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Implementation of Direct Torque Control Scheme for Induction Machines with Variable Structure Controllers
TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 13/20 pp593-597 Volume 10, Number 5, October 2005
Implementation of Direct Torque Control Scheme for Induction Machines with Variable Structure Controllers LI Jian (ळ
), YANG Geng (ཷ ٖ) **, WANG Huan’gang (ฆܵ)ر, XU Wenli (༗ำो) Department of Automation, Tsinghua University, Beijing 100084, China
Abstract: A torque control scheme for high-performance induction machine drives was developed to overcome some disadvantages of direct torque control (DTC). In the improved DTC method, the stator flux and the torque controllers use variable-structure control theory which does not require information about the rotor speed. Space vector modulation is applied to the voltage source inverter to reduce the torque, stator flux, and current ripples. The digital signal processor-based implementation is described in detail. The experimental results show that the system has good torque and stator flux response with small ripples. Key words: variable-structure control; torque control; induction machines; flux estimation
Introduction The development of high-performance control strategies for induction machine (IM) drives needed by industry has evolved rapidly during the last two decades. The two high-performance control strategies for induction motor drives are field-oriented control (FOC) and direct torque control (DTC)[1,2]. Both can achieve good transient torque performance, but have some essential disadvantages[3-5]. In traditional DTC, torque and current ripples are caused by random switching times, and the performance at lower speeds is not satisfactory. The FOC scheme, however, is very sensitive to variation of the rotor parameters. Many improvements have been proposed to overcome the disadvantages of DTC. One of them is the use of space vector modulation (SVM) instead of improving the DTC look-up table[6,7]. Hoang[4] added a discrete time sliding mode to the torque and flux control of DTC, but the implementation needs several motor parameters and more complicated calculations. Received: 2004-03-22; revised: 2004-05-17
γγ To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62792512
Lascu and Andrzej[7] used a torque controller combining the sliding mode and space vector modulation. The tests showed a fast dynamic response with small torque and stator flux ripples at static state. However, the method uses rotor parameters to construct the stator flux observer, so the system performance is also sensitive to parameters. Furthermore, these two papers do not verify that the algorithms guarantee system stability for all operating conditions. Recently, Wang et al.[8] presented a new variablestructure torque control scheme for high-performance induction machine drives. In this scheme, two variable-structure controllers regulate the stator flux, the amplitude, and the torque. As with traditional DTC, only the stator resistance is required to estimate the stator flux so the scheme is less sensitive to parameter variations. In addition, the control algorithms do not require motor speed information and they verified that the system was stable for all conditions. This paper describes how the control scheme is implemented and gives test results for the system. Methods given to construct the variable-structure controllers for the torque and stator flux are also described.
Tsinghua Science and Technology, October 2005, 10(5): 593̢597
594
1
torque can be regulated by the stator voltage components, usd and usq . The highly nonlinear and
Control Scheme
The dynamic model of IM in the fixed stator reference frame (D , E ) can be described by the stator flux vector M s and the rotor flux vector
°Ms ° ® °M °¯ r where
Ls
Rs RM M s s M r us , V Ls V Ls Lr
Rr M
V Ls Lr
and
M r as:
Lr
r
Ms , \ r
The system block diagram in Fig. 1 includes variable-structure stator flux and torque controllers, a voltage vector generator, an SVM modulator, and stator flux and torque estimators. The first two blocks differ from the traditional DTC as follows.
Rr
V Lr
M r Zr J M r
ª0 1º «1 0 » . ¬ ¼
Variable-structure controllers for the stator flux and the torque are expressed as:
M r , and U s
us , then in
usd ° ® °usq ¯
the stator flux reference frame, the torque Te can be expressed as
Te
s
are the stator and rotor
Ms
1 ( M 2 /( Ls Lr )), and
Defining\ s
r
disturbances regulating the stator flux and the torque[8]. Thus, variable-structure controllers can be designed for \ s and Te .
Rr the stator and rotor resistances, Z r the rotor
J
s
(1)
self-inductances, M the mutual inductance, Rs and speed, V
coupled dynamics in Eq. (3) complicate the design of a simple, high-performance system with conventional linear controllers. However, if the stator flux amplitude \ s is controlled to be constant, the variables \ cos(T T ) and T can be analyzed as bounded
M \ \ sin(T s T r ) V Ls Lr s r
(2)
(3)
RsTeREF
\ sREF
KTe eTe H Te sgn eTe
(4)
where, 2 Rs ° K\ t 0, KTe t 0, H\ t V L \ sREF , s ° ® 2 · 2V Ls Lr § Rr °H ! Z \ TeREF ¸ ¨ TeREF r sREF 2 ° Te M \ sREF © V Lr ¹ (5) ¯
and the system described by Eqs. (1) and (2) can then be rewritten as: Rs Rs M °\ s V L \ s V L L \ r cos(Ts T r ) usd , ° s s r ® 2 \ \ °T s T s u s sq °¯ e Rs Rs
K\ e\ s H\ sgn e\ s ,
in which \ sREF and TeREF are reference values of the stator flux amplitude and the torque. K\ , H\ , KTe ,
U s sin(Tu Ts ) . T s ,
and H Te are positive constants. Since the controller
T r , and Tu denote the stator flux, rotor flux, and
parameters H\ and H Te can be acquired from the
stator voltage angles. The induction machine is assumed to have only one pole pair to simplify the analysis.
inequalities (5) by analyzing the variation of the machine parameters, the tuning of the variablestructure controllers is independent of the accuracy of the machine parameters except for the stator resistance.
where usd
U s cos(Tu Ts ) , usq
From Eq. (3), the stator flux amplitude and the
Fig. 1
Block diagram of the system ( e\ s
\ sREF \ s ˈ eTe
TeREF Te ).
LI Jian ळ
et al˖Implementation of Direct Torque Control Scheme for Induction Machines ĂĂ
The voltage components [usd , usq ]T in Eq. (4) can be transformed into the stationary reference frame D , E by Eq. (6). Consequently, [usD , usE ]T can be modulated with SVM to drive a voltage-source inverter (VSI) with the switching frequency set to constant. ªusD º ªcos Ts sin T s º ªusd º us « » « (6) »« » ¬usE ¼ ¬ sin T s cos T s ¼ ¬usq ¼ The stator flux values MsD , M sE , and \ s can be obtained by integrating the back electromotive force. M sD ° ® °¯M sE
\s
³ (u D i D R )dt , ³ (u E i E R )dt
(7)
Ms2D Ms2E
(8)
s
s
s
s
s
s
°sin T s M sE /\ s , ® °¯cosT s MsD /\ s
(9)
The torque is calculated using Eq. (10): Te
isE MsD isD M sE
(10)
Wang et al.[8] demonstrated that these algorithms can guarantee system stability for all operating conditions.
2
Implementation
2.1
System structure and hardware configuration
595
permanent magnetic AC motor and its controller were used to supply the load torques. The rotor speed detected by an encoder was used only for the speed feedback control. The system included protection functions such as over or under voltage protection and over current protection. The double-CPU board had TMS320F240 and 80C196KC CPUs. The current signals were sent to the A/D conversion inputs of the digital signal processor (DSP). The rotor position was transmitted from the 80C196KC to the DSP. The flux and torque commands and the regulator parameters could be set with the keyboard. The servo system functions were displayed on the display board. The position feedback was only used for speed control. The system sampling time was 0.2 ms and the PWM frequency was 7.6 kHz. The control scheme parameters were: K\ =10, KTe =30, H\ =100, and H Te =100. 2.2
Software implementation
The algorithms required by the variable-structure torque control and the inverter control using the SVM technique were implemented in software as shown in Fig. 3.
The system configuration is shown in Fig. 2. The motor parameters were: 3 phases, 4 poles, 3 kW, 380 V, 6.8 A, and Rs = 1.87 ȍ.
Fig. 3 Software implementation
Fig. 2 System structure and hardware configuration (PM motor: Permanent magnetic motor)
The servo system included a double-CPU board, a power inverter board, and a general induction motor with an optical encoder. The power board included a rectifier, an intelligent power module (IPM) with its drive circuit, and current measuring circuits. A
2.2.1 Stator flux estimate The stator flux estimation is an important task in the control scheme. Its accuracy directly affects the motor drive performance. The flux calculation was determined theoretically by Eq. (7), but the implementation of the integrator for the motor flux estimate is difficult. A pure integrator has DC drift and initial value problems. One improvement is to construct a stator flux observer[7], using the rotor parameters, but the estimated flux values are then sensitive to variations of the rotor parameters. Another common
Tsinghua Science and Technology, October 2005, 10(5): 593̢597
596
solution to the DC drift problem of the pure integrator is to replace it with a first-order low-pass (LP) filter, which can be expressed as
M s | (Ts 1)1 (us is Rs )
(11)
where T is a time constant. This equation can be realized in digital form as[9]
M s (n) G M s (n 1) [us (n 1) Rs is (n 1)] (12) where M s (n) is a discrete vector of the state flux and
2.2.2 Excitation In this control scheme, the stator flux amplitude must be constant. However, during excitation, the stator flux direction is not controlled which creates problems since the control scheme works with a stator having fixed amplitude and direction. Simple implementation of the flux-loop does not solve the problem because of the calculation errors at low speeds. An improved method is illustrated in Fig. 5.
G is a gain slightly less than 1. The selection of G has a small influence on the system performance as shown in Fig. 4. Te and \ s are the actual torque and actual stator flux, while Tˆe and \ˆ s are the torque and flux calculated by Eqs. (8), (10), and (12). The errors 'Te Te Tˆe and '\ s \ s \ˆ s vary with changes of the rotor speed and G . Since the measurement of the actual motor voltage us is difficult, the voltage command us* was used in the controller instead of voltage measurements. The error between the voltage command and the actual voltage is mainly produced by the dead-band time of the inverter[10]. This error can be reduced by compensating for the time in the actual motor voltage or by calculating the actual voltage using C us | us* udeadtim e
(13)
C where udeadtime is the voltage error caused by the
dead-band time.
Fig. 4
Fig. 5
Excitation process
First, a voltage with fixed magnitude and direction is applied to the stator for about 20 ms. The magnitude is not very large and the direction is the same as the specified flux’s direction. Secondly, close the flux loop. The flux loop assures that the excitation result is the desired value. Finally, close the torque loop after about 100 ms.
3
Experimental Results
In the following experiments, the PWM serial ports of the DSP change the waveforms of the estimated stator flux and machine torque. Then, the LP filters with delay times of about 10 ms filter the output signals. As a result, the real responses are quicker than these shown here. Figure 6 shows the stator flux response for the entire excitation process described in Section 2.2.2. The three parts are clear in the figure. Figure 7 shows the phase current during steady state. Figure 8 shows the static waveform of the motor stator flux. Figure 9 shows the response to a step torque change. The torque approaches the step change quickly without error, but has small ripples during steady state.
Influence of LP filter Fig. 6
Response to step flux change (\sREF˙0.5 Wb)
LI Jian ळ
et al˖Implementation of Direct Torque Control Scheme for Induction Machines ĂĂ
597
References [1] Ortega R, Barabanov N, Valderrama G E. Direct torque control of induction motors: Stability analysis and performance improvement. IEEE Trans. on Automatic Control, 2001, 46(8): 1209-1222. Fig. 7
Phase current during steady state
[2] Buja G, Casadei D, Serra G. Direct stator flux and torque control of an induction motor: Theoretical analysis and experimental results. In: Annual Conference of the IEEE Industrial Electronics Society IECON. Aachen, Germany, 1998: T50-T64. [3] Telford D, Dunnigan M W, Williams B W. A comparison of vector control and direct torque control of an induction machine. In: Conference Record of IEEE PESC’00. 2000, 1: 421-426. [4] Hoang Le-Huy. Comparison of field-oriented control and direct torque control for induction motor drives. In: Conference Record of IEEE IAS’99. 1999, 2: 1245-1252. [5] Casadei D, Profumo F, Serra G, Tani A. FOC and DTC: Two viable schemes for induction motors torque control.
Fig. 8
Stator flux during steady state (\sREF˙0.5 Wb)
IEEE Trans. on Power Electronics, 2002, 17(5): 779-787. [6] Neves F S, Landim R P, Habetler T G. Induction motor DTC strategy using discrete-time sliding mode control. In: Conference Record of IEEE IAS’99. 1999, 1: 79-85. [7] Lascu C, Andrzej M T. Combing the principles of sliding mode, direct torque control, and space vector modulation in
a
high-performance
sensorless
AC
drives.
In:
Conference Record of IEEE IAS’ 02. 2002, 3: 2073-2079. [8] Wang Huan’gang, Xu Wenli, Yang Geng, Li Jian. Variable-structure direct torque control of induction motors using space vector modulation. Electrical Engineering, 2005, 87(2): 93-102.. Fig. 9 Response to step torque change (TeREF˙ 5.7 Ngm, \sREF˙0.6 Wb)
4
Conclusions
A new torque control scheme was implemented for high-performance induction machine drives which combines variable-structure controllers and space vector modulation. This paper describes the control algorithms and the improved method to estimate the stator flux. The implementation shows the simplicity and practicality of the scheme. The experimental results illustrate that the system has good torque and stator flux response with small ripples.
[9] Hurst K D, Habetler T G, Griva G, Profumo F. Zero-speed tacho-less IM torque control: Simply a matter of stator voltage integration. IEEE Trans. Industry Applications, 1998, 34(4): 790-795. [10] Kim Hyun-Soo, Kim Kyeong-Hwa, Youm Myung-Joong. On-line dead-time compensation method on time delay control. IEEE Trans. Control Systems Technology, 2003, 11(2): 279-285.
Implementation of Direct Torque Control Scheme for Induction Machines with Variable Structure Controllers LI Jian (ळ
), YANG Geng (ཷ ٖ) **, WANG Huan’gang (ฆܵ)ر, XU Wenli (༗ำो) Department of Automation, Tsinghua University, Beijing 100084, China
Abstract: A torque control scheme for high-performance induction machine drives was developed to overcome some disadvantages of direct torque control (DTC). In the improved DTC method, the stator flux and the torque controllers use variable-structure control theory which does not require information about the rotor speed. Space vector modulation is applied to the voltage source inverter to reduce the torque, stator flux, and current ripples. The digital signal processor-based implementation is described in detail. The experimental results show that the system has good torque and stator flux response with small ripples. Key words: variable-structure control; torque control; induction machines; flux estimation
Introduction The development of high-performance control strategies for induction machine (IM) drives needed by industry has evolved rapidly during the last two decades. The two high-performance control strategies for induction motor drives are field-oriented control (FOC) and direct torque control (DTC)[1,2]. Both can achieve good transient torque performance, but have some essential disadvantages[3-5]. In traditional DTC, torque and current ripples are caused by random switching times, and the performance at lower speeds is not satisfactory. The FOC scheme, however, is very sensitive to variation of the rotor parameters. Many improvements have been proposed to overcome the disadvantages of DTC. One of them is the use of space vector modulation (SVM) instead of improving the DTC look-up table[6,7]. Hoang[4] added a discrete time sliding mode to the torque and flux control of DTC, but the implementation needs several motor parameters and more complicated calculations. Received: 2004-03-22; revised: 2004-05-17
γγ To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62792512
Lascu and Andrzej[7] used a torque controller combining the sliding mode and space vector modulation. The tests showed a fast dynamic response with small torque and stator flux ripples at static state. However, the method uses rotor parameters to construct the stator flux observer, so the system performance is also sensitive to parameters. Furthermore, these two papers do not verify that the algorithms guarantee system stability for all operating conditions. Recently, Wang et al.[8] presented a new variablestructure torque control scheme for high-performance induction machine drives. In this scheme, two variable-structure controllers regulate the stator flux, the amplitude, and the torque. As with traditional DTC, only the stator resistance is required to estimate the stator flux so the scheme is less sensitive to parameter variations. In addition, the control algorithms do not require motor speed information and they verified that the system was stable for all conditions. This paper describes how the control scheme is implemented and gives test results for the system. Methods given to construct the variable-structure controllers for the torque and stator flux are also described.
Tsinghua Science and Technology, October 2005, 10(5): 593̢597
594
1
torque can be regulated by the stator voltage components, usd and usq . The highly nonlinear and
Control Scheme
The dynamic model of IM in the fixed stator reference frame (D , E ) can be described by the stator flux vector M s and the rotor flux vector
°Ms ° ® °M °¯ r where
Ls
Rs RM M s s M r us , V Ls V Ls Lr
Rr M
V Ls Lr
and
M r as:
Lr
r
Ms , \ r
The system block diagram in Fig. 1 includes variable-structure stator flux and torque controllers, a voltage vector generator, an SVM modulator, and stator flux and torque estimators. The first two blocks differ from the traditional DTC as follows.
Rr
V Lr
M r Zr J M r
ª0 1º «1 0 » . ¬ ¼
Variable-structure controllers for the stator flux and the torque are expressed as:
M r , and U s
us , then in
usd ° ® °usq ¯
the stator flux reference frame, the torque Te can be expressed as
Te
s
are the stator and rotor
Ms
1 ( M 2 /( Ls Lr )), and
Defining\ s
r
disturbances regulating the stator flux and the torque[8]. Thus, variable-structure controllers can be designed for \ s and Te .
Rr the stator and rotor resistances, Z r the rotor
J
s
(1)
self-inductances, M the mutual inductance, Rs and speed, V
coupled dynamics in Eq. (3) complicate the design of a simple, high-performance system with conventional linear controllers. However, if the stator flux amplitude \ s is controlled to be constant, the variables \ cos(T T ) and T can be analyzed as bounded
M \ \ sin(T s T r ) V Ls Lr s r
(2)
(3)
RsTeREF
\ sREF
KTe eTe H Te sgn eTe
(4)
where, 2 Rs ° K\ t 0, KTe t 0, H\ t V L \ sREF , s ° ® 2 · 2V Ls Lr § Rr °H ! Z \ TeREF ¸ ¨ TeREF r sREF 2 ° Te M \ sREF © V Lr ¹ (5) ¯
and the system described by Eqs. (1) and (2) can then be rewritten as: Rs Rs M °\ s V L \ s V L L \ r cos(Ts T r ) usd , ° s s r ® 2 \ \ °T s T s u s sq °¯ e Rs Rs
K\ e\ s H\ sgn e\ s ,
in which \ sREF and TeREF are reference values of the stator flux amplitude and the torque. K\ , H\ , KTe ,
U s sin(Tu Ts ) . T s ,
and H Te are positive constants. Since the controller
T r , and Tu denote the stator flux, rotor flux, and
parameters H\ and H Te can be acquired from the
stator voltage angles. The induction machine is assumed to have only one pole pair to simplify the analysis.
inequalities (5) by analyzing the variation of the machine parameters, the tuning of the variablestructure controllers is independent of the accuracy of the machine parameters except for the stator resistance.
where usd
U s cos(Tu Ts ) , usq
From Eq. (3), the stator flux amplitude and the
Fig. 1
Block diagram of the system ( e\ s
\ sREF \ s ˈ eTe
TeREF Te ).
LI Jian ळ
et al˖Implementation of Direct Torque Control Scheme for Induction Machines ĂĂ
The voltage components [usd , usq ]T in Eq. (4) can be transformed into the stationary reference frame D , E by Eq. (6). Consequently, [usD , usE ]T can be modulated with SVM to drive a voltage-source inverter (VSI) with the switching frequency set to constant. ªusD º ªcos Ts sin T s º ªusd º us « » « (6) »« » ¬usE ¼ ¬ sin T s cos T s ¼ ¬usq ¼ The stator flux values MsD , M sE , and \ s can be obtained by integrating the back electromotive force. M sD ° ® °¯M sE
\s
³ (u D i D R )dt , ³ (u E i E R )dt
(7)
Ms2D Ms2E
(8)
s
s
s
s
s
s
°sin T s M sE /\ s , ® °¯cosT s MsD /\ s
(9)
The torque is calculated using Eq. (10): Te
isE MsD isD M sE
(10)
Wang et al.[8] demonstrated that these algorithms can guarantee system stability for all operating conditions.
2
Implementation
2.1
System structure and hardware configuration
595
permanent magnetic AC motor and its controller were used to supply the load torques. The rotor speed detected by an encoder was used only for the speed feedback control. The system included protection functions such as over or under voltage protection and over current protection. The double-CPU board had TMS320F240 and 80C196KC CPUs. The current signals were sent to the A/D conversion inputs of the digital signal processor (DSP). The rotor position was transmitted from the 80C196KC to the DSP. The flux and torque commands and the regulator parameters could be set with the keyboard. The servo system functions were displayed on the display board. The position feedback was only used for speed control. The system sampling time was 0.2 ms and the PWM frequency was 7.6 kHz. The control scheme parameters were: K\ =10, KTe =30, H\ =100, and H Te =100. 2.2
Software implementation
The algorithms required by the variable-structure torque control and the inverter control using the SVM technique were implemented in software as shown in Fig. 3.
The system configuration is shown in Fig. 2. The motor parameters were: 3 phases, 4 poles, 3 kW, 380 V, 6.8 A, and Rs = 1.87 ȍ.
Fig. 3 Software implementation
Fig. 2 System structure and hardware configuration (PM motor: Permanent magnetic motor)
The servo system included a double-CPU board, a power inverter board, and a general induction motor with an optical encoder. The power board included a rectifier, an intelligent power module (IPM) with its drive circuit, and current measuring circuits. A
2.2.1 Stator flux estimate The stator flux estimation is an important task in the control scheme. Its accuracy directly affects the motor drive performance. The flux calculation was determined theoretically by Eq. (7), but the implementation of the integrator for the motor flux estimate is difficult. A pure integrator has DC drift and initial value problems. One improvement is to construct a stator flux observer[7], using the rotor parameters, but the estimated flux values are then sensitive to variations of the rotor parameters. Another common
Tsinghua Science and Technology, October 2005, 10(5): 593̢597
596
solution to the DC drift problem of the pure integrator is to replace it with a first-order low-pass (LP) filter, which can be expressed as
M s | (Ts 1)1 (us is Rs )
(11)
where T is a time constant. This equation can be realized in digital form as[9]
M s (n) G M s (n 1) [us (n 1) Rs is (n 1)] (12) where M s (n) is a discrete vector of the state flux and
2.2.2 Excitation In this control scheme, the stator flux amplitude must be constant. However, during excitation, the stator flux direction is not controlled which creates problems since the control scheme works with a stator having fixed amplitude and direction. Simple implementation of the flux-loop does not solve the problem because of the calculation errors at low speeds. An improved method is illustrated in Fig. 5.
G is a gain slightly less than 1. The selection of G has a small influence on the system performance as shown in Fig. 4. Te and \ s are the actual torque and actual stator flux, while Tˆe and \ˆ s are the torque and flux calculated by Eqs. (8), (10), and (12). The errors 'Te Te Tˆe and '\ s \ s \ˆ s vary with changes of the rotor speed and G . Since the measurement of the actual motor voltage us is difficult, the voltage command us* was used in the controller instead of voltage measurements. The error between the voltage command and the actual voltage is mainly produced by the dead-band time of the inverter[10]. This error can be reduced by compensating for the time in the actual motor voltage or by calculating the actual voltage using C us | us* udeadtim e
(13)
C where udeadtime is the voltage error caused by the
dead-band time.
Fig. 4
Fig. 5
Excitation process
First, a voltage with fixed magnitude and direction is applied to the stator for about 20 ms. The magnitude is not very large and the direction is the same as the specified flux’s direction. Secondly, close the flux loop. The flux loop assures that the excitation result is the desired value. Finally, close the torque loop after about 100 ms.
3
Experimental Results
In the following experiments, the PWM serial ports of the DSP change the waveforms of the estimated stator flux and machine torque. Then, the LP filters with delay times of about 10 ms filter the output signals. As a result, the real responses are quicker than these shown here. Figure 6 shows the stator flux response for the entire excitation process described in Section 2.2.2. The three parts are clear in the figure. Figure 7 shows the phase current during steady state. Figure 8 shows the static waveform of the motor stator flux. Figure 9 shows the response to a step torque change. The torque approaches the step change quickly without error, but has small ripples during steady state.
Influence of LP filter Fig. 6
Response to step flux change (\sREF˙0.5 Wb)
LI Jian ळ
et al˖Implementation of Direct Torque Control Scheme for Induction Machines ĂĂ
597
References [1] Ortega R, Barabanov N, Valderrama G E. Direct torque control of induction motors: Stability analysis and performance improvement. IEEE Trans. on Automatic Control, 2001, 46(8): 1209-1222. Fig. 7
Phase current during steady state
[2] Buja G, Casadei D, Serra G. Direct stator flux and torque control of an induction motor: Theoretical analysis and experimental results. In: Annual Conference of the IEEE Industrial Electronics Society IECON. Aachen, Germany, 1998: T50-T64. [3] Telford D, Dunnigan M W, Williams B W. A comparison of vector control and direct torque control of an induction machine. In: Conference Record of IEEE PESC’00. 2000, 1: 421-426. [4] Hoang Le-Huy. Comparison of field-oriented control and direct torque control for induction motor drives. In: Conference Record of IEEE IAS’99. 1999, 2: 1245-1252. [5] Casadei D, Profumo F, Serra G, Tani A. FOC and DTC: Two viable schemes for induction motors torque control.
Fig. 8
Stator flux during steady state (\sREF˙0.5 Wb)
IEEE Trans. on Power Electronics, 2002, 17(5): 779-787. [6] Neves F S, Landim R P, Habetler T G. Induction motor DTC strategy using discrete-time sliding mode control. In: Conference Record of IEEE IAS’99. 1999, 1: 79-85. [7] Lascu C, Andrzej M T. Combing the principles of sliding mode, direct torque control, and space vector modulation in
a
high-performance
sensorless
AC
drives.
In:
Conference Record of IEEE IAS’ 02. 2002, 3: 2073-2079. [8] Wang Huan’gang, Xu Wenli, Yang Geng, Li Jian. Variable-structure direct torque control of induction motors using space vector modulation. Electrical Engineering, 2005, 87(2): 93-102.. Fig. 9 Response to step torque change (TeREF˙ 5.7 Ngm, \sREF˙0.6 Wb)
4
Conclusions
A new torque control scheme was implemented for high-performance induction machine drives which combines variable-structure controllers and space vector modulation. This paper describes the control algorithms and the improved method to estimate the stator flux. The implementation shows the simplicity and practicality of the scheme. The experimental results illustrate that the system has good torque and stator flux response with small ripples.
[9] Hurst K D, Habetler T G, Griva G, Profumo F. Zero-speed tacho-less IM torque control: Simply a matter of stator voltage integration. IEEE Trans. Industry Applications, 1998, 34(4): 790-795. [10] Kim Hyun-Soo, Kim Kyeong-Hwa, Youm Myung-Joong. On-line dead-time compensation method on time delay control. IEEE Trans. Control Systems Technology, 2003, 11(2): 279-285.