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Implementation of Space Vector Modulation Technique for Two-Level Inverter feeding Induction Motor

This thesis discusses the implementation of a space vector modulation technique for a two-level inverter in order to improve the performance of an induction motor. It covers topics such as power electronics, types of inverters, pulse width modulation, and the relationship between inverter switching and space vector switching timings.

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Implementation of Space Vector Modulation Technique for Two-Level Inverter feeding Induction Motor

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  1. Implementation of Space Vector Modulation Technique for two-level Inverter feeding Induction Motor Guided By:Submitted by: G.PRANAVA RAMESH CHANDRA. P Dept. of Electrical Engineering KLCE

  2. Organization of Thesis This thesis is organized as follows: Chapter 1 introduces the review of power electronics, introduction of inverters and types of Pulse Width Modulated inverters. Chapter 2 introduces the classifications of Inverters, analysis of Induction Motor drives, analysis of Pulse Width Modulation and its control. Chapter 3 describes the technique of Space Vector Modulation, Gating signal generation in Space Vector Modulated PWM scheme and relationship between the Inverter leg Switching timings and the Space Vector Switching timings. Chapter 4 describes the Result & discussion. Chapter 5 describes the conclusion.

  3. INTRODUCTION Inverter: A device that converts dc power into ac power at desired output voltage and frequency is called an inverter. Some industrial applications of inverters are for adjustable speed ac drives, induction heating, stand by aircraft power supplies, uninterruptible power supplies for computers, HVDC transmission lines etc. • As Inverters are employed to get a variable frequency supply from a dc supply, Stepped-wave inverters of figure.1 can be designed to behave as voltage source or current source Steeped wave Inverters

  4. Variable frequency and variable voltage ac is directly obtained from fixed voltage dc when the inverter is controlled by Pulse Width Modulation (PWM) as shown in Figure • The Pulse Width Modulation control also reduces harmonics in the output voltage. Pulse Width Modulated Inverters

  5. Pulse Width modulated Inverters: • Pulse Width Modulated inverters are gradually taking over other types of inverters in industrial applications.Pulse Width Modulation techniques are characterized by constant amplitude pulses. The width of these pulses is, however, modulated to obtain inverter output voltage control and to reduce its harmonic content. • Different Pulse Width Modulation techniques are as under: • Single-pulse modulation • Multiple- pulse modulation • Sinusoidal- pulse modulation

  6. Analysis of Induction Motor fed from Non-sinusoidal voltage supply: Consider the fundamental phase voltage components with the phase sequence ABC. VAN = V1 Sin ωt VBN = V1 Sin (ωt-2π/3) VCN = V1 Sin (ωt-4π/3) The corresponding 5th and 7th harmonic voltages are: VAN = V5 Sin 5ωt VBN = V5 Sin 5(ωt-2π/3) = V5 Sin (5ωt-10π/3) VCN = V5 Sin 5(ωt-4π/3) = V5 Sin (5ωt-20π/3) and VAN = V7 Sin 7ωt VBN = V7 Sin 7(ωt-2π/3) = V5 Sin (7ωt-14π/3) VCN = V7 Sin 7(ωt-4π/3) = V5 Sin (7ωt-28π/3)

  7. The harmonic voltages and currents of the order n = 6k+1 are of positive sequence n = 6k-1 are of negative sequence. n = 3k are of zero sequence. (where k is an integer) Classification of inverters: Inverters can be broadly classified into two types: • Voltage Source Inverter (VSI) • Current Source Inverter (CSI)

  8. VSI Induction Motor drives VSI controlled IM drive using chopper VSI controlled IM drive using controlled rectifier

  9. VSI controlled IM drive using dc supply VSI controlled IM drive using ac supply

  10. Drawbacks of Stepped wave Inverters: • Because of low frequency harmonics, the motor losses are increased at all speeds causing derating of the motor. • Motor develops pulsating torques due to fifth, seventh, eleventh and thirteenth harmonics that cause jerky motion of the rotor at low speeds. • Harmonic content in motor current increases at low speeds. The machine saturates at light loads at low speeds due to high (v/f) ratio. These two effects overheat the machine at low speeds, thus limiting lowest speed to around 40% of base speed.

  11. Pulse width modulation control: • The advantages of Pulse Width Modulation technique are as under: • The output voltage control with this method can be obtained without any additional components. • With this method, lower order harmonics can be eliminated or minimized along with its output voltage control. As higher order harmonics can be filtered easily, the filtering requirements are minimized. Disadvantage: The main disadvantage of this method is that the SCRs are expensive as they must possess low turn-on and turn-off times.

  12. SPACE VECTOR MODULATION Conventional Induction motor drive using a two-level inverter

  13. The ratio of the peak value of the modulating signal and the peak value of the carrier signal is defined as the amplitude modulation ratio (also called modulation index) and is denoted as ma The space vector constituted by the pole voltages, and is defined as: Vs = VAO + VBO .exp [j (2π/3)] + VCO .exp [j (4π/3)] The relationship between the phase voltages,, and the pole voltages , and is given by: VAO = VA N + VN O; VBO = VB N + VN O; VCO = VCN + VN O; Since , VAN+VBN+VCN =0 VNO = (VAO + VBO + VCO) / 3 Where,VNO is the common mode voltage.

  14. The space vector can also be resolved into two rectangular components namely V and V . It is customary to place the -axis along the A-phase axis of the induction motor. Hence: • The relationship between (V,V) and the instantaneous phase voltages (VAN,VBN,VCN) is given by the conventional ABC- transformation as below using the Clark’s Transformation(3-phase to 2-phase transformation):

  15. Each pole in a two-level inverter can independently assume two values namely 0 and Vdc. Therefore, the total number of states a two-level inverter can assume is 8 . • These states are graphically illustrated in Figure below for State 2(+, +, -)

  16. The switching states from 1 to 6 are known as “Active” states and the states 7 and 8 are known as “Null” or “Zero” or “Passive” state. Table 1: Switching States of two-level Inverter in all the sectors

  17. The following example illustrates the method of determination of the space vector location for a given state. When the inverter assumes a state of ‘2’ (+ + -) as shown in Figure, then the pole voltages are: VAO = (Vdc/2); VBO = (Vdc/2); VC0 = - (Vdc/2) Hence the space vector for this state is given by Vs = VAO + VBO .exp [j (2π/3)] + VCO .exp [j (4π/3)] Vs = (Vdc/2) + (Vdc/2). exp [j (2π/3)] - (Vdc/2). exp [j (4π/3)] = (Vdc/2) + (Vdc/2). [-(1/2) + j (√3/2)] - (Vdc/2). [-(1/2) - j √3/2)] = Vdc. [(1/2) + j (√3/2)] = Vdc at an angle 60 degrees For the states 8 (- - -) and 7(+ + +) the motor phases are short-circuited and therefore are not connected to the source. These states are called the zero states or null states during which there is no power flow from the source to the motor. Hence, by controlling the duration of these zero state intervals, we can control the output voltage magnitude

  18. Space vector locations for a two-level inverter

  19. Gating signal generation in Space Vector Modulated PWM scheme:

  20. Switching sequences for two-level inverter in all the sectors for Space Vector Modulation

  21. The conventional implementation of the Space Vector PWM involves the following steps: • Sector identification • Calculation of the active vector switching time periods and using equation. • Translation of the active vector switching time periods and into the inverter leg switching timings Tga,Tgb and Tgc. • Generation of the gating signals for the individual power devices using the inverter leg switching timings Tga,Tgb and Tgc.

  22. Space vector Switching Timings

  23. Inverter leg Switching Timings

  24. RESULTSFor Modulation Index m=0.8 Sampling time

  25. Pole Voltage (Vao)

  26. Phase voltage (Vab)

  27. Phase to Neutral voltage (Van)

  28. Normalized Harmonic Spectra of phase voltage when Inverter is operated in the range of linear modulation with ma = 0.8

  29. For Modulation Index m=1 Switching times PoleVoltage

  30. Phase voltage Phase to Neutral voltage Normalized Harmonic Spectra

  31. Conclusion A classical sinusoidal modulation limits the phase duty cycle signal to the inner circle. The space vector modulation schemes extend this limit to the hexagon by injecting the signal third harmonic. The result is about 10% (2/1.73 x 100%) higher phase voltage signal at the inverter output. The PWM modulation chops alternatively two adjacent phase voltage and zero voltage signals in a certain pattern producing the switching impulses for the inverter Sa, Sb and Sc.

  32. SCOPE FOR FUTURE WORK This thesis can be extended for the speed control of Induction Motor as we are getting varying voltage levels depending upon the switching timings of the Inverter and also can be used for harmonic analysis of higher order. This can be further implemented using DSP for more-level Inverters.

  33. References [1] “Power Electronics, Circuits,Devices and Applications”, second edition, by MH.Rashid. [2]“Fundamentals of Electrical Drives” by Gopal k Dubey, Narosa Publishing House second edition 2001. [3]“Power Semi-Conductor Controlled Drives” by Gopal K. Dubey, Prentice-Hall International Limited. [4 “Power Electronics and Variable Frequency Drives” by Bimal K. Bose, IEEE PRESS 2000 Edition. [5]“Electric Motor drives modeling Analysis, and control” by R.Krishnan, Pearson education Inc 2003 edition. [6] “MATLAB Programming for Engineers” Second Edition by Stephen J. Chapman [7] Sidney R. Bowes and Yen-Shin Lai," The Relation between Space-Vector Modulation and Regular-Sampled PWM", IEEE Trans. Industrial Electronics. , vol. 44, no. 5, Oct. 1997, pp. 670 - 679. [8] Sandor Halasz, Bin T. Huu and Alexei Zakharov, “Two-Phase Modulation Technique for Three-Level Inverter-Fed AC Drives”, IEEE Trans. Ind. Electron., Vol.47, No.6, Dec 2000, pp.1200 - 1211. [9] Fei Wang, “Sine-Triangle versus Space- Vector Modulation for Three-level PWM voltage Source Inverters”, IEEE Trans.Ind.Applicat., vol.38.no.2, Mar/Apr 2002 pp.500-506. [10] T.A. Meynard, M.Fadel and N.Aouda, “Modeling of Multilevel Converters”, IEEE Trans. On Industry. Electronics. vol. 44, no.3, Jun 1997, pp.356 – 364. [11] www.mathworks.com

  34. Thank U

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