SENSORLESS SPEED ESTIMATION OF INDUCTION MOTOR IN A DIRECT TORQUE CONTROL SYSTEM

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1 TẠP CHÍ PHÁT TIỂN KH&CN, TẬP 9, Ố ENOE PEED ETIMATION OF INDUCTION MOTO IN A DIECT TOQUE CONTO YTEM Pha Dinh Truc (1),Hoang Dang Khoa (2) (1) University of Technology, VNU-HCM (2) Ho Chi Minh City University of Industry (Manuscript eceived on January 26 th, 2006, Manuscript evised Deceber 04 th, 2006) ABTACT: Fast and robust torque control in a very wide range of speed is very needed by various industrial AC drive applications. Therefore, since 1986 [1], direct torque control (DTC) has been introduced to satisfy this desire. In order to achieve ore econoical control, conventional speed sensor has been replacing by sensorless speed estiation. The sensorless schees are used to iprove reliability and decrease aintenance requireents. In this paper, concerned sensorless techniques of induction achine controlled by DTC algorith are openloop estiators and MA schees [2], [3]. To deonstrate clearly the advantages and disadvantages between two kinds of sensorless techniques, obtained siulation results are copared. By enhancing speed estiation, the pure integrator is replaced by a low-pass filter to avoid DC drift and saturation probles [2]. Keywords: ensorless, DTC (Direct Torque Control), IM (Induction Motor). 1. INTODUCTION Nowadays, coparing with the field oriented control (FOC), direct torque control (DTC) is known as a sipler and easier schee to perfor [2], [4], [5]. With DTC technique, the instantaneous values of flux and torque are estiated fro the stator voltages and currents in order to coparing with the coand values. esidual results are used to deterine the optiu inverter switch states through a look-up table to supply for induction otor. As a result, torque can be controlled directly. The stator voltage being used for the estiation is obtained fro DC link voltage the switching states of the inverter (The inverter is assued to be supplied fro an ideal AC-DC converter which takes AC voltage fro the AC grid and provides the inverter a constant DC input). The switching states of the inverter is controlled directly by the central processor, therefore, the On-Off states of the transistors in the inverter are available to the processor. Fro the pre-defined value of DC link voltage, the processor can deterine the stator phase voltage space vectors corresponding to those switching states. The stator phase currents are obtained fro current sensors. In conventional speed control of DTC the actual value of rotor speed is required. The controller receives the signals of rotor speed fro the speed sensors. Unfortunately, the accuracy of the control syste will decrease with the appearance of noises, causing low reliability. Furtherore, the conventional sensors ake the higher cost, increase the coplexity of the systes because of noise filtering. The filtering will help to iprove the quality of feedback speed, however, additional digital filters require higher coputing capacities for faster signal processing and transission. Therefore, they ount additional costs on the overall systes. ecently, any researches have been carried out for the design of speed sensorless control schees [2], [3]. In these new schees the speed is obtained fro the deterined stator voltages and easured stator currents instead of using a sensor. In this paper, two sensorless techniques are presented, an open loop and a close-loop (MA) schee, which can overcoe the necessity of the speed sensor. Trang 33

2 cience & Technology Developent, Vol 9, No This paper is organized as follows. First, DTC algorith is introduced in ection 2. Then, the proposed rotor speed estiation is presented in ection 3. In the ection 4, soe siulation results are presented. Finally, soe concluding rearks are stated in the last ection. 2. THE DTC AGOITHM 2.1 Dynaic Model of Induction Machine All the equations in this paper are defined in stationary reference frae. Voltage and flux equations: U dψ =.I + (1) dψ 0 =.I jω. ψ + (2) ψ =.I +.I (3) ψ =. I +.I (4) Torque equations: 3 Te = p ψ ψ (5) 2 2 (. ) dω Te T = J (6) The sybol in equation (5) denotes vector product. By using space vector theory, there are eight voltage space vectors (six non-zero and two zero vectors) for an induction feeding by a conventional voltage inverter [2], [4]. 2.2 ynthesis of the Takahashi s DTC Approach In the equation (1), by neglecting the effect of stator voltage drop across stator resistance, stator flux can be estiated directly fro the stator voltage and the change of the stator flux depends on the change of the stator voltage. d ψ U (7) By defining σ=(1-2 ), T = and rearranging the equation fro (1) to (4), a forula. of the rotor flux is obtained. ψ = ψ (8) 1+ s.t. σ j. ω.t. σ Equation (8) iplies that rotor flux is varied by the change of stator flux. Because of the lager of rotor tie constant (T ), rotor flux vector is assued stationary during a sall tie interval when stator flux vector is rotating. The torque value in the equation (5) can be expressed in another way. T e = 2 3 p.( 1 )( ). σ. ψ ψ.sin(δ -δ ) (9) Trang 34

3 TẠP CHÍ PHÁT TIỂN KH&CN, TẬP 9, Ố Fro (7) to (9), it is iportant to note that, the torque of an induction achine is produced by the interaction between rotor and stator flux space vectors. In a tie interval sall enough, rotating of the stator flux space vector in appropriate direction, according to the deanded torque, can result in a rapid and desirable changes of actual electroagnetic torque. It should be ephasized that the suitable stator flux space vector will be obtained by using the optiized inverter switching look-up table of Takahashi [1], [2], [5]. Figure 1.The changes of stator flux with the choosing switching states. Table 1. The Takahashi s optiized switching table. Order T e T e Not order T e ψ V k+1 V k-1 V 0,7 ψ V k+2 V k-2 V 0,7 3. THE ENOE ETIMATION TECHNIQUE 3.1 Open-loop speed estiation The open-loop speed estiation is based on the residual between the speed of rotor flux and the slip speed [2]. The schee described below uses the onitored stator voltages and currents to reconstruct the rotor flux, torque by equations fro (1) to (5). d d ψα ψβ ψβ ψα ω = Te (10) ( ψ ) + ( ψ ) 3 p ψ α β This schee requires several achine paraeters, soe of which vary with teperature, skin effect and saturation. Thus, the speed can only be obtained accurately if these paraeters are accurately known. 3.2 Model eference Adaptive yste (MA) In a MA syste, rotor flux vector is estiated in a reference odel, which is independent of speed, and then copared with the one estiated by using an adaptive odel, which is using speed as a paraeter. The reference and adaptive odel are obtained by rearranging the equations fro (1) to (4). eference odel equations: Trang 35

4 cience & Technology Developent, Vol 9, No ψ = [( U. I )- σ.. I ] (11) Adaptive odel equations: ψ = [( j. )..I ] 1 + ω ψ + (12) T T Error betwen two odel: ε = I( ψ. ψ ) = ( ψ. ψ ψ. ψ ) (13) ω β α α β Figure 2. MA-based speed estiator schee The error between two odels is the input of a PI control whose output is the estiated rotor speed, this estiated speed is used to adjust the adaptive odel until satisfactory perforance obtained [2], [4]. ince the MA is an close-loop syste, the accuracy can be increased. However, the odels contain pure intergrators, which cause inaccuration of the estiation syste because of DC drift and saturation at the outputs of the intergrators. To avoid the proble, low-pass filters are used. 4. IMUATION EUT Two sensorless high perforance drives of induction achines using DTC are siulated, one with open-loop speed estiator and one with closed loop MA speed estiator. Both systes are siulated for the first two seconds, including acceleration during starting fro zero speed to rated speed, steady state at rated speed, and deceleration fro rated speed to about 10 rad/s. oad rejection tests are carried out during steady states at rated speed as well as low speed. These tests are aied at investigating the disturbance rejection ability of the controller by aintaining the actual speed at the coanded value of the controller when load suddenly increases and decreases. MATAB/IMUINK is used to carry out the siulations above. This software allows digital siulation of the systes using analogue expression of the ordinary differential equations in the dynaic achine odel as well as the controller. The nuerical ethod for solving the equations is unge-kutta ethod. Fixed-step ode is chosen for the coputational tie interval, this will eulate the fixed sapling frequency of the real-tie control. The sapling period is 1ìs. This sapling frequency is higher than the actual sapling frequency of industrial DTC controllers, which is usually about 100kHz due to the liitation on the switching frequencies of power electronic coponents. However, to verify the correctness of Trang 36

TẠP CHÍ PHÁT TIỂN KH&CN, TẬP 9, Ố12-2006 the suggested algoriths, highly accurate data is necessary. Noinal paraeters for the test otor are provided in Table 2.

5 TẠP CHÍ PHÁT TIỂN KH&CN, TẬP 9, Ố the suggested algoriths, highly accurate data is necessary. Noinal paraeters for the test otor are provided in Table 2. Table 2. IM test otor paraeters Paraeter Value H H H 1.37Ω 1.1Ω T e 26.5N J 0.1(kg.N/) P 2 U 240(V) Flux eference Wb Figure 3. eference load and speed Figure 4. oror speed with the open-loop estiation schee. Figure 5. Estiated peed with the open-loop estiation schee at stabe state (Zoo F.4). Figure 6. Torque response with the open-loop sche6 Trang 37

6 cience & Technology Developent, Vol 9, No Figure 7. oror speed with the MA estiation schee Figure 8. Transient of rotor speed when add and reove load at high-speed Figure 9. Estiation peed with the MA estiation schee at stabilty state. Figure 10. Torque response with the MA schee. Figure11. oror speed with the MA when replacing pure integrators by low-pass filters. 6. CONCUION Fro the result above, the advantage and disadvantage between two schees have been analysed. Both open-loop and closed-loop schee have good speed responses during loading or Trang 38

7 TẠP CHÍ PHÁT TIỂN KH&CN, TẬP 9, Ố unloading, even at the low speed. The electroagnetic torque has fast responses because of the advantage of DTC technique (figure 6 and figure 10). It is significant that the closed-loop schee is ore accurate than the open-loop one due to the presence of the PI controller in the closed-loop syste that balances the ripper of rotor speed. One of concerned proble is the swiching frequency, the open-loop syste requires higher switching frequency than closed-loop schee because of the lack of the PI controller (figure 5 and figure 9). This can be solved by using an Butterworth filter or other low-pass filter. Both open and closed-loop schees rely on the stability of achine paraeters for a high accuracy. Although the closed-loop estiator is sensitive to paraeter fluctuation, it is still effected by the variation of stator resistance due to theral effect. To solve this proble, a theral odel of induction achine should be concerned. By replacing the pure integrator in the MA schee with a low-pass filter, the practical ipleentation will be ore effective because of the eliination of DC drift and saturation probles (figure 11). ƯỚC ƯỢNG VẬN TỐC ĐỘNG CƠ KHÔNG DÙNG CẢM BIẾN TONG HỆ THỐNG ĐIỀU KHIỂN TỰC TIẾP MOMENT Phạ Đình Trực (1), Hoàng Đăng Khoa (2) (1)Trường Đại học Bách khoa, ĐHQG-HCM (2)Trường Đại học Công nghiệp Tp.HCM TÓM TẮT: Bài báo trình bày các phương pháp ước lượng vận tốc dùng trong điều khiển trực tiếp oent (DTC) của động cơ không đồng bộ. Hai nhó phương pháp chủ yếu được dùng là phương pháp ước lượng vận tốc ạch hở và phương pháp ước lượng vận tốc ạch hồi tiếp. Điều khiển thích ứng ô hình được dùng trong phương pháp ước lượng vận tốc ạch hồi tiếp. Điều khiển thích ứng ô hình cho phép giả tối thiểu ảnh hưởng của sai số cũng như sự dao động của các giá trị tha số động cơ lên kết quả ước lượng vận tốc. Các kết quả ô phỏng của hai phương pháp trên sẽ được so sánh. Để kết quả ô phỏng bá sát thực tế,các ạch tích phân trong bộ điều khiển sẽ được thay thế bởi các ạch lọc tần số thấp trong các ô phỏng. EFEENCE [1]. I.Takahashi and T. Noguchi, A New Quick-esponse and High-Efficiency Control trategy of an Induction Motor, IEEE Trans.Ind Appl, Vol. IA-22, No.5, pp , epteber, (1986). [2]. P.Vas, ensorless Vector and Direct Torque Control, Oxford University Press. pp , (1998). [3]. P.Vas, ensorless Drivers, tate-of-art. University of Aberdeen, (2002). [4]. Bial K.Bose, Modern Power Electronics and AC Drivers, Prentice Hall PT. pp , (2002). [5]. Direct Torque Control- the world's ost advanced AC drive technology, Technical Guide No. 1, ABB website. Trang 39

DIRECT TORQUE CONTROL OF INDUCTION MACHINES CONSIDERING THE IRON LOSSES

DIRECT TORQUE CONTROL OF INDUCTION MACHINES CONSIDERING THE IRON LOSSES TRUC PHAM-DINH A thesis subitted in partial fulfilent of the requireents of Liverpool John Moores University for the degree of Doctor