Direct Torque Control of Induction Motor Drive Fed from Hybrid Multilevel Inverter

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142903-6868-IJECS-IJENS © June 2014 IJENS I J E N S

Direct Torque Control of Induction Motor Drive Fed

from Hybrid Multilevel Inverter

H.G.Zaini, M.K. Metwally, Mahrous Ahmed

Faculty of Engineering, Electrical Engineering, Dept, Taif University, KSA [email protected] , [email protected], [email protected]

Abstract-- This paper presents a torque and flux control method for an induction motor drive. The control method uses the torque and speed estimation to control the load angle and to obtain the appropriate flux vector trajectory from which the voltage vector is directly derived based on direct torque control methods. The voltage vector is then generated by a hybrid multilevel inverter with lower power electronic components. The inverter high quality output voltage which leads to a high quality IM performances. Besides, the MLI switching losses is very low due to most of the power cell switches are operating at nearly fundamental frequency. Some selected simulation results for are presented.

Index Term-- Direct Torque Control, Induction Motor drive,

hybrid multilevel inverter.

1. INTRODUCTION

In the 18th century DC motor was extensively used. After the invention of separately excited DC motor it becomes main workhorse of the industry. But it requires more maintenance and it has more commutation problem. In 1891 at the Frankfurt exhibition, Nikola Tesla first presented a polyphase induction motor. Since then great improvements have been made in the design and performance of the induction motors and numerous types of polyphase and single-phase induction motors have been developed. Due to its simple, rugged and

inexpensive construction and excellent operating

characteristics induction motor has become very popular in industrial applications. As a rough estimate nearly 90 percent of the world‟s AC motors are polyphase induction motors. After the invention of DC motor controller for speed and torque control, DC machine again back in action till development in power electronics for induction motor. Because of power electronics drive technology, induction motor becomes main workhorse of industry. Those were scalar control methods which has good steady state response but poor dynamic response. To achieve good dynamic response as well as good steady state response, vector control was introduced. But it has complexity in construction and control. In recent years several studies have been carried out for the purpose to find out alternative solution of field oriented control drive to achieve accurate and fast response of flux and torque and also to reduce the complexity of the control system of the drive. This was “direct torque control” or “direct torque and flux control” drive.

Since, DTC (direct torque control) introduced in 1985, the DTC was widely use for Induction Motor Drives with fast dynamics. Despite its simplicity, DTC is able to produce very fast torque and flux control, if the torque and the flux are correctly estimated, is robust with respect to motor parameters and

perturbations [1], [2], [3]. Unlike FOC (field oriented control), DTC does not require any current regulator, coordinate transformation and PWM signals generator. In spite of its simplicity, DTC allows a good torque control in steady-state and transient operating conditions to be obtained. The problem is to quantify how good the torque control is with respect to FOC. In addition, this controller is very little sensible to the parameters variations in comparison with FOC [4], [5]. FOC makes decoupling of stator current to produce independent control of torque and flux. FOC is very sensitive to flux variations, which is mainly affected by parameter variations. It is greatly influenced on the performance of induction motor. Instead of FOC, DTC directly control flux and torque without depending on parameter variation [6].

In recent years, there has been great interest in multilevel inverters (MLIs) technology. Special attention has been paid for cascaded H-bridge inverter [7] – [11]. Generally, there are many advantages in the applications of MLIs inverters over conventional two-level inverters. The series connection of power converter modules reduces the voltage stress of each converter module (or increases the voltage capability of the overall converter structure). Besides, the resolution of the staircase waveform of the output voltage increases with the number of voltage steps of capacitor voltage sources available in the multilevel inverter. As a result of the improved resolution in the voltage harmonic content, filtering efforts and the level of the electromagnetic interference (EM) generated by the switching operation of the converter can be reduced.

FOC and DTC can be applied for multilevel inverter-fed drives, only the modulation method has to be upgraded to multilevel pulse width modulation (PWM) (with multiple carrier arrangements) or multilevel space vector modulation (SVM).

In this paper DTC drive for an IM based hybrid multi level inverter (HMLI) has been developed using MATLAB SIMULINK. Stator current, rotor speed, electromagnetic torque and flux plot which show the performance of DTC with HMLI. DTC has also track the required speed and torque successfully which represents the successful design of the DTC drive.

2. THE HYBRID MLI POWER CIRCUIT

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International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:14 No:03 7

142903-6868-IJECS-IJENS © June 2014 IJENS I J E N S switch inverter. Each auxiliary cell consists of two switches

and single dc input voltage. The basic auxiliary cell of the proposed inverter includes two switches are always operating in a complementary mode and single input dc voltage to generate two levels output voltage waveform 0 and its input dc source. Therefore the auxiliary cell givesV_o 0, when the switch S1 is ON and it gives its input voltage when S1 is OFF.

To avoid short circuit condition, it should be kept in mind that both of the switches (S1 and S2) never be switched on at a

time. c b Phase Arm ‘A’ S11 S12 S21 S22 2Vdc a Auxiliary stage Main stage 4Vdc IM S31 S32 Vdc Auxiliary stage ‘N’

Fig. 1. Eight level line-to-line hybrid MLI

Using two auxiliary cell with the main cell results in generating 8 levels output phase to the pole of the dc source and 15 levels for the line-to-line-voltage. It can be noted that the main cell dc source is 4 Vdc and the auxiliary cell dc

source is 2Vdc and Vdc, respectively, therefore VaN has eight

states (0, Vdc, 2Vdc, 3Vdc, 4Vdc, 5Vdc, 6Vdc, 7Vdc). The load

line-to-line voltages can be calculated as follows

v

_ab



v

_aN



v

_bN (1)

Therefore the load line-to-line voltages can have (7Vdc, 6Vdc,

5Vdc, 4Vdc, 3Vdc, 2Vdc, 1Vdc, 0, -1Vdc, -2Vdc, -3Vdc, -4Vdc,

-5Vdc, -6Vdc, -7Vdc). And the load phase voltages Van, Vbn and

Vcn can be calculated as in (2)

                                   cN bN aN cn bn an 2 1 1 -1 2 1 -1 1 2 3 1 v v v v v v (2)

Table 1 summarizes the output voltage levels for 4 levels using only single auxiliary cell with the main cell. The space vector as described in [12] will be employed which is the convenient modulation control to DTC. Figure 2 shows the generation of 15 levels line-to-line voltages and the corresponding voltage generation of each cell of the MLI. It can be noted that the auxiliary cell voltage are operating at low switching frequency while only switches of the main cell

operate at switching frequency. This leads to reduced switching losses and thus increasing system performances.

TABLE I

Switching States of phase VaN

MLI Pole voltage

Switches of arm ‘A’

vaN Sa11 Sa12 Sa21 Sa22 Sa31 Sa32

0Vdc 0 1 0 1 0 1

1Vdc 0 1 0 1 1 0

2Vdc 0 1 1 0 0 1

3Vdc 0 1 1 0 1 0

4Vdc 1 0 0 1 0 1

5Vdc 1 0 0 1 1 0

6Vdc 1 0 1 0 0 1

7Vdc 1 0 1 0 1 0

0 0.005 0.01 0.015 0.02 0.025 0.03

-200 0 200 400

0 0.005 0.01 0.015 0.02 0.025 0.03

-100 0 100 200

0 0.005 0.01 0.015 0.02 0.025 0.03

-50 0 50 100

0 0.005 0.01 0.015 0.02 0.025 0.03

0 200 400 600

0 0.005 0.01 0.015 0.02 0.025 0.03 -500 -400 -300 -200 -100 0 100 200 300 400 500

Fig. 2. generation of auxiliary cells voltages, pole voltage and line-to-line voltage.

3. DIRECT TORQUE CONTROL PRINCIPLES

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142903-6868-IJECS-IJENS © June 2014 IJENS I J E N S torque ripple and slow transient response to the step changes

in torque during start-up.

IM

VSI

Inverter switching table

+-Flux comparator

Torque comparator

+-VDC

C

Motor currents

Motor voltages

Stator flux & torque estimation tref

Flux linkage vector section

Te

|ψ| |ψref|

α

dψ

dte

Fig. 3. Block diagram of basic DTC drive

Figure 3 shows the schematic of the basic functional blocks used to implement the DTC of induction motor drive. A voltage source inverter (VSI) supplies the motor and it is possible to control directly the stator flux and the electromagnetic torque by the selection of optimum inverter switching modes. This control strategy uses two level inverter suggested by Takahashi, to control the stator flux and the electromagnetic torque of the induction motor.

The DTC scheme consists of torque and flux comparator (hysteresis controllers), torque and flux estimator and a switching table. It is much simpler than the vector control system due to the absence of coordinate transformation between stationary frame and synchronous frame and PI regulators. DTC does not need a pulse width modulator and a position encoder, which introduce delays and requires mechanical transducers respectively. DTC based drives are controlled in the manner of a closed loop system without using the current regulation loop. DTC scheme uses a stationary d-q reference frame having its d-axis aligned with the stator q-axis. Torque and flux are controlled by the stator voltage space vector defined in this reference frame [13]. The basic concept of DTC is to control directly both the stator flux linkage and electromagnetic torque of machine simultaneously by the selection of optimum inverter switching modes. The DTC controller consists of two hysteresis comparator (flux and torque) to select the switching voltage vector in order to maintain flux and torque between upper and lower limit. DTC explained in this paper is closed loop drive. Here flux and torque measured from the induction motor using proper electrical transducer. Then flux and torque errors are found out by equation (3) and (4) [14].

dΨ=Ψref – Ψ (3)

dte=tref – te (4)

Using flux and torque comparator flux and torque command obtained respectively. From these commands, drive can know flux has to increase or decrease and torque has to increase, make constant or decrease. Then by finding field angle, drive can find sector of flux linkage vector. After acquiring signals from flux comparator (dΨ), torque comparator (dte) and flux-linkage vector section (α), control algorithm of DTC was developed means gate pulse required for inverter is developed using switching table 1 [6].

As shown in figure 4, particular logic level signals developed. Here U0 indicates (000) and U7 indicates (111).

Sector can be obtained as per the flux vector angle as shown below in figure 4 as in [13]:

-30°≤α<30° 30°≤α<90° 90°≤α<150° 150°≤α<210° 210°≤α<270° 270°≤α<330°

Where, dΨ=1; Increase in flux =0; Decrease in flux

dt=1; Increase in torque =0; No change

=-1; Decrease in torque

Fig. 4. DTC vector diagram.

gate pulses for the inverter is obtained as per figure 3. For example, if switching vector in switching table selected is U6 (101) then it indicates first switch in phase-A is on +ve terminal, second switch in phase-B is on –ve terminal and third switch in phase-C is on +ve terminal. All signals selected as per this switching table. After selection of the gate pulse required performance of the DTC drive will obtained.

4. MATLAB SIMULATION OF DTC

In this paper for case study, 3HP, 220V, 50 Hz, 3-phase induction motor used for simulating DTC drive. Induction motor parameters are given in table II.

Table II Induction motor parameters

Stator resistance (Ohms) 0.435

Stator inductance (Henry) 2.0e-3

Rotor resistance (Ohms) 0.816

Rotor inductance (Henry) 2.0e-3

Mutual inductance (Henry) 69.31e-3

Inertia 0.082

Friction Factor 0

Pairs of poles 2

Here required speed is 1500 rpm and required torque is 10 N.m to which drive has to track. DTC of induction motor is

r

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142903-6868-IJECS-IJENS © June 2014 IJENS I J E N S simulated for the sample time of 1e-6 second. Simulation time

is 2.0 second.

In this configuration, main objective is to design DTC controller for hybrid multilevel inverter fed IM drive. This controller design block is given in figure 5. Required signal for this controller is obtained from the speed controller which is given in figure 6. As shown in figure 5, first of all stator flux magnitude and angle is obtained from the measurement.

Torque and flux error is obtained, from which drive can decide either flux has to increase or decrease, also torque has to increase, decrease or remains constant. From the stator flux angle, sector will decided. From the flux command, torque command and sector, switching table 1 developed. This table is useful for generating gate pulses for inverter. These gate pulses creates required signal which decouples the torque and flux from stator currents.

Fig. 5. MATLAB SIMULINK model of DTC of induction motor

Fig. 6. DTC controller

Fig. 7. DTC speed controller

5. SIMULATION RESULTS

The simulation results are done at rotor speed 1500 rpm and load torque changed from no load to full load torque at time instant t= 1sec.

Figure 8 and 9 show the hybrid MLI performances, they show the line-to-line voltages and the IM currents, respectively. The dc voltages of isolated batteries for the inverter have been chosen 60V, 120V and 240V, therefore their dc sum is 420 V. as mentioned before the load line-to-line voltages has (7Vdc, 6Vdc, 5Vdc, 4Vdc, 3Vdc, 2Vdc, 1Vdc, 0,

-1Vdc, -2Vdc, -3Vdc, -4Vdc, -5Vdc, -6Vdc, -7Vdc). Figure 9 shows

the motor currents which are very almost sinusoidal due to the high quality of the hybrid MLI output voltage and the natural low-pass load filter.

Figures 10 and 11 show the IM torque and speed profiles due to this specific loading condition. They give the conventional and well-know profiles, the torque tracks its reference with an very good performance. On the other hand, the speed builds up with an acceptable performance.

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142903-6868-IJECS-IJENS © June 2014 IJENS I J E N S

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 -500

0 500

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 -500

0 500

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 -500

0 500

Fig. 8. line-to-line inverter output voltages

1.13 1.14 1.15 1.16 1.17 1.18 1.19 -6

-4 -2 0 2 4 6

Time [sec]

cu

rr

e

n

ts

[A

m

p

]

Fig. 9. Stator currents

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

0 200 400 600 800 1000 1200 1400 1600

Time [sec]

S

p

e

d

[

rp

m

]

Fig. 10. Motor speed

1 1.2 1.4 1.6 1.8 2

-2 0 2 4 6 8 10 12

Time [sec]

T

orqu

e

[N

.m

]

Fig. 11. Motor torque response

Fig. 12. - plot Stator flux

5. CONCLUSION

Torque and flux control for induction motor drives are presented. The DTC drive which has been employed has achieved the reference speed and torque properly. Also ripples in torque and stator currents are very small due to the high quality of the hybrid MLI used as a power circuit. The provided simulation results show that the DTC drive works successfully. Another achievement of the power circuit is the reduced switching losses of the hybrid MLI and thus increasing overall system performances.

REFERENCES

[1] Z. Sorchini, P. Krein “Formal Derivation of Direct Torque Control for Induction Machines” IEEE Transactions on Power Electronics, Vol. 21, No.5, September 2006, pp. 1428 – 1436.

[2] M. Vasudevan, R. Arumugam, S. Paramasivam: “High-performance Adaptive Intelligent Direct Torque Control Schemes for Induction Motor Drives”, Serbian Journal of Electrical Engineering, Vol. 2, No. 1, May 2005, pp. 93 – 116.

[3] Anjana Manuel, Jebin Francis “Simulation of Direct Torque Controlled Induction Motor Drive by using Space Vector Pulse Width Modulation for Torque Ripple Reduction” International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, Vol 2, Sept 2013, pp. 4471- 4478,. [4] T.A. Wolbank, A. Moucka, J.L. Machl “A Comparative Study of

Field-oriented Control and Direct-torque Control of Induction Motors Reference to Shaft-sensorless Control at Low and Zero-speed” IEEE International Symposium on Intelligent Control, Oct. 2002, pp. 391 – 396.

[5] D. Casadei, F. Profumo, G. Serra, A. Tani “FOC and DTC: Two Viable Schemes for Induction Motors Torque Control” IEEE Transaction on Power Electronics, Vol. 17, No. 5, Sept. 2002, pp. 779 – 787.

[6] Abdesselam Chikhi1, Mohamed Djarallah1, Khaled Chikhil “A Comparative Study of Field-Oriented Control and Direct-Torque Control of Induction Motors Using An Adaptive Flux Observer” SERBIAN JOURNAL OF ELECTRICAL ENGINEERING Vol. 7, No. 1, May 2010, 41-55

[7] V. Ambrozic, M. Bertoluzzo, G. Buja, and R. Menis, “An assessment of the inverter switching characteristics in DTC induction motor drives,” IEEE Trans. Power Electron., vol. 20, no. 2, pp. 457–465, Mar. 2005.

[8] Z. Sorchini and P. T. Krein, “Formal derivation of direct torque control for induction machines,” IEEE Trans. Power Electron., vol. 21, no. 5, pp. 1428–1436, Sep. 2006.

[9] Xiao Q. Wu and Andreas Steimel, “Direct Self Control of Induction Machines Fed by a Double Three-Level Inverter”, IEEE Transactions on Industrial Electronics, Vol. 44, No. 4, August 1997.

[10] Baruschka, L., Mertens, A., “Comparison of Cascaded H-Bridge and Modular Multilevel Converters for BESS application,” IEEE Energy Conversion Congress and Exposition (ECCE), pp. 909 – 916, 2011.

[11] H. Sepahvand, J. Liao, M. Ferdowsi, “Investigation on Capacitor Voltage Regulation in Cascaded H-Bridge Multilevel Converters With Fundamental Frequency Switching”, IEEE Trans. Ind. Electron., Vol. 58, pp. 5102 – 5111, 2011.

[12] Md Mubashwar Hasan, Saad Mekhilef, Mahrous Ahmed “A Three-Phase Hybrid Multi-Level Inverter With Less Power Electronic Components Employing A Space Vector Control scheme,” IET Power Electronics (in press).

[13] H.F. Abdul Wahab and H. Sanusi “Simulink Model of Direct Torque Control of Induction Machine” American Journal of Applied Sciences 5 (8): 1083-1090, 2008 ISSN 1546-9239 © 2008 Science Publications

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Hatim Ghazi Zaini received the Bachelor's degree/higher diploma in 1993 from Electrical & Computer Engineering, Faculty of Engineering, Umm Al-Qura University, Mecca, Saudi Arabia. The Master degree and the Ph.D. from Florida Institute of Technology, Melbourne, FL, United States in 1998 and 2003, respectively. In 2005, he was cooperative with Faculty of Applied Computer and Science,Umm Al Qura University, Makkah, Saudi Arabia as an assistant professor. In 2007, 2008, and 2009 he become a supervisor Cisco Networking Academy, Member of the College Committee of Quality Control and Quality Assurance, Member of the College Higher Committee of Consultation at Technical Collage at Makkah, Department of Computer Technology, Saudi Arabia, respectively. In June 2011 and January 2013 he become assistant professor and ascoicate professor, respectively, at Taif University, Department of Electrical Engineering, Saudi Arabia. His research interests include programming, computation algorithms and real time control systems.

Dr. M. K. Metwally: received his doctoral degree in electrical engineering from Vienna University of Technology, Austria in March 2009. He is a lecturer in the Department of Electrical Engineering, Minoufiya University, Egypt. Presently he is working as Assistant Professor in Electrical Engineering department, Taif University, Kingdom of Saudi Arabia. His research interests cover AC machines control, transient excitation of AC machines, sensorless control techniques, and signals processing techniques.