Reduce Torque Ripple of IM by Approach Method DTC

This paper presents a new methodology for direct torque control (DTC) with improvement method control for induction motor (IM) systems. The application of approached method DTC (AMDTC), allows by using two hysteresis controllers to regulate torque and flux with the information of the angular location to address the switching table. The principal advantage of this AMDTC (optimization method of DTC) enables the minimization of the electromagnetic torque ripple and the reduction of the output current total harmonic distortion (THDI). Furthermore, it improved consumption quality of IM and it improved the lifespan of the motor. The switching characteristics of an inverter feeding an IM controlled with the AMDTC system are assessed in steady state.The proposed method of AMDTC is illustrated by computer simulations.


Introduction
The DTC is widely applied in industry of manufacturing and electromechanical energy utilization .The principal advantage of AMDTC, improve the difficulty of large torque ripple and inconstant inverter switching frequency in the conventional DTC, however, many researchers have given attentions to these difficulty.The AMDTC of the three-phase asynchronous machine is subjected to values of the successive voltages, which realizes the protecting of its insulators of motor and amelioration the lifespan of IM [1,3,[6][7][8][9][10].Takahashi (1989) and Depenbrock (1988) have developed the DTC specifically for IM [7,8].Because the DTC was applied in 1980s for IM drives, the technique has also been used to the commands asynchronous motors.The work application of different approach DTC of asynchronous motor, allows by using two hysteresis controllers to regulate flux and torque with temperature evolution of insulated gate bipolar transistor (IGBT).The voltages of the inverter, by using two hysteresis controllers (torque and flux), are determined by comparing between; the reference flux and the estimated flux, at the same time with the reference torque and the estimated torque.The remainder of this article is organized as follows; principles of the proposed method in Section 2, performance enhancement of AMDTC is discussed in Section 3, thermal modeling of the power hybrid module is presented in Section 4, the simulations results and discussion are dealt with in Section 5 and conclusion is presented in Section 6.

Principles of the Proposed Method
Generally, in the AMDTC system of IM; the equations of the three phases IM are modified in the equivalents two-phase components [1,3,4,[6][7][8][9].The equations for the IM are given by the following equations.
For squirrel-cage IM; The components, stator flux and rotor flux, are bound to the currents (stator and rotor).These fluxes are given by the following equations.
The absolute value and phase angle of stator flux are given by the following equation.
The electromagnetic torque is given by the following equation.
where from P = 2 is the number of pole-pairs, θ is the angle between the stator flux and stator current.
3 sin The electromagnetic torque is given by the following equation.From Figure 1, the voltage V dc is the DC bus voltage, the commands (S 1 , S 2 and S 3 ) are the outputs signals of model switching table (inputs signals of IGBTs) and they are appropriate to the AMDTC strategy.
The commands (S 1 , S 2 , and S 3 ) can be either 1 or 0, the voltage vectors d-q-0 axes are given by the following equations.

Performance Enhancement of AMDTC
The AMDTC of Figure 2, represents three cases ((a), (b) and (c)) of the switching frequency of IGBT to command an IM.The switching state of the inverter is resulted from the calculated stator voltage.An AMDTC is investigated in this technique, which features in very low flux undulate and improving torque ripple with almost fixed switching frequency.This work uses an improvement torque ripple, the reduction of the THDI and optimization of consumption quality of IM [20,22,25].The input reference control of the torque and input reference control of the flux are the entries of the AMDTC.One can indicate that the voltage space vector can be used via Figure 3 and by the switches S 1 , S 2 and S 3 as follows.
The electromagnetic torque (T e ) and stator current by the IM; are given by following equations.

For
, with The position θ β of Figure 4, is represented the angle difference between the rotor flux and stator flux; use a critical role in scheming output torque.Mathematically the couple is given by the following equations.Conforming to the approach of DTC, the information of a voltage vector is used to regulate the stator flux and torque within the limits of two hysteresis controllers (torque and flux).Allows by the output of tow hysteresis bands with the address of the angular flux location is used to data of the switching Table 1.The proposed technique; improvements RMS of torque ripple, THD of IM and the reduction strategy consisted by approach method for trimming minimum of torque ripple [5] The torque and flux errors (ε e , ε φ ), are realized by comparing between; the reference flux and the estimated flux, at the same time with the reference torque and the estimated torque [1].The AMDTC strategy is to choose the technical of better voltage vector that propels the flux magnitude (Φ s ) to arrive a value of reference flux (Φ ref ) and by the same time in turns over flux to an adjustment corresponding to a reference torque T ref .
The AMDTC, allows to make the instantaneous torque equal with the reference torque at the end of the cycle and the technique is given by the following equation.

 
The Table 2 of AMDTC represented the commutations number of switches between 0 s to 1 s.The switches commutations are recapitulated in the following Table 2.
The fluxes of the induction motor are determined by the following equation.
The fluxes of (27) are expressed in the discrete form as.
where t cp is controlled period of a small value and k + 1, the sampling instants.In our work, the root-mean-square (RMS) value of the torque ripple can be expressed as follows.With T ond is the RMS value of the torque ripple.The total harmonic distortion (THD) is defined as the RMS value of the current (I KRMS ), divided by the RMS value of its fundamental current (I 1RMS ).The quality of the current is measured with the THD I .

Thermal Modeling of the Power Hybrid Module
The equivalent electrical circuit shown in [23][24][25] can represent the finite element method (FEM), the finite difference method (FDM) and the thermal model of the material for IGBT.The electric model used for the Semikron module SKM 75 GB 123 D of IGBT is used in AMDTC with a temperature without cooling; the power dissipated (P dis ), with voltage drop at the boundaries (V CEsat(t) ) and internal resistance (R CE(Tj) ).The technique investigation that was performed with the Semikron module SKM 75 GB 123 D (75 A/1200 V) is presented by the following equations

Simulation Results and Discussion
The output voltage space vector with reduce torque ripple of IM by AMDTC; can be realized via switching  (b) and (c)) one finds the same forms.With these approaches one finds; the selection seven-state voltagevector switching Table 1 is prepared via the Equation (18) of inverter.One can indicate that the voltage space vector can be used via Figure 7 (V q in function of V d ) and by the switches (IGBTs).The voltage maximal V q equal 265.581V and the voltage maximal V d equal 306.666V.One notices the passages enter the vectors V j with j = 0, 1, 2, •••, 7. Finally one notes the passages enter the vectors, one finds with each vector contains seven of possible combinations.
The output voltage of inverter via commutation switch, provoke reduce torque ripple of IM by AMDTC.To consider the results of the approaches, they are necessary a system of temperature cooling of IGBTs.
Figures 7-12, illustrates ameliorations respectively the torque ripples, the currents of IM and temperatures evolutions of three approaches scenario (a, b and c).The idea is improved the DTC by the operation of commutations number and modification voltage space vector.The following Table 3 determines the results AMDTC of the IM.

Conclusions
In this paper, the comparison of AMDTC strategy for approaches method (a, b and c) via advantages of Table 3 are realized; by the minimizing THD, improve torque     3, recapitulate advantages of the AMDTC for the IM.

Figure 3 .
Figure 3.The stationary plane of inverter voltage vectors.

Figure 4 .
Figure 4.The principle of stator flux compared to the rotor flux.

Figure 7 .
Figure 7.The response of voltages (V d and V q ), for AMDTC of cases ((a), (b) and (c)).

Figure 11 ,
affirm of improving respectively the torque ripple by the proposed AMDTC under the same IM condition; realized torque ripple at ±9.55 Nm of case (a), torque ripple at ±3 Nm of case (b), and torque ripple at ± 0.4 Nm of case (b), The torque ripple minimization method for AMDTC of induction motor.If compared with the cases (a) and (b), the proposed case (c) has the advantage of global minimum RMS torque ripple.For Figure 12 (a2), the maximum junction temperature of IGBT1 at 0.15 s via approaches DTC; provoke case (a) equal 324.588K with case (b) equal 332.296K and case (c) equal 404.533K. Thus the Figure 12 (a3), represent the maximum case temperature of IGBT1 at 0.15s for Figure 12 (a3); with case (a) equal 323.654K, case (b) equal 332.089K and case (c) equal 385.395K.
ripples and thereafter improve quality of consumption IM.The proposed scenario using AMDTC enables to reduce torque ripple and obtain improvements inputs currents of IM, while guarding torque dynamic response to follow one reference torque and switching frequency to follow Figure 7.The effective of in Figures 7-12 has been realized with different approaches ((a), (b) and (c)).We have according to the Figure 11 an approximately 95.81% reduction in torque ripple in AMDTC.The phase current with THD I of approach (c) operation contains lowest THD to that of approaches ((a) and (b)) operations.The Table

Table 2
and determined by the two basic variables T 

Table 2 ,
represent the commutations number of switches between 0 s to 1s.The switches commutations are recapitulated in the response Copyright © 2011 SciRes.ICA