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Considerations about direct torque control and encoder-less operation of induction motor drives Viorica Spoiala*, Helga Silaghi* and Dragos Cristian Spoiala* * Department of Electrical Drives and Automatics , University of Oradea, Faculty of Electrical Engineering and Information Technology, Universitatii Street, no.1, Oradea, Romania, E-Mail: vspoiala@uoradea.ro, dspoiala@uoradea.ro, hsilaghi@uoradea.ro Abstract – The paper presents the principle of direct the stator flux linkagespace vector determine the stator r torque control and encoder-less operation of the voltage space vector u s that is applied to the motor induction machines. Direct Torque Control - or DTC - is the most advanced AC drive technology developed by during each sampling interval ΔT. any manufacturer in the world. Comparative to vector control, direct torque control (DTC) techniques need no axis transformation and the electromagnetic torque and the stator flux are estimated and directly controlled by applying the appropriate stator voltage vector. Aditionally, it is possible to estimate the rotor speed without the use of an encoder. Keywords: direct torque control, speed control, space vector I. INTRODUCTION There are many electromechanical systems where it is very important to precisely control their torque, speed or position. Direct Torque Control technique is a part of the advanced electric drive systems absolutely Figure 1. Block diagram of DTC necesssary in different fields of activity: elevators in highrise buildings, industrial robots in automated Estimating the electromagnetic torque and the stator factories, pumps and compressors systems, wind electric flux linkage vector requires measuring the stator systems, hybrid-electric vehicles, etc. currents and the stator phase voltages. The latter This paper is structured as follows: first is presented quantities are indirectly calculated by measuring the dc- the block diagram of the DTC system, then the principle bus voltage and knowing within the digital controller the of encoder-less DTC operation, the calculation of the status of the inverter switches. main quantities (stator flux, rotor flux, electromagnetic torque and rotor speed), the calculation of the stator voltage space vector and finally are presented some II. THEORETICAL CONSIDERATIONS results of simulation, conclusions and references. A. PRINCIPLE OF ENCODER-LESS DTC A. SYSTEM BLOCK DIAGRAM OPERATION The block diagram of the system is presented in The various steps in the estimator block of Figure 1 Figure 1, which includes the speed and the torque loops, can be detailed in the following relations, where all wthout a speed encoder. The estimated speed Ωest is space vectors are implicitly expressed in electrical substracted from the reference speed Ω* and the error radians with respect to the stator a-axis as the reference between the two acts on the PI-controller to generate the axis: 1. From the measured stator voltages and currents, ˆ torque reference signal for the stator flux linkage Ψ * r s calculate the stator flux linkage space vector Ψs : which is compared with the estimated stator flux linkage r r ˆ 2. From Ψs and is calculate the rotor flux space Ψs,est . The errors in the electromagnetic torque and the r stator flux, combined with the angular position ∠θ s of vector Ψr and hence the speed of the rotor flux linkage 125 vector, where ΔTΩ is a sampling time for speed Therefore, the rotor flux linkage space vector in Eq. calculation: (6) can be written as: r r r L r r r 3. From Ψs and is , calculate the estimated Ψr = r (Ψs − σL s is ) = Ψr e jθr (8) electromagnetic torque M: Lm r We observe that similar to Eq. (1) for the stator flux 4. From Ψr and M est estimate the slip speed Ωslip linkage vector, the rotor flux linkage space vector can be and the rotor speed Ωm: expressed as follows, knowing that the rotor voltage in a In the stator voltage selection block of Fig.1, an squirrel-cage rotor is zero: appropriate stator voltage vector is calculated to be t r r r applied for the next sampling interval ΔT based on the A errors in the torque and the stator flux, in order to keep ΨrA ( t ) = ΨrA ( t − ΔT ) + ∫ (−R r irA ) ⋅ dτ = Ψr e jθr ˆ (9) t − ΔT them within a hysteretic band. where the space vectors and angles (in electrical radians) r r are expressed with the respect to the rotor A-axis shown B. CALCULATION OF Ψs , Ψr , M AND Ω m in Fig.2. The expression (9) shows that the rotor flux r changes very slowly with time only due to a small 1.Calculation of the Stator Flux Ψs voltage drop across the rotor resistance. The stator voltage equation with the stator a-axis as the reference is: r r d r u s = R s is + Ψs (1) dt The stator flux linkage space vector at time t can be calculated in terms of the flux linkage at the previous sampling time as: Figure 2. The position of the stator and rotor flux r r t r linkage vectors r ∫ ˆ Ψs ( t ) = Ψs ( t − ΔT ) + (u s − R s is ) ⋅ dτ = Ψs e jθs (2) t − ΔT 3. Calculation of the Electromagnetic Torque M where τ is the variable of integration, the applied stator voltage remains constant during the sampling interval The expression of the electromagnetic torque ΔT and the stator current value is that measured at the depends on the magnitude of the stator and the rotor previous time step. fluxes and the angle between the two space vectors. p Lm ˆ ˆ r M= Ψs Ψr sin θ sr (10) 2. Calculation of the Rotor Flux Ψr 2 L2σ where Lσ is the machine leakage inductance and It is known that [1]: θsr = θs - θr (11) r r The angles in the expressions (10) and (11) are r Ψs = L s is + L m ir (3) expressed in electrical radians with respect to the stator r r r a-axis, as shown in Fig.2. and Ψr = L r ir + L m is (4) Eq. (1) and Fig. 2 show that the torque can be r Calculating ir from eq. (3), results: controlled quickly by rapidly changing the position of r r the stator flux linkage space vector ( θs ), by applying an Ψs L r ir = − s is (5) appropriate voltage space vector during the sampling Lm Lm interval ΔT, while the rotor flux space vector position θr and subtituting it into eq. (4): changes relatively slowly. Expression (10) shows that r L r L L r r the desired change in torque is obtained by the change in Ψr = r Ψs − s r is + L m is = θsr. Lm Lm To estimate the torque, a better expression is ⎡ ⎤ presented below [1]: ⎢ r⎛ 2 ⎞⎥ (6) p r r L ⎢r L ⎥ M = Im(Ψsconj is ) = r ⎢Ψs − L s is ⎜1 − m ⎟⎥ 2 (12) Lm ⎜ Ls Lr ⎟ ⎢ ⎝ 4243 ⎥ 1 ⎠ where does not appear the rotor flux linkage. ⎢ ⎣ ( =σ) ⎥ ⎦ where the leakage factor σ is defined as: 4.Calculation of the Rotor Speed Ωm L2 m σ = 1− (7) The speed of the rotor flux (in electrical rad/s) is Ls Lr calculated from the phase angle of the rotor flux space vector (Eq. 13) as follows: 126 d θ ( t ) − θ r ( t − ΔTΩ ) Ωr = θr = r (13) dt ΔTΩ where the sampling interval ΔTΩ may be smaller, for example equal to 1 ms. The slip speed is given by the following expression: 1 ⎛ Lm ⎞ Ω slip = R r ⎜ L i sq ⎟ ⎜ Ψrd ⎝ r ⎟ (14) ⎠ The electromagnetic torque has the expression: p ⎛L ⎞ M = Ψrd ⎜ m i sq ⎟ ⎜L ⎟ (15) 2 ⎝ r ⎠ Figure 4 Calculating isq from Eq. (15) and substituting it into ˆ The roles of various voltage vectors can be tabulated in Eq. (14) and knowing that Ψ = 2 / 3Ψ [4], we rd r the following table, assuming that the stator flux linkage obtain the slip speed: space vector is along the central vector: 2⎛3 ⎜ R M ⎟ ⎞ Ω slip = r (16) TABLE 1. Effect of Voltage Vector on the Stator Flux-Linkage p⎜2 ⎝ Ψr2 ⎟ ˆ ⎠ Vector in Sector 1 So, the rotor speed can be calculated from (13) and r (16), as: us M ˆ Ψs Ωm = Ωr - Ωslip (17) r increase u3 increase where all speeds are in electrical radians per second. r u2 increase decrease C.CALCULATION OF THE STATOR VOLTAGE r decrease SPACE VECTOR u4 decrease r decrease u5 increase In DTC often the torque and the stator flux amplitude are controlled with a hysteretic band around The voltage vectors would have the same effects as the desired values [3]. Therefore, at a sampling time the tabulated above, provided the stator flux-linkage space decision to change the voltage space vector is r implemented only if the torque and/or the stator flux vector is anywhere in sector 1. The use of vectors u 1 amplitude are outside their range. r and u 6 is avoided because their effect depends on Selection of the new voltage vector depends on the where the stator flux-linkage vector is in sector 1. A signs of the torque and the flux errors and the sector in similar table can be generated for all other sectors. which the stator flux linkage vector lies. r r Use of zero vectors u 0 (000) and u 7 (111) results The plane of the stator voltage space vector is divided into six sectors as shown in Figure 3. The in the stator flux linkage vector esentially unchanged in central vectors for each sector, which lie in the middle of amplitude and in the angular position θs. a sector, are the basic inverter vectors as shown in Fig.3. In the torque expression of Eq. (10) for small values of θsr in electrical radians: sin θsr ≈ θs - θr (18) With a zero voltage vector applied, assuming that the amplitudes of the stator and the rotor flux linkage vectors remain constant: M ≈ k (θ s − θ r ) (19) where k is a constant. The position of the stator flux-linkage vector remains essentially constant, thus Δθ s ≅ 0 . Similarly, the position of the rotor flux-linkage vector, with respect to the rotor a-axis remains essentially constant, that is, Δθ A ≅ 0 . However, Δθ r = Δθ m + Δθ A (Fig.2). r r Therefore the position of the rotor flux-linkage vector changes, slowly, and the change in torque in Eq. (19) Figure 3 Inverter basic vectors and sectors can be expressed as: ΔM = −k (Δθ m ) (20) The choice of the voltage space vector for sector 1 is explained below based on Fig.4 and Eq. (10) Eq. (20) shows that applying a zero stator voltage and (11). space vector causes change in torque in adirection opposite to that of Ωm. Therefore, with the rotor rating in 127 a positive direction for example, it may be preferable to Torque error vs.Time apply a zero voltage vector to decrease torque in order to 10 keep it with hysteretic band. 5 III. SIMULATION RESULTS 0 The Simulink model is made based on the block 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 diagram of DTC control of AC drives shown in Figure 1. Figure 7 The block diagram shows that DTC has two Torque vs. Time fundamental sections: the Torque Control Loop and the Speed Control Loop. 15 Some parameters of the induction motor are: stator resistance Rs = 1,56 Ω, rotor resistance Rr = 1,23 Ω, 10 stator reactance Xls = 4,93 Ω, rotor reactance Xlr = 5,11 Ω, mutual reactance Xm = 141 Ω, number of pairs of poles p = 4. 5 The simulations were made in Matlab/Simulink, the 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 motor having the model presented in Figure 5 and DTC estimator model is presented in Figure 6 (the hole model Figure 8 is too large): Speed vs. Time 164 162 160 158 156 154 152 150 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Figure 9 Figure 5. Simulink motor model 1.79 Flux vs. Time 1.78 1.77 1.76 1.75 1.74 1.73 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Figure 10 Figure 6. DTC estimator model We assumed a steady state motor operation At time t Better product quality, which can be partly achieved = 0.2 s , a load-torque disturbance causes it to reduce to with improved speed accuracy and faster torque control, one-half of its initial value. The objective is to keep the is obtained with DTC. load speed constant at its initial value. Some simulation • Less down time which means a drive that will not trip results are presented in Figure 7, 8, 9 and 10. unnecessarily, a drive that is not complicated by Because torque and flux are motor parameters that expensive feedback devices and a drive which is not are being directly controlled, there is no need for a greatly affected by interferences like harmonics and modulator, as used in PWM drives, to control the RFI. frequency and voltage. This, in effect, cuts out the • Fewer products can be realized; one drive capable of middle man and dramatically speeds up the response of meeting all application needs whether AC, DC or servo. the drive to changes in required torque. DTC also That is a truly “universal” drive. provides precise torque control without the need for a • A comfortable working environment with a drive that feedback device[2]. produces much lower audible noise is available. 128 III. CONCLUSIONS • Controlling variables are taken directly from the motor DTC techniques are very performant, having the • The fast processing speeds of the DSP and following advantages: Optimum Pulse Selector hardware • Fast torque response: this significantly reduces the • No modulator is needed . speed drop time during a load transient, bringing much improved process control and a more consistent product quality. REFERENCES • Torque control at low frequencies: compared to PWM flux vector drives, DTC brings the cost saving benefit [1] Ned Mohan, Advanced electric drives. Analysis, Control that no tachometer is needed. and Modeling using Simulink, Mnpere Minneapolis USA, • Torque linearity: this is important in precision 2001. applications. [2] Richard Crowder, Electric drives and electromechanical systems, Newnes Elesevier, 2006 • Dynamic speed accuracy: after a sudden load change, [3] Viorica Spoiala, Helga Silaghi, D.C. Spoiala, ″Aspects the motor can recover to a stable state remarkably fast. about direct torque control of induction machine″, in DTC technology allows a drive to adjust itself to varying Proceedings of the 9th International Conference on application needs. Engineering of Modern Electric Systems, Oradea, • DTC not need a tachometer or position encoder to tell Romania, pp. 244 – 247, 2007. it precisely where the motor shaft is at all times, because [4] N.R.N. Idris, ″Improved direct torque control of induction of: machines ″, PhD Thesis, Universiti Teknologi Malaysia, • The accuracy of the motor model 2000. [5] ***** ABB DTCguide, 2009 129

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direct torque control, torque control, induction motor, load torque, torque ripple, rotor flux, ac drives, synchronous motor, speed accuracy, permanent magnet, rotor position, ac induction motor, vector control, motor torque, rotor speed

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posted: | 4/8/2010 |

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