Vector control of AC Motor Drive for Active Damping of Output using Passive filter Resonance

Transcription

1 315 Vector control of AC Motor Drive for Active Damping of Output using Passive filter Resonance Ankita Nandanwar*, Miss. R. A. Keswani** *IDC (M.Tech) 4th Sem, Dept. of Electrical Engg., Priyadarshini College of Engineering, Nagpur, India ** Professor, Dept. of Electrical, Priyadarshini College of Engineering, Nagpur, India Abstract This project presents simulation of vector controlled VSI-Fed AC Motor i.e Induction Motor Drives by using damping technique of output Passive filter resonance.ac machines is to be fed by sinusoidal voltages which increases life and gives better accuracy of control. This is achieved by connecting an passive filter i.e LC filter or LCL filter. A simple active damping is used for lossless damping of vector controlled ac motor with an Passive i.e LC filter. The Control technique is carried out in the three-phase domain for accuracy of the control. The proposed technique reduces the Total harmonic distortions giving sinusoidal outputs. A simulation model will be developed and analyzed for induction motor without filter and with filter. Index Terms: Squirrel Cage Induction Motor, LC filter, vector control, Modelling and Simulation in MATLAB software, Torque, Speed, Stator Currents. 1. INTRODUCTION In industry more than 80% Induction motor is being used. They have a simple construction; manufacturing cost is low compared to other motors, reliable operation in extreme condition. Due to the invent of Power Electronic devices it has become more reliable to control the Induction motor. This work covers the voltage source converter technologies, including pulse widthmodulated voltage-source inverters (VSIs). Due to high dv/dt of the VSI output voltages, bearing failure, insulation failure of the motor windings, and issues related to electromagnetic compatibility/interference are common. Passive dv/dt filters, common-mode filters, and Pulse width-modulation (PWM) techniques have been used to mitigate the problems. However, for longer life of the motor, it is always desirable to operate the machine with sinusoidal voltages. One common practise is to connect an LC filter between the inverter and the machine. The LC filter smoothens the output voltage of Voltage Source Inverter and supplies sinusoidal voltage into the motor. In this paper simulation of IM fed with three phase PWM Voltage Source Inverter by applying vector control method using Filter and without filter have been developed and analyzed in detail using MATLAB. The speed & torque of motor is controlled.the main objective is reducing harmonics in motor currents and Total harmonic distortion along with filter and without filter are analysed using MATLAB-SIMULINK. The most widely used IM is squirrel cage motor because of its advantages such as mechanical robustness, simple construction and less maintenance. These applications include pumps and fans, paper and textile mills, home appliances, electric and hybrid vehicles,heat pumps and air conditioners, rolling mills, wind generation systems etc. Table 1.Input Parameters for 3 Ph Squirrel cage Induction Motor. Input parameters Values Rated power 2HP Poles 4 Input Voltage 220V rms Frequency 50Hz Reference speed 1500rpm Torque 9Nm Stator Resistance 0.66ohm Stator Inductance 1.62mH Rotor Resistance 0.21ohm Rotor Inductance 1.62mH Mutual Inductance 38.8mH Table2. Filter parameters Input parameters Values Inductance 2mH Capacitance 30µF 2. PRINCIPLE OF VECTOR CONTROL The fundamentals of vector control implementation can be explained with the help of figure-1 where the machine model is represented in a synchronously rotating reference frame. The inverter is not shown in the figure, assuming that it has unity current gain, that is,it generates currents ia, ib and ic as dictated by the corresponding command currents ia*,ib* and ic* from the controller. A machine model with internal conversions is shown on the right. The machine terminal

2 316 phase currents ia, ib and ic are converted to i s s ds and i qs components by 3-phase and 2-phase transformation. These are then converted to synchronously rotating frame by the unit vector components cosθe and sinθe before applying them to the d e -q e machine model as shown. The controller makes two stages of inverse transformation, as shown, so that the control currents ids* and iqs* correspond to the machine currents ids and iqs respectively. In addition unit vector assures correct alignment of ids current with the flux vector ψ r and iqs perpendicular to it, as shown. Note that the transformation and inverse transformation including the inverter ideally do not Incorporate any dynamics and therefore the response to ids and iqs is instantaneous. b. Transform them to the 2-phase system (α,β) using a Clarke transformation c. Calculate the rotor flux space vector magnitude and position angle d. Transform stator currents to the d-q coordinate system using a Park transformation e. The stator current torque- (isq) and flux- (isd) producing components are separately controlled f. The output stator voltage space vector is calculated using the decoupling block g. An inverse Park transformation transforms the stator voltage space vector back from the d-q coordinate system to the 2-phase system fixed with the stator h. Using the space vector modulation, the output 3-phase voltage is generated. In Figure.4 the standard vector control technique is shown. Figure 1: Vector Control Implementation Principle with Machine d e -q e Model There are essentially two general methods of vector control: 1. Direct or Feedback method 2. Indirect or Feed-forward method In this paper Space vector Direct Vector control is used. These methods are different essentially by how the unit vector (cosθe and sinθe) is generated for the control. The control is usually performed in the reference frame (d-q) attached to the rotor flux space vector. That s why the implementation of vector control requires information on the modulus and the space angle (position) of the rotor flux space vector. The stator currents of the induction machine are separated into flux- and torque-producing components by utilizing transformation to the d-q coordinate system, whose direct axis (d) is aligned with the rotor flux space vector. That means that the q-axis component of the rotor flux space vector is always zero: Figure 2: Block diagram of Vector Controlled AC Induction Machine 3. CONTROL TECHNIQUE Resonant-frequency capacitor voltages are essential for the control. In 3.1 resonant frequency signal is described and in 3.2 the procedure to generate compensating signals is discussed. rq 0 and d rq 0 dt 2.1. Computation of Vector Control Figure 2. shows the basic structure of the vector control of the AC induction motor. To perform vector control, steps are as follows: a. Measure the motor quantities (phase voltages and currents) Figure 3: Resonant-Frequency Extraction block

3 Resonant-Frequency Extraction Block The terminal voltage of machine contains fundamental (ω f ) and resonant-frequency (ω n ) signals. When the switching frequency of inverter is high the switching frequency component in the capacitor voltages are comparatively lower in magnitude than the resonantfrequency components. Machine-per-phase voltages v sr, v sy, and v sb are fed to extract resonant capacitor voltages. These voltages are transformed into d-q domain. In the transformed d-q voltages v sd and v sq both the fundamental components vsdf and vsqf and the resonant components v sd and v sq are present. vsdf and vsqf are dc quantities and v sd and v sq are ac quantities that are close to resonant-frequency i.e. v sd = vsdf + v sd v sq = vsqf + v sq. v sd and v sq are filtered using low pass filters.the outout of the low pass filter are vsdf and vsqf. They are substracted from v sd and v sq to extract v sd and v sq.the extracted resonant frequency component v sd and v sq have a frequency (ωn ωf ) due to the d-q transformation. The frequency of v sd and v sq varies with the variation of ωf. To get rid of the difference of ωf in v sd and v sq they are transformed back to three phase domain. The outputs of the reverse transform are v sr, v sy, and v sb. Hence, the extracted per phase resonantfrequency capacitor voltage v sr, v sy, and v sb are exactly at ωn Compensating Signals As shown in figure.4 the extracted resonant capacitor voltages v sr, v sy, and v sb are integrated to obtain v sr _int, v sy _int, and v sb _int signals.a low pass filter is used to generate v sr _int, v sy _int, and v sb _int as it does not cause any phase shift. When the inverter switching frequency is close to resonant frequency, the inverter introduces considerable amount of phase delay to the compensating signals v sr _int, v sy _int, and v sb _int.to compensate the inverter phase lag the phases are advanced of v sr _int, v sy _int, and v sb _int. The capacitor voltages v sr, v sy, and v sb are lags v sr _int, v sy _int, and v sb_int by 90.. v sr _int, v sy _int, and v sb _int signals are phase advanced by ωnts/2 to construct per-phase compensating signals v r_comp, v y _comp, and vb_comp.this phase advancement compensates the delay of ωnts/2 introduced by the inverter.the inverter switching frequency is fs and the inverter time constant is Ts/2, where Ts = 1/fs. vr_comp is obtained from vr_comp = v sr_int cos (ωnts/2) + v sr sin (ωnts/2) Figure 4: Complete Control Block Diagram

4 318 As cos (ωnts/2) and sin (ωnts/2) are fixed numbers, the compensation for the inverter delay can be easily and accurately introduced. These v r_comp, v y _comp, and vb_comp signals are multiplied by scaling factor K damp to emulate the resistance drop i.e. v invr_res = K damp v r_comp. v invr_res, v invy_res, and v invb_res signals are directly added to inverter voltage references v invr*, v invy*, and v invb* generated from standard vector control block 4. SIMULATION RESULTS The Simulation of Induction Motor along with Vector conrol is done on MATLAB SIMULINK using Filter and without filter. The results for both the cases are compared and FFT analysis is done as below. 5. SIMULATION OF IM APPLYING VECTOR CONTROL WITHOUT FILTER Figure 5: Simulink Model of Resonant-Frequency Extraction Block. The resonant frequency taken is 828Hz and K damp is 0.6. Figure 7.Simulink Model of Vector controlled Induction Motor without using Filter. A. Case-1 Figure 8: Speed of Induction Motor without filter. Settling time 0.7s Figure 6: Simulink Model of Vector Control Figure 9: Stator Current without filter.

5 319 Figure 10: Torque at 9Nm of Induction Motor without filter Figure 13: Speed of Induction Motor with filter. Settling time 2.5s. Figure 14: Torque at 9Nm of Induction Motor with filter Figure 11: Bar graph showing Magnitude of Harmonics without Filter. B. CASE-2:- 6. SIMULATION OF INDUCTION MOTOR APPLYING VECTOR CONTROLWITH FILTER Figure15: Stator Curent with filter. Figure 16: Bar graph showing Magnitude of Harmonics with Filter. Figure 12: Simulink Model of Vector controlled Induction Motor using Filter.

6 320 Table 2.Total Harmonic Distortion Harmonic Without Filter With Filter Order Overall 12.11% 5.90% Harmonic 5 th Harmonic 1.97% t Harmonic 1.51% 0.80% 7. CONCLUSION The motor current and voltage waveforms are close to sinusoidal and do not contain any voltage steps with high dv/dt. The vector control ensures extraction of resonant-frequency signal giving appropriate damping. Though the settling time for speed is 2.5s with filter and without filter is 0.5s but it reduces ripples, distortion and gives more efficiency and steady state operations of Induction motor. Thus the speed and Torque is controlled. By using filter Total harmonic distortion is also reduced from 12.11% to 5.90% giving sinusoidal outputs. Oxford University Press, Oxford. [8] J. K. Steinke, Use of an LC filter to achieve a motor-friendly performance of the PWM voltage source inverter, IEEE Trans. Energy Convers., vol. 14, pp , Sep [9] A. H. Bonnett, Analysis of the impact of pulsewidth modulated inverter voltage waveforms on ac induction motors, IEEE Trans. Ind. Appl., vol. 32, no. 2, pp , Mar./Apr [10] H. Akagi and S. Tamura, A passive EMI filter for eliminating both beaing current and ground leakage current from an inverter- driven motor, IEEE Trans. Ind. Electron., vol. 21, no. 5, pp , Sep REFRENCES [1] Abdesselam Chikhi, Mohamed Djarallah A Comparative Study of Field-Oriented Control and Direct-Torque Control of Induction Motors Using An Adaptive Flux Observer, SERBIAN JOURNAL OF ELECTRICAL ENGINEERING, Vol. 7, No. 1, pp. May 2010, [2] JanneSalomäki and JormaLuomi Vector Control of an Induction Motor Fed by a PWM Inverter with Output LC Filter, pp.2011 January/ Febraury [3] Jaroslaw Guzinski, Closed Loop Control of AC Drive with LC Filter,International Power Electronics and Motion Control Conference (EPE-PEMC 2008),pp th. [4] AnirudhAcharya, Shankar Murthy, VinodJoh, Filters for Active Front End Motor Drives, Dept. of Electrical Engineering, pp. May 2009 [5] Pekik Argo Dahono, A Method to Damp Oscillations on the Input LC Filter of Current-Type AC- DC PWM Converters by Using A Virtual Resistor, Department of Electrical Engineering, Bandung Institute of Technology, pp [6] A. Nabae, H. Nakano, and Y. Okamura, A novel control strategy of the inverter with sinusoidal voltage and current outputs, in Proc. IEEE PESC 94, vol. 1, Taipei, Taiwan, June 1994, pp [7] Vas, P "Vector Control of AC Machines",