Simulation research of direct torque open-loop control system based on MATLAB

Guo Qing

School of Electric and Control Engineering Xi’an University of Science and Technology

Xi’an, China e-mail: guoqing504011279@126.com

Gao Yun

School of Electric and Control Engineering Xi’an University of Science and Technology

Xi’an, China e-mail: xkdgaoyun @xust.edu.cn

Abstract-In the asynchronous motor direct torque control system ,the size of flux tolerance determines the switching time of non-zero voltage vector,and affect the hexagonal flux trajectory.If the flux unchanged,through a given torque and actual torque difference to adjust the voltage vector in each sector of the imposed time to keep the torque stability within the tolerance range.The size of the torque tolerance affect switching state of the converter switching frequency and torque ripple size.Use MATLAB/simulink simulation model to establish the open-loop direct torque control system in this paper.The simulation results show that the smaller tolerance,the greater inverter switching frequency and greater pulse of systerm.In the conduction of constant speed, increase the system load, it need to increase applied time of the non-zero voltage vector in the same sector.

Keywords-direct torque control; voltage vector; MATLAB/simulink;open-loop

I. INTRODUCTION

Direct torque control(DTC) is based on the torlerance of flux and torque to adjust given time of zero and non-zero voltage vector (switching state),direct control the inverter switching state ato ensure the torque is stable within the. range of torlerance

In the direct torque control system, torque and flux tolerance size is controlled to output a corresponding voltage vector to control the torque and speed of the system. Flux tolerance determines the size of the sector switch, torque tolerance is to ensure the stability of the electromagnetic torque, the applied voltage vector role in the stator windings to influence the angle between the rotor flux and stater flux ,and adjust the torque [1,2,3]. When speed is constant , how to modulate the applied voltage vector based on different flux and torque tolerance, then affecting the switching frequency and proportion of non-zero or zero voltage vector. When flux and speed is constant ,under different loads, how to adjust the duration of action of the applied voltage vector to ensure that the speed is constant and stable system. In this paper use MATLAB / Simulink simulation, through open-loop direct torque control system to simulate the flux constant and different tolerance differential regulation of the switching device ,the result is when flux constant, the tolerance greater,the switching frequency smaller (the ratio of the non-zero,andzero-voltage vector is 1), speed is the same.

II. BASIC PRINCIPLE[4,5]

Flux tolerance and torque tolerance through the definition of Schmitt trigger to set the high and low threshold to get the high and low level state of the output torque switch signal TQ and flux output singal S� to control the switching state of the converter, when input greater than the tolerance limit ,then output is 0,when below the tolerance lower limit,then output is 1, as shown in Figure 1 and 2.The six voltage vector U1 ,U2, U3, U4, U5, U6, counterclockwise in the regular hexagon formed in the stator flux space vector ts� ,as shown in Figure 3, zero

state U0 located in the center of the hexagon. Flux tolerance to determine the regular hexagon side length (sector) size, the size of torque tolerance decide to a zero voltage vector imposed time within each sector. The relationship of voltage vector and the magnetic chain is as follows (1)

The stator flux : � �=s s s su i R dt� �� (1)

Where, su is the statar voltage, si is current and sR is

resistance ,it can formed regular hexagon flux trajectory

when | |s si R is small enough to | |su .

2 g�

- g� + g�

S�

Figure 1. flux regulater

Tei�

Figure 2. torque regulater

Figure 4 (a) applied to the inverter in turn six non-zero voltage vector derived from the motor input phase voltage waveform ,the diagram of the voltage is imposed by the six non-zero voltage state in accordance with the counterclockwise order of the flux formed hexagonal trajectory, hexagon size is determined in accordance with

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2012 3rd International Conference on System Science, Engineering Design and Manufacturing Informatization

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� �s t�g�

Figure 3. flux hexagon formed by the voltage vector

the flux tolerance of non-zero voltage vector applied. Speed of n; (b) for each sector imposed the zero vector into the same non-zero voltage vector, the insert moment is determined byt the size of torque tolerance, speed is set to n '(the same cycle). Final enquation:

n’=kn (0 k 1) 2

Figure 4 (b) by adding the zero voltage vector, so the smaller the role of each non-zero voltage vector time in the cycle unchanged, the formation of hexagonal trajectory of the flux is smaller, So the speed will decrease.

a Without non-zero voltage vector

(b) Add to the zero voltage vector Figure 4. Phase voltage waveform

Figure 4 (b) in the sector of a non-zero voltage vector, use the zero voltage vector in torque tolerance adjustment process under load TL , illustrates in Figure 5.

Figure 5 (b) tolerance is twice than (a). when torque drop to the lower tolerance limit

T-� of regulator in t1

moment, the regulater output signal TQ is in state 1,imposed the corresponding voltage space vector, then torque rise, To time t2, the torque rise to the upper limit

T� of tolerance, in

the role of TQ=0 , the zero-voltage added to the motor, the

stator flux is stationary, the torque decreases. To the time t3,and repeated the process of time t1. through tolerance of torque regulator switching, the torque ripple limit within a given tolerance range,achieve direct torque control goals. voltage vector time and the entire cycle is constant in (a) and (b) figure.and flux and speed remain unchanged.

Figure 6 for constant torque and flux tolerance,when Load (b) TL2> TL1 (a) ,the adjustment process in a particular sector. After the load becomes large, non-zero voltage vector increases, the zero voltage vector time is reduced, the cycle becomes smaller, the flux and speed remains unchanged.

s�

12 T�LT1T�

1- T�

a smaller tolerance

s��

22 T�LT

2T�

2T��

(b) larger tolerance Figure 5. Torque adjustment under different tolerance

s�

s��

12 T�L1T1T�

1T��

(a) larger load

s�

s��

12 T�L2T1T�

1T��

(b) smaller load Figure 6. Different load torque regulation

The size of the flux hexagon trajectory is determined by the non-zero voltage vector time in each cycle, imposed the voltage vector at different times which changed by the inverter switching state.The formation of hexagonal cycle determines the system frequency, and affect the speed of the system. Figure 5 (a)and (b) is When the load is constant, the torque tolerance changes in the adjustment process. It can be seen from Figure 5, with the tolerance becomes larger,the role in one side of the hexagonal flux trajectory (a sector), non-zero voltage vector synthesis time constant, and the

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cycle remains the same, so the flux is constant, but the torque ripple has increase [6]. Figure 6 (a), (b) is when the torque tolerance unchanged, the adjustment process after the load becomes larger. (b) compared to (a) to increase non-zero vector imposed time and reduce the cycle, the flux remains unchanged, the speed of the system unchanged. When the cycle and load remain constant, non-zero voltage vector is applied to the longer time, the speed.become higher. Direct torque switching frequency is not fixed in the whole process [7].

Under different load and torque tolerance, imposed different times of the non-zero voltage vectors and zero vector to keep the flux and speed constant. In this paper, through following open-loop torque control system to verify the simulation.

III. DIRECT TORQUE OPEN-LOOP SIMULATION SYSTEM

In order to research different torque tolerance and load adjust inverter switch state on the influence of the system speed, Reference [8] of mathematical model of asynchronousn motor,using each modules in MATLAB7.0/Simulink ‘s Power System Blocksets, Connection and established the simulation model of the open-loop direct torque.

Analog inputs of the open-loop simulation system waveform,obtained torque,speed and flux waveforms. Through motor model with 3/2 transform method in the two-phase stationary coordinates -� ,using the flux -

voltage model method ( u-i flux model method) to observe

the flux trajectory. Specific simulation model shown in Figure 7.

Figure 7. Direct Torque open-loop control system simulation structure

IV. THE SIMULATION RESULTS

Simulation model parameters: power P 2kW ,voltage

U 380V ,frequency f 50HZ ,Pole number P 2 ,Stator

resistanceSR 3.52 � , rotor resistance R 4.32r � , stator

inductanceSL 1.2165mH ,rotor inductance

rL 1.2165mH , mutual inductance L 0.8mHm , moment

of inertia 20.0073kg mJ � ,

In order to verify the load changes, the speed and flux unchanged, the corresponding non-zero voltage vector and the frequency changes, compare with the Figure 8 and Figure 9. Figure 8 (a) ~ (d) were given same load torque 1N • m, impose a certain percentage of the voltage vector (30%)

and zero voltage vector (70%) of the three-phase voltage, torque, flux, speed of the system simulation results, the speed for 930 1minr �� . Figure 9 shown the simulation

results of the increase load torque for 2.5N • m ,add non-zero voltage vector applied time (35%) and zero vector for 65%, change the cycle of 90% of the original. Both figure 8 and 9 are applied voltage vecto four times each sector .

For a given load in the different tolerance, impose corresponding voltage vector. Figure 10 shown when the system load torque is 1N • m, under five percent of the rated tolerance,the three-phase voltage, torque, speed of the simulation waveforms , speed is 1000 1minr �� .Doubled

(10%) of the rating tolerance of Figure 10, the simulation results shoen in Figure 11.

-2000

200 Ua/ V

-2000

200

U/ bV

1.02 1.025 1.03 1.035

-2000

200

time /s

Uc/ V

0 1 2-5

0

5

10

15

time /s

Tor

que

/N.m

T

0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

d Axis flux amplitude /Wb q

Axi

s flu

x am

plitu

de

/Wb

0 0.5 1 1.5 2-200

0

200

400

600

800

1000

time /s

Spe

ed

/r/m

in

a (b) (c) (d)

Figure 8. simulation results of load torque 1 N.m

78

-2000

200 Ua/ V

2000

200U/ bV

1 1.005 1.01-200

0200

time /s

Uc/ V

0 0.5 1 1.5 2

-10

-5

0

5

10

15

time /s

Tor

que

/N.m

0 0.5 1 1.5

-1.5

-1

-0.5

0

0.5

d Axis flux amplitude /Wbq A

xis

flux

ampl

itude

/Wb

0 0.5 1 1.5 2

-200

0

200

400

600

800

1000

time /s

Spe

ed /

r/m

in

n

a (b) (c) (d)

Figure 9. simulation results of load torque 2.5N·m

-2000

200 Ua/ V

-2000

200U/ bV

1.02 1.025 1.03 1.035-200

0200

time /s

Uc/ V

0 0.5 1 1.5 2-10

0

10

20

30

time /s

Tor

que

/N

.m

T

0 1 2-500

0

500

1000

time /s

Spe

ed

r/m

in

a b c

Figure 10. simulation results of 5% tolerance ratings

-2000

200 Ua/ V

-2000

200U/ bV

1.02 1.025 1.03 1.035-200

0200

time /s

Uc/ V

0 0.5 1 1.5 2-10

0

10

20

30

time /s

Tor

que

/N

.m

T

0 0.5 1 1.5 2-200

0

200

400

600

800

1000

time /s

Spe

ed

/Wb

n

a b c

Figure 11. simulation results of 10% tolerance rating

Compare with figure 8 and figure 9 ,we can conclude that When the load increases, to maintain the speed and flux of the system unchanged, it need imposes a longer time of non-zero voltage vector. Because this is a Open-loop simulation, the starting of the flux trajectory is not begin from a regular hexagon center, but from the side. Figure 11 shown twice tolerance of Figure 10 , the latter torque waveform width smaller significantly, speed waveform get smoother ,The pulse of the system is smaller, the speed and flux remains same, the inverter switching frequency decreases double.

V. CONCLUSION

In this paper, according to the principle of direct torque control,using MATLAB to build the open-loop Direct Torque Simulation System,Directly applied different times of the voltage vector to the system ,it is concluded that when the load is constant, different torque tolerances size can control the voltage vector applied moment to control the inverter switching frequency, And the speed and flux of the system are unchanged. When the load and frequency constant, non-zero voltage vector imposed longer, the higher speed of the system, the greater relative flux hexagonal trajectory.when load increases, increase the imposed time of

non-zero voltage vector and reduce the cycle of the system to maintain speed and flux constant.

REFERENCES

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[2] Dong Fang,Jun-Yong Zhang, “Simulink research of direct torque control system based on Matlab/simulink,” Manufacturing Automation, vol.32, jan. 2001

[3] Feng Liang and Fang-Lin Wang, “Research of hysteresis controller for direct torque control of induction motor,” ELECTRIC DRIVE,2008

[4] Wei Xin,Da-Yue Chen and Chun-Yu Zhao, “A new direct torque control strategy of induction motors based on duty ratio control technique ,” Proceedings of the CSEE, vol. 25, Jul. 2005

[5] De-Hua Li, AC adjustable speed control system [M].Publishing House of Electronics Industry,2003 ,pp.179–2031

[6] Yu-Lin Zhang,Ding-Guo Jiang and Zhu-Qing Jiao, “Research on direct torque control of induction motor with low torque ripple,” ELECTRIC DRIVE, vol. 38. 2005

[7] Yen-Shin Lai, Jian-Ho Chen. “A new approach to direct torque control of induction motor drives for constant inverter switching frequency and torque ripple reduction,” IEEE Transanction Energy Conersion ,2001,16(3). pp. 220-227

[8] Li-Quan Zhou and Ji-Quan Shen, “Mldeling and simulation of induction motor based on MATLAB/SIMULINK,” TECHNOLOGY OF ELECTRIC MOTOR AND APPLIANV, vol.4. 2003

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