting of the DC supply. Thelength of this delay time is usually about 1.5-2 times themaximum tum-off time [11] of the transistor. The interruptsservice routine then reads the variables, reloads the timer withnew value according to the duty cycle, applies the sequence onthe inverter drive, clears the interrupt flag, starts the timeragain and ends. The length of this interrupt service routine isusually very small because only a few instructions areperformed whereas the dead time is small relative to the maininterrupt overflow time. The time period of the main interruptrequired for complete execution of its ISR can be calculatedusing (7).

Fig. 7. The optimized algorithm for the closed loop, constant VIf, SVM baseddrive.

II Initialize Bto 0and Sector to 1at start of main

Start

Br =OJrx(Tmaill_ISR)

BNEW = Bold +BrTraverse:

if(BNE W > lrj3){

BNEW = BNEW - lrj3

Sector++;

Traverse;

if(Sector > 6)

Sector = Sector % 6

else

end

end

Fig. 8. The algorithm for determining the sector.

Fig. 9. Another variation of the algorithm for the closed loop, constant V/f,SVM based drive.

VI. CONCLUSIONS

An efficient implementation scheme for the closed-loop,constant VIf and space vector modulation based speed controlof induction motor, on an 8-bit general purpose MCU, has beenintroduced. The algorithm presented for the systemimplementation uses two interrupts where the low priorityinterrupt implements the speed measurement, talks to user andPC, controls the speed and does the SVM calculations. Thesecond high priority interrupt implements the space vector

l II'I II II I j

Channel A Channel S 0 TriggerSource Channel A

V/Div 1.00 1.00 Horizontal Level 0.00Offset 0 0 Source Trace Coupling DCInvert normal normal Position 24.00 mS Edge RisingCoupling DC DC S/Div 2.00 mS Mode Auto

Fig. lO(a). Pulse patterns generated for gate I.

II III

I

Channel A ChannelS 0 Trigger

Source Channel AV/Div 1.00 1.00 Horizontal Level 0.00Offset 0 0 Source Trace Coupling DCInvert normal normal Position 24.00 mS Edge RisingCoupling DC DC S/Div 2.00 mS Mode Auto

Fig. lO(b). The line-to-line voltage, Vab

modulation. Necessary measurements to avoid low-priorityISR staying incomplete are also discussed. The scheme alsoimplements the hardware interlock in addition to the softwareprovision of dead-time , over-current protection and thesoft-starting of the motor. The algorithm provides superiorperformance in terms of achieving higher switching speeds,better dynamic response and requiring lesser hardwareresources.

REFERENCES

(1] M.H. Rashid, "Power Electronic s: Circuits, Devices and Applications",Pearson Education, Singapore, 2005.

[2] A. von Jouanne, P. Enjeiti and W. Gray, "Applications Issues for PWMAdjustable Speed AC Motors," IEEE Industry Applications Magazine ,Vol. 2, No.5, September/October 1996, pp. 10-18.

[3] H.W. Van der Broeck, H.C. Skudclny and G.V. Stanke, "A nalysis andRealization of Pulse-Width Modulator based on Voltage SpaceVectors,"IEEE Transactions on Industry Application s, Vol. 24, No. I ,January/February, 1988, pp. 142-150.

[4J J. I-10hz , "Pulse Width Modulation - A Survey," IEEE Transactions onIndustrial Electronics," Vol. 39, October 1992, pp. 410-420.

[5J B.K. Bose, "Modem Power Electronics and AC Drives," Prentice Hall,Upper Saddle River, NJ 07458 , 2002.

[6J A. Schonung and H. Stemmler , "Static Frequency Changers withSubharmonic Contro l in conjunction with Reversible Variable Speed ACDrives," Brown-Boveri Rev, Vol. 51, pp. 555-577 ,1964.

[7J G. PfatT, A. Weschta and A.F. Wick, "Design and Experimental Resultsof a Brushless AC Servo Drives", IEEE Transactions on IndustryApplications, Vol. lA-20, No. 4, pp 814-821, 1984.

[8] D.G. Holmes, "The General Relationship between Regula r-Sampled Pulse­Width-Modulation and Space Vector Modulat ion for Hard SwitchedConverters", IEEE Industrial Society Annual Meeting , Vol. I, pp 1002­1009,1992

[9J Atmel Corporat ion, "AVR495: AC Induction Motor Control using theConstant V/fPrinciple and a Space Vector PWM Algorithm," 2005.

[10] R. Parekh , "VI' Control of three phase Induction Motor using SpaceVector Modulation " Microchip Technolog y Incorporated, 2005.

[II] S.G. Jeong and M.H. Park, "The Analysis and Compensation of Dead­Time Effects in PWM Inverter," IEEE Transaction s on IndustrialElectronics, Vol. 38, Issue 2, April 1991 pp. 108-114.

APPENDIXThe image of the prototype is given below.