Unit- 4 inverters (part-2)
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1) There are several methods to control the output voltage of single phase inverters including external control of AC output voltage, external control of DC input voltage, and internal control of the inverter. 2) Internal control of the inverter through pulse width modulation is commonly used as it requires no additional components. Pulse width modulation controls the output voltage by adjusting the ON and OFF periods of the inverter components. 3) Harmonic reduction can be achieved through techniques like multiple pulse modulation, sinusoidal pulse modulation, and combining output voltages from multiple inverters with transformer connections. Internal control of the inverter through advanced PWM techniques is effective in minimizing harmonics in the output voltage.
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Unit- 4 inverters (part-2)
- 1. Inverters Prepared by, Mr. A.Johny Renoald M.E., Ph.D,
- 2. Voltage Control in Single Phase Inverters β’ AC loads may require a constant or variable voltage at their input terminals. β’ Methods of controlling the output voltage are, β’ External control of AC output voltage β’ External control of DC input voltage β’ Internal control of inverter β’ The first two methods require additional components. But third method requires no additional components. Inverter DC AC
- 3. External Control of AC Output Voltage AC Voltage Control β’ The output voltage of the inverter is controlled by using an AC voltage controller. β’ The output contains more harmonics when the output voltage is low. β’ Hence it is rarely used. Inverte r AC Voltage Controlle r AC Load Controlle d AC voltage Consta nt DC voltage Consta nt AC voltage
- 4. External Control of AC Output Voltage Series Inverter Control β’ Two or more inverters are connected in parallel. β’ Output of inverters are connected to transformers whose secondary windings are connected in series. β’ Frequency of output voltages V01 & V02 must be same. Ο΄ V 02 V 01 V O
- 5. External Control of DC Input Voltage Fully Controlle d Rectifier Filter Inverte r Controlle d AC voltage Controll ed DC voltage Consta nt AC voltage Uncontro lled Rectifier Filter Inverte r Controll ed AC voltage Chop per AC Voltage Controll er Filte r Inverte r Controll ed DC voltage Uncontro lled Rectifier Chopp er Filter Inverte r Controll ed AC voltage Controll ed DC voltage Control led DC voltage Controll ed DC voltage Consta nt AC voltage Controll ed AC voltage Consta nt AC voltage
- 6. Internal Control of Inverter β’ Output voltage of inverter is adjusted by controlling the inverter itself. β’ This method of controlling the output voltage is called Pulse Width Modulation. β’ It is obtained by adjusting the ON and OFF periods of the inverter components. Advantages οΌ No additional components are required. οΌ Lower order harmonics can be eliminated along with voltage control. οΌ Filter requirements are minimized.
- 7. Pulse Width Modulated Inverters β’ These inverters can produce ac voltages of variable magnitude as well as variable frequency. β’ The quality of output voltage can also be greatly enhanced, when compared with those of square wave inverters. β’ Pulse Width Modulation is the process of modifying the width of the pulses to obtain variation in the o/p voltage with reduced harmonic content. β’ The main aim of using different PWM techniques is to
- 8. Different types of PWM inverters οΆ Single pulse modulation οΆ Multiple pulse modulation οΆ Sinusoidal pulse modulation οΆ Modified sinusoidal pulse modulation οΆ Space vector pulse width modulation
- 9. Single Pulse Width Modulation β’ In single pulse modulation, there is only one pulse exists per half cycle. β’ The width of this pulse is varied to control the inverter output voltage.
- 10. β’ Frequency of the reference signal determines the frequency of output voltage. β’ The ratio of Ar to AC, called modulation index, controls the output voltage. Ar > AC Ar < AC
- 11. β’ The output voltage of the inverter with modulation is given by, π π = π=1,3,5 β 4π π ππ sin ππ 2 sin ππ sin πππ‘ π π = 4π π π sin π sin ππ‘ β 1 3 sin 3π sin 3ππ‘ + 1 5 sin 5π s ππ1 = 4π π π sin π 2 sin π sin ππ‘ = 4π π π sin π sin π ππ1π = 4π π π sin π ββββββββ βπ΄ β’ If nd = Ο or d=Ο/n, then nth harmonic will be e the inverter output voltage. β’ For example, for eliminating third harmonic, 3d width, 2π = 2π/3 = 1200 .
- 12. Inference from Single PWM β’ 3rd, 5th & 7th harmonics dominate when the voltage is reduced. β’ A large amount of harmonics is introduced at lower output voltages. β’ Harmonic content can be reduced by having many pulses in each half cycle of output voltage.
- 13. Multiple Pulse Modulation β’ In this method, many pulses having equal widths are produced per every half cycle. β’ The gating signals are produced by comparing reference signal with triangular carrier wave. πΈ = π β ππ π + π π
- 14. Multiple Pulse Modulation β’ Frequency of the reference signal determines the frequency of output voltage. β’ The ratio of Ar to AC, called modulation index, controls the output voltage.
- 15. β’ The output voltage waveform can be expressed in Fourier s π π = π=1,3,5 β 8π π ππ sin nΞ³ sin ππ 2 sin πππ‘ π π = 8π π π sin πΎ sin π 2 sin ππ‘ β 1 3 sin 3πΎ sin 3π 2 sin + 1 5 sin 5πΎ sin 5π 2 sin 5ππ‘ β¦ . . ππ1 = 8π π π sin Ξ³ sin π 2 sin ππ‘ ππ1π = 8π π π sin Ξ³ sin π 2 βββββ βπ΅
- 16. β’ For example, take pulse width 2d = 720. β’ In single pulse modulation, the peak value of fundamental ππ1π = 4ππ π sin π = 4ππ π sin 36 = π. ππππ π½ β’ In two pulse modulation, the peak value of fundamental vo ππ1π = 8ππ π sin πΎ sin π 2 πΎ = 180 β 72 3 + 36 2 = 540 ππ1π = 8ππ π sin 54 sin 18 = π. πππ π½πΊ
- 17. Multiple Pulse Modulation β’ It is seen from the above that the fundamental component of output voltage is low for two pulse modulation than it is for single pulse modulation. β’ But lower order harmonics are eliminated and higher order harmonics are increased. But higher order harmonics can be filtered easily. β’ This scheme is advantageous than single pulse modulation. β’ But large number of pulses per half cycle requires
- 18. Sinusoidal Pulse Modulation β’ In this method, several pulses per half cycle are used as in the case of multiple pulse modulation. β’ But width of each pulse is modulated proportional to the amplitude of sine wave. β’ Gate pulses are generated by comparing sinusoidal reference signal with triangular carrier signal. β’ Frequency of reference signal (fr) decides the frequency of output voltage.
- 20. Sinusoidal Pulse Modulation What is Modulation Index? οΆ Modulation index is the ratio of peak magni modulating waveform and the carrier waveform π = π π π π οΆ MI controls the harmonic content in the outpu
- 21. Summary οΆ By increasing the number of pulses (N) per half cycle, the lower order harmonics get cancelled. But higher order harmonics will get increased. οΆ Higher order harmonics can be filtered out easily. οΆ Higher value of N results in more switching
- 22. Sinusoidal Pulse Modulation What is Over Modulation? β’ When the peak magnitude of modulating signal exceeds the peak magnitude of carrier signal, the PWM inverter operates under over-modulation. β’ During over-modulation the output voltage increases slightly.
- 23. Reduction of Harmonics in the Inverter O/P β’ Harmonics of 5% is allowable in an inverter output voltage. β’ But inverter output voltage contains more than 5% of harmonics. β’ Filters can be used to reduce the harmonic content. β’ Small size filter is enough for reducing higher order harmonics. β’ But a bigger size filter is required for reducing lower
- 24. Harmonic Reduction by PWM β’ Several pulses per half cycle reduces the lower order harmonics. β’ As the waveform is symmetrical during every quarter cycle, an=0.
- 25. β’ If 3rd and 5th harmonics are to be eliminated, β’ Using Ξ±1 and Ξ±2, voltages of 7th, 9th and 11th harmonics are found as,
- 26. β’ The amplitude of the fundamental component for these values of Ξ±1 and Ξ±2 is, β’ The amplitude of the fundamental component of un modulated output voltage wave is, β’ The amplitude of the fundamental voltage is 83.91% of the un modulated wave. So inverter is de-rated by 16.09%. β’ Additional eight commutation per cycle increases switching losses.
- 27. Harmonic Reduction by Transformer Connections β’ The output of two or more inverters are combined using transformers to get a net voltage with reduced harmonic content. β’ The voltage waveform should be similar but phase shifted from each other.
- 28. Harmonic Reduction by Transformer Connections β’ The Fourier analysis of V01 and V02 gives, β’ The amplitude of the fundamental output voltage with phase shift, β’ The amplitude of the fundamental output voltage with no phase shift, β’ In this method, the inverters are de-rated by 13% but this is less compared to the previous method
- 29. Harmonic Reduction by Stepped Wave Inverters β’ In this method, pulses of different widths and heights are super imposed to get a resultant stepped wave with reduced harmonic content.
- 30. Voltage Source Inverters Input voltage is constant Output voltage does not depend on load Magnitude of load current & its shape depends on load Current Source Inverters Input current is constant Output current does not depend on load Magnitude of load voltage & its shape depends on load
- 31. Single Phase Current Source Inverter