Direct Torque Control of Induction Machine Using Matrix Converter
K. Jagadeesh1Asst. Professor, P.V. Dhinakar Reddy2PG Student
Dept. of EEE, Samskruthi College of Engineering & Technology,Ghatkaser,Hyderabad , Andhra Pradesh, India.
Dept. of EEE,Vardhaman College of Engineering & Technology., Kacharam,Shamshabad, Andhra Pradesh, India
Abstract
In this paper, a new control method for matrix converters is proposed which allows, under the constraint of unity input power factor, the generation of the voltage vectors required to implement the direct torque control (DTC) of induction machines. Using this control method, it is possible to combine the advantages of matrix converters with the advantages of the DTC schemes. Some numerical simulations are carried out, showing the effectiveness of the proposed method in steady-state and transient conditions. Some experimental tests were also carried out demonstrating the practical feasibility of this control scheme.
Keywords—AC–AC Power Conversion, Induction Motor Drives, DirectTorque Control
I. Introduction
THREE-PHASE matrix converters have received considerable attention in recent years because they may become a good alternative to voltage-source inverter pulse width-modulation (VSI-PWM) converters [1]–[6]. In fact, the matrix converterprovides bidirectional power flow, sinusoidal input/output waveforms, and controllable input power factor. Furthermore, the matrix converter allows a compact design due to the lack of dc-link capacitors for energy storage. With reference to the control methods, two approaches are widely used. The first one is based on transfer function analysis and has been proposed in [1]. The second one is based on space-vector modulation (SVM) technique, which has some advantages, such as immediate comprehension of the required commutation processes, simplified control algorithm, and maximum voltage transfer ratio without adding third harmonic components [5], [7]–[9].
The direct torque control (DTC) technique for inductionmotors was initially proposed as DTC [10] or directself-control [11], then the method was generalized to current-source-inverter-fed induction motors and to VSI-fed andcurrent-source-inverter-fed synchronous machines [12]. Themain advantages of DTC are robust and fast torque response, norequirements for coordinate transformation, no requirementsfor PWM pulse generation and current regulators. In [13] and[14], a control scheme for induction motors based on DTChas been analyzed, but the rotor flux is assumed as reference,instead of stator flux, in order to achieve the highest pull-outtorque. Using a VSI, different vector selection criteria can beemployed to control the torque and the flux leading to differentswitching strategies. Each strategy affects the drive behaviorin terms of torque and current ripple, switching frequency, andtwo- or four-quadrant operation capability [15]–[17]. In [18],aspeed-dependent switching strategy has been proposed in orderto achieve fast torque response in a wide speed range.
In this paper, a new control method for matrix convertersis proposed which allows, under the constraint of unity inputpower factor, the generation of the voltage vectors required toimplement the DTC of induction machines. The appropriateswitching configuration of the matrix converter is directly selected,at each sampling period, using an opportune switchingtable. The table is entered by the outputs of three hysteresiscontrollers applied to the errors of stator flux, electromagnetictorque, and input power factor, respectively. Using this controlmethod, it is possible to combine the advantages of matrix converterswith the advantages of DTC schemes.
Figure.1. Schematic representation of a matrix converter
The good performance of the proposed scheme has beentested using a realistic numerical simulation of the wholedrive. The steady-state and the transient behavior have beeninvestigated. In both cases, the results obtained emphasize theeffectiveness of the proposed drive system.
II. Direct Torque Control by Matrix Converter
A. Matrix Converter Theory
In three-phase/three-phase matrix converters, the nine bidirectionalswitches allow any output phase to be connected toany input phase as schematically represented in Figure. 1.
There are 27 possible switching configurations; amongthese, only 21 can be usefully employed in the DTC algorithm.These configurations are summarized in Table I. The first 18switching configurations (named ±1, ±2 … ±9) have thecommon feature of connecting two output phases to the sameinput phase. The corresponding output line-to-neutral voltagevector and input line current vector, have fixed directions,as represented in Figure. 2 and 3, and will be named “activeconfigurations.” The magnitude of these vectors dependsupon the instantaneous values of the input line-to-neutralvoltages and output line currents respectively as shown inTable I. Three switching configurations determine zero inputcurrent and output voltage vectors and will be named “zeroconfigurations.”
The remaining six switching configurations have the threeoutput phases connected to a different input phase. In this case,the output voltage and input current vectors have variable directionand cannot be usefully used.
It should be noted that the voltage vectors produced by a matrixconverter can be utilized using the SVM technique to synthesizethe instantaneous voltage vector required by field-orientedcontrol of induction motors [5]–[9].
B. Basic DTC Principles
In principle, the DTC is a hysteresis stator flux and torquecontrol that directly selects one of the six nonzero and two zerovoltage vectors generated by a VSI (Figure. 4), in order to maintainthe estimated stator flux and torque within the hysteresisbands. In particular, the stator flux is controlled by a two-levelhysteresis comparator, whereas the torque by a three-level hysteresiscomparator, as shown in Figure. 5 and 6, respectively. Onthe basis of the hysteresis comparator outputs and the stator fluxsector number, the most opportune VSI voltage vector is selectedat each sampling period, according to the switching tablegiven in Table II.
Figure. 2 Output line-to-neutral voltage vector configurations
As an example, considering the stator flux vector lying in sector-1, the voltage vectors V2 and V6 can be selected in order to increase the flux while V3 and V5 can be applied to decrease the flux. Among these, V2 and V3 determine a torque increase, while V5 and V6 a torque decrease. The zero-voltage vectors are selected when the output of the torque comparator is zero, irrespective of the stator flux condition. Using the switching table given in Table II, it is possible to implement DTC schemes having good performance.
Figure. 3 Input line current vector configurations
Figure. 4 VSI output line-to-neutral voltage vectors and corresponding stator flux variations in a period ∆t
Figure. 5 Flux hysteresis comparator
TABLE 1. Switching Configurations used in the Proposed Control Scheme
C. DTC Principles Using Matrix Converters
From the previous considerations, it appears that the matrixconverter generates a higher number of output voltage vectorswith respect to VSI. This feature can be utilized to keep undercontrol a further variable in addition to stator flux and torque.In the proposed control method, the average value of the sineof the displacement angle ψi between the input line-to-neutralvoltage vector and the corresponding input line current vectorhas been chosen as a third variable.
In principle, the proposed control technique of the matrix converter selects, at each sampling period, the proper switching configuration, which allows the compensation of instantaneous errors in flux magnitude, and torque, under the constraint of unity input power factor. This last requirement of the input sideof the matrix converter is intrinsically satisfied if the averagevalue of sin(ψi) is maintained close to zero. The hysteresis regulatorshown in Figure.6 directly control this variable. The average value of sin(ψi)is obtained by applying a low-pass filter to its instantaneous value.
The criteria utilized to implement the switching table for the matrix converter can be explained referring to an example.We can assume that V1is the VSI output voltage vector selected by the DTC algorithm in a given switching period. From Figure. 2 and 4 and from Table I it appears that in order to generate a voltage vector similar to V1, one of the matrix converter switching configurations ±1, ±2, ±3 must be chosen. The magnitude and the direction of the corresponding output voltage vectors depend on the input line-to-neutral voltage vector. Among the six vectors, those having the same direction of V1 and the maximum magnitude are considered. If the input line-to-neutral voltage vector lies in sector-1, then the switching configurations, which can be utilized, are +1 and-3. Both these switching configurations satisfy the torque andflux requirements.
As can be noted from Table I and Figure. 3, these configurations determine input current vectors lying on the directions adjacent to sectors 1 and 4. Then, if the average value of sin(ψi) has to be decreased, the switching configuration -3 has to be applied. On the contrary, if the average value of sin(ψi) has to be increased, the switching configuration +1 has to be applied. The switching table based on these principles is shown inTable III. The first column contains the voltage vectors selectedby the basic DTC scheme to keep the stator flux and torquewithin the limits of the corresponding hysteresis bands. Theother six columns are related to the sector in which the input line-to-neutral voltage vector is lying. Depending on the output value of the hysteresis comparator, the left or the right subcolumn has to be used in selecting the switching configuration of the matrix converter. When a zero-voltage vector Cψis required from Table II, the zero configuration of the matrix converter, which minimize the number of commutations, is selected.
A schematic diagram of the proposed drive system is represented in Figure.6. The reference values of torque and stator flux are compared with the estimated values.The output of the hysteresis comparators, together with the numbers of the sectors of the stator flux vector and input line-to-neutral voltage vector, are the input to the switching configuration selection algorithm (Tables II and III).
Figure. 6 Block diagram of the DTC scheme with matrix converter
TABLE 3 Matrix Converter Switching Table
1 / 2 / 3 / 4 / 5 / 6Cψ / +1 / -1 / +1 / -1 / +1 / -1 / +1 / -1 / +1 / -1 / +1 / -1
V1 / -3 / 1 / 2 / -3 / -1 / 2 / 3 / -1 / -2 / 3 / 1 / -2
V2 / 9 / -7 / -8 / 9 / 7 / -8 / -9 / 7 / 8 / -9 / -7 / 8
V3 / -6 / 4 / 5 / -6 / -4 / 5 / 6 / -4 / -5 / 6 / 4 / -5
V4 / 3 / -1 / -2 / 3 / 1 / -2 / -3 / 1 / 2 / -3 / -1 / 2
V5 / -9 / 7 / 8 / -9 / -7 / 8 / 9 / -7 / -8 / 9 / 7 / -8
V6 / 6 / -4 / -5 / 6 / 4 / -5 / -6 / 4 / 5 / -6 / -4 / 5
In the lower part of the diagram are shown the estimators of electromagnetic torque, stator flux, and average value of sin(ψi). These estimators require the knowledge of input and output voltages and currents. However, only the input voltages and output currents are measured, while the other quantities are calculated on the basis of the switching states of the matrix converter.
III. Numerical Simulations
The drive system proposed in this paper has been tested bysome numerical simulations in order to verify the steady-stateand dynamic performance.
To analyze real phenomena such as the influence of discretization,the delay caused by the sampling of signals, and theeffects of sensors and analog-to-digital converters, a numericalsimulation of the whole system has been carried out. Thetest machine is a standard 3.73kW four-pole 460-V 60-Hz cageinduction motor having the following parameters:
Rs=1.115Ω
Lls=0.005974H
Rr=1.083Ω
Llr=0.005974H
Lm=0.2037H
J=0.02Kg-m
Figure. 7. Input 3-Ф Supply Voltage
Figure. 8 Input 3-Ф Currents
Figure.9 Switching Pulses SAa, SBa & SCa
Figure.10 Speed response at No-Load
Figure.11 Torque profile at No-Load
Figure.12 Stator Currents
Figure.13 Rotor Currents
Figure.14 Output Voltage
Figure.15 Output Currents
Figure.16 Speed at Load condition at 0.15sec
Figure.17 Torque profile at Load of 25 N-m at 0.15sec
Figure. 18dq-axes stator flux linkages
IV. Conclusions
In this paper, a new induction motor drive scheme has been proposed in which a matrix converter is employed in driving an induction motor using the DTC technique. A switching table, which allows direct control of the matrixconverter on the basis of the motor control requirements, hasbeen defined. Input line-to-neutral voltage and input line currentwaveforms with 25-N-m torque command. Over traditional VSI-PWM converters have been combined withthe advantages of the DTC technique. The result is a high-performanceinduction motor drive system with intrinsic regenerativebreaking and unity input power factor operation capability.
The proposed scheme has been tested in steady-state conditionsin the low- and high-speed ranges, performing some numericalsimulation. The current and torque waveforms emphasizethe effectiveness of the control scheme.
The dynamic behavior has been tested during the transientfrom motor to regenerative breaking operating condition. Theresults show a high dynamic response with decoupled action onflux and torque. Furthermore, it has been verified that, duringthe regenerative breaking, the drive system acts as a nearly sinusoidal,unity input power factor generator.
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K. Jagadeesh received the B.Tech (Electrical and Electronics Engineering) degree from the Jawaharlal Nehru Technological University, Hyderabad in 2007 and pursuing M.Tech (Power Electronics) in from the Jawaharlal Nehru Technological University, Anantapur. His field of interest includes Matrix Converters, Space Vector Modulation,Power Electronics and ElectricalDrivesandControlSystems
P V Dhinakar Reddy received the B.Tech (Electrical and Electronics Engineering) degree from the Jawaharlal Nehru Technological University, Hyderabad in 2007 and received M.Tech (Power Electronics) in 2011 from the Jawaharlal Nehru Technological University, Hyderabad. His field of interest includes Matrix Converters, Power Electrical & Drives and Control Systems.