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A novel switching scheme for torque ripple reduction in Direct Torque Control of induction motor.

Introduction

Advanced control of electrical machines requires an independent control of magnetic flux and torque. For that reason it was not surprising, that the DC machine played an important role in the early days of high performance electrical drive systems, since the magnetic flux and torque are easily controlled by the stator and rotor current, respectively. The introduction of Field Oriented Control [1] meant a huge turn in the field of electrical drives, since with this type of control the robust induction machine can be controlled with a high performance. Later in the eighties a new control method for induction machines was introduced: The Direct Torque Control (DTC) method is characterized by its simple implementation and a fast dynamic response. Furthermore, the inverter is directly controlled by the algorithm, i.e. a modulation technique for the inverter is not needed. However if the control is implemented on a digital system (which can be considered as a standard nowadays); the actual values of flux and torque could cross their boundaries too far [2] [3], which is based on an independent hysteresis control of flux and torque. The main advantages of DTC are absence of coordinate transformation and current regulator absence of separate voltage modulation block. Common disadvantages of conventional DTC are a sluggish response (slow response) in both starts up and changes in either flux or torque, large and small errors in flux and torque are not distinguished. In other words, the same vectors are used during start up and step changes and during steady state. In order to overcome the mentioned drawbacks, there are different solutions, which can be classified as follows, modification of the switching table, so modified DTC (M_DTC) and twelve sectors DTC (12_DTC). In this paper a comparison of various direct torque control methodologies (Conventional TC, M_DTC, and 12_DTC) have been presented with evaluation of the influence on the transient performances of induction motor.

Direct Torque Control with Three-level Inverter

The basic functional blocks used to implement the DTC scheme are represented in Figure.1. The instantaneous values of the stator flux and torque are calculated from stator variable by using a closed loop estimator [1]. Stator flux and torque can be controlled directly and independently by properly selecting the inverter switching configuration.

[FIGURE 1 OMITTED]

DTC Twelve Sector Table (12_DTC)

In Conventional DTC there are two states per sector that present a torque ambiguity. Therefore, they are never used. In a similar way, in the modified DTC there are two states per sector that introduce flux ambiguity, so they are never used either. It seems a good idea that if the stator flux locus is divided into twelve sectors instead of just six, all six active states will be used per sector. Consequently, it is arisen the idea of the twelve sector modified DTC (12_DTC). This novel stator flux locus is introduced in Fig.2 [4].

[FIGURE 2 OMITTED]

FD/FI: Flux Decrease/Increase. TD/TI: Torque Decrease/Increase.

TsD/TsI: Torque small Decrease/Increase.

Notice how all six voltage vectors can be used in all twelve sectors, disappearing all ambiguities.

Table.1 can be written when a twelve-sector locus is used.

It is necessary to define small and large variations. It is obvious that V1 will produce a large increase in flux and a small increase in torque in sector S12. On the contrary, V2 will increase the torque in large proportion and the flux in a small one. It is reasonable to deduce that the torque error should be divided in the number of intervals that later on will be measured. Therefore, the hysteresis block should have four hysteresis levels at is suggested in Tab.2 [5] [6] [7].

Simulation Results

The results of simulation of Conventional DTC (C_DTC), Modified DTC (M_DTC) and proposed Twelve sector DTC (12_DTC) of induction motor is shown in figures 3,4,5 and 6. The simulation was done using MATLAB Simulink. The Induction motor used in this case study is a 119 KW,460V , 3600 rpm, 2-pole, 50 Hz, 3-phase induction motor having the following parameters

Rs = 14.85 X [10.sup.-3] ohm, Rr = 9.295 X [10.sup.-3] ohm

Ls = Lr = 0.3027 X [10.sup.-3] H, Lm = 10.46 X [10.sup.-3] H

From 0.02s to 0.25s, the fan speed increases because of the 600 Nm acceleration torque produced by the induction motor. At t=0.25s, the electromagnetic torque jumps down to 0 Nm and the speed decreases because of the load torque opposed by the fan. At t = 0.5s, the motor torque develops a -600 Nm torque and allows braking of fan. During braking mode, power is sent back to the DC bus and the bus voltage increases. As planned, the braking chopper limits the DC bus voltage to 700V. At t = 0.75s, the electromagnetic torque jumps back to 0 Nm and the speed settles around -10 rpm. The flux stays around 0.8Wb throughout the simulation. The flux and torque oscillation amplitudes are slightly higher than 0.02 Wb and 10 Nm respectively as specified in the user interface. This is due to the combined effects of the 15 [micro]s DTC controller sampling time, the hysteresis control, and the switching frequency limitation. The simulation results show that the torque responses are very good dynamic response for three DTC methods, but the response of the torque conventional DTC and modified DTC presented the ripple. In 12 sector DTC shown in figure 6, the ripple of torque is reduced remarkably compared with conventional and modified DTC. Figure 7 shows the comparison of different DTC ripples.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Note: X-axis represents "Time" in seconds Y-axis represents "Electromagnetic Torque" in Nm

[FIGURE 7 OMITTED]

Conclusion

The proposed work presents various DTC methodologies (Conventional DTC (C_DTC), modified DTC (M_DTC) and twelve sector DTC (12_DTC)) and also evaluate their influence on the motor operating condition (transient state performance).An increased emphasis on torque ripple has been done. The simulation results suggest that modification of conventional DTC of induction motor can achieve precise control of the torque. Compared to conventional DTC, the presented method can be easily implemented and the steady performances of ripple of torque are shown to considerably improve.

To summarize, the main improvements shown in this paper are

(1) Reduction of torque ripple, both in transient and steady state response.

(2) No flux droopings caused by sector circular trajectory changes.

(3) Faster stator flux response in transient state.

References

[1] P. Vas (1995), "DSP controlled Intelligent High performance AC Drives, Present and Future", IEE, Savoy place, London, WC2R OBL, pp 1-8.

[2] Giuseppe Buja (1997), "Direct Torque Control of Induction motor drives", Proceedings of the ISIE'97 Int.Conf, pp. TU2-TU8.

[3] Luis A (1997), " Learning Techniques to Train Neural Networks as a State selector for Inverter fed Induction Machines using Direct Torque Control", IEEE Transactions on Power Electronics, VOL 12, NO.5, pp 788-799.

[4] S. K. Panda (1999), "Direct Torque Control of Induction Motor-Variable Switching Sectors", Proceedings of the PEDS'99 Int. Conf on Power Electronics and Drive System, pp. 80-85.

[5] Yen Shin-Lai (2001), "A New Approach to Direct Torque Control of Induction Motor Drives for Constant Inverter Switching Frequency and Torque Ripple Reduction", IEEE Transactions on Energy Conversion, VOL 16, NO.3, pp 220-227.

[6] Domenico Casadei (2002), "FOC and DTC: Two Viable Schemes for Induction Motors Torque Control", IEEE Transactions on Power Electronics, VOL 17, NO.5, pp 779-787.

[7] Marian P.Kazmierkowski (2004), "Direct Torque Control of PWM Inverter-Fed AC Motors-A Survey", IEEE Transactions on Industrial Electronics, VOL 51, NO.4, pp 744-757.

A. Sivasubramanian (1) and B. Jayanand (2)

(1) Ph.D Research Scholar, Dr. M.G.R. University, Chennai, Tamil Nadu, India E-mail: sivapollachi2002@yahoo.co.in

(2) Department of Electrical and Electronics Engineering, Government Engineering College, Thrissur, Kerala, India E-mail: jayanandb@gmail.com
Table. 1 Table for sectors 12 and 1 in the 12_DTC.
Notice how all six voltage vectors can be used
in all sectors disappearing al ambiguities.

[S.sub.12]    INCREASE
Stator Flux   [V.sub.1], [V.sub.2], [V.sub.6]
Torque        [V.sub.1], [V.sub.2], [V.sub.3]

[S.sub.1]     I INCREASE
Stator Flux   [V.sub.1], [V.sub.2], [V.sub.6]
Torque        [V.sub.2], [V.sub.3], [V.sub.4]

[S.sub.12]    DECREASE
Stator Flux   [V.sub.3], [V.sub.4], [V.sub.5]
Torque        [V.sub.4], [V.sub.5], [V.sub.6]

[S.sub.1]     DECREASE
Stator Flux   [V.sub.3], [V.sub.4], [V.sub.5]
Torque        [V.sub.5], [V.sub.6], [V.sub.1]

Table. 2 Switching tablefor the 12_DTC.

[PHI]                               F1
[tau]           TI           TsI           TsD           TD

[S.sub.1]    [V.sub.2]   * [V.sub.2]   [V.sub.1]     [V.sub.6]
[S.sub.2]    [V.sub.3]   [V.sub.2]     * [V.sub.1]   [V.sub.1]
[S.sub.3]    [V.sub.3]   * [V.sub.3]   [V.sub.2]     [V.sub.1]
[S.sub.4]    [V.sub.4]   [V.sub.3]     * [V.sub.2]   [V.sub.2]
[S.sub.5]    [V.sub.4]   * [V.sub.4]   [V.sub.3]     [V.sub.2]
[S.sub.6]    [V.sub.5]   [V.sub.4]     * [V.sub.3]   [V.sub.3]
[S.sub.7]    [V.sub.5]   * [V.sub.5]   [V.sub.4]     [V.sub.3]
[S.sub.8]    [V.sub.6]   [V.sub.5]     * [V.sub.4]   [V.sub.4]
[S.sub.9]    [V.sub.6]   * [V.sub.6]   [V.sub.5]     [V.sub.4]
[S.sub.10]   [V.sub.1]   [V.sub.6]     * [V.sub.5]   [V.sub.5]
[S.sub.11]   [V.sub.1]   * [V.sub.1]   [V.sub.6]     [V.sub.5]
[S.sub.12]   [V.sub.2]   [V.sub.1]     * [V.sub.6]   [V.sub.6]

[PHI]                               FD
[tau]           TI           TsI           TsD        TD

[S.sub.1]    [V.sub.3]   [V.sub.4]     [V.sub.7]   [V.sub.5]
[S.sub.2]    [V.sub.4]   * [V.sub.4]   [V.sub.5]   [V.sub.6]
[S.sub.3]    [V.sub.4]   [V.sub.5]     [V.sub.0]   [V.sub.6]
[S.sub.4]    [V.sub.5]   * [V.sub.5]   [V.sub.6]   [V.sub.1]
[S.sub.5]    [V.sub.5]   [V.sub.6]     [V.sub.7]   [V.sub.1]
[S.sub.6]    [V.sub.6]   * [V.sub.6]   [V.sub.1]   [V.sub.2]
[S.sub.7]    [V.sub.6]   [V.sub.1]     [V.sub.0]   [V.sub.2]
[S.sub.8]    [V.sub.1]   * [V.sub.1]   [V.sub.2]   [V.sub.3]
[S.sub.9]    [V.sub.1]   [V.sub.2]     [V.sub.7]   [V.sub.3]
[S.sub.10]   [V.sub.2]   * [V.sub.2]   [V.sub.3]   [V.sub.4]
[S.sub.11]   [V.sub.2]   [V.sub.3]     [V.sub.0]   [V.sub.4]
[S.sub.12]   [V.sub.3]   * [V.sub.3]   [V.sub.4]   [V.sub.5]

FD/FI: Flux Decrease/Increase

TD/=/I: Torque Decrease(Equal/Increase.

(* there is no suitable state. It has been chosen the second
most suitable).
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Author:Sivasubramanian, A.; Jayanand, B.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Feb 1, 2009
Words:1867
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