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UNIT-IV

UNIT-IV. INVERTERS. Single-Phase Inverters. Half-Bridge Inverter One of the simplest types of inverter . Produces a square wave output. Single-Phase Inverters (cont’d). Full Bridge (H-bridge) Inverter Two half-bridge inverters combined. Allows for four quadrant operation.

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UNIT-IV

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  1. UNIT-IV INVERTERS

  2. Single-Phase Inverters Half-Bridge Inverter One of the simplest types of inverter. Produces a square wave output. EE2301-POWER ELECTRONICS

  3. Single-Phase Inverters (cont’d) Full Bridge (H-bridge) Inverter Two half-bridge inverters combined. Allows for four quadrant operation. EE2301-POWER ELECTRONICS

  4. Single-Phase Inverters (cont’d) Quadrant 1: Positive step-down converter (forward motoring) Q1-On; Q2 - Chopping; D3,Q1 freewheeling EE2301-POWER ELECTRONICS

  5. Single-Phase Inverters (cont’d) Quadrant 2: Positive step-up converter (forward regeneration) Q4 - Chopping; D2,D1 freewheeling EE2301-POWER ELECTRONICS

  6. Single-Phase Inverters (cont’d) Quadrant 3: Negative step-down converter (reverse motoring) Q3-On; Q4 - Chopping; D1,Q3 freewheeling EE2301-POWER ELECTRONICS

  7. Single-Phase Inverters (cont’d) Quadrant 4: Negative step-up converter (reverse regeneration) Q2 - Chopping; D3,D4 freewheeling EE2301-POWER ELECTRONICS

  8. Single-Phase Inverters (cont’d) Phase-Shift Voltage Control - the output of the H-bridge inverter can be controlled by phase shifting the control of the component half-bridges. See waveforms on next slide. EE2301-POWER ELECTRONICS

  9. Single-Phase Inverters (cont’d) EE2301-POWER ELECTRONICS

  10. Single-Phase Inverters (cont’d) The waveform of the output voltage vab is a quasi-square wave of pulse width . The Fourier series of vab is given by: The value of the fundamental, a1= The harmonic components as a function of phase angle are shown in the next slide. EE2301-POWER ELECTRONICS

  11. Single-Phase Inverters (cont’d) EE2301-POWER ELECTRONICS

  12. Three-Phase Bridge Inverters Three-phase bridge inverters are widely used for ac motor drives. Two modes of operation - square wave and six-step. The topology is basically three half-bridge inverters, each phase-shifted by 2/3, driving each of the phase windings. EE2301-POWER ELECTRONICS

  13. Three-Phase Bridge Inverters (cont’d) EE2301-POWER ELECTRONICS

  14. Three-Phase Bridge Inverters (cont’d) EE2301-POWER ELECTRONICS

  15. Three-Phase Bridge Inverters (cont’d) The three square-wave phase voltages can be expressed in terms of the dc supply voltage, Vd, by Fourier series as: EE2301-POWER ELECTRONICS

  16. Three-Phase Bridge Inverters (cont’d) The line voltages can then be expressed as: EE2301-POWER ELECTRONICS

  17. Three-Phase Bridge Inverters (cont’d) The line voltages are six-step waveforms and have characteristic harmonics of 6n1, where n is an integer. This type of inverter is referred to as a six-step inverter. The three-phase fundamental and harmonics are balanced with a mutual phase shift of 2/3. EE2301-POWER ELECTRONICS

  18. Three-Phase Bridge Inverters (cont’d) If the three-phase load neutral n is isolated from the the center tap of the dc voltage supply (as is normally the case in an ac machine) the equivalent circuit is shown below. EE2301-POWER ELECTRONICS

  19. Three-Phase Bridge Inverters (cont’d) In this case the isolated neutral-phase voltages are also six-step waveforms with the fundamental component phase-shifted by /6 from that of the respective line voltage. Also, in this case, the triplen harmonics are suppressed. EE2301-POWER ELECTRONICS

  20. Three-Phase Bridge Inverters (cont’d) For a linear and balanced 3 load, the line currents are also balanced. The individual line current components can be obtained from the Fourier series of the line voltage. The total current can be obtained by addition of the individual currents. A typical line current wave with inductive load is shown below. EE2301-POWER ELECTRONICS

  21. Three-Phase Bridge Inverters (cont’d) The inverter can operate in the usual inverting or motoring mode. If the phase current wave, ia, is assumed to be perfectly filtered and lags the phase voltage by /3 the voltage and current waveforms are as shown below: EE2301-POWER ELECTRONICS

  22. Three-Phase Bridge Inverters The inverter can also operate in rectification or regeneration mode in which power is pushed back to the dc side from the ac side. The waveforms corresponding to this mode of operation with phase angle = 2/3 are shown below: EE2301-POWER ELECTRONICS

  23. Three-Phase Bridge Inverters (cont’d) The phase-shift voltage control principle described earlier for the single-phase inverter can be extended to control the output voltage of a three-phase inverter. EE2301-POWER ELECTRONICS

  24. Three-Phase Bridge Inverters (cont’d) EE2301-POWER ELECTRONICS

  25. Three-Phase Bridge Inverters (cont’d) The three waveforms va0,vb0, and vc0 are of amplitude 0.5Vd and are mutually phase-shifted by 2/3. The three waveforms ve0,vf0, and vg0 are of similar but phase shifted by . EE2301-POWER ELECTRONICS

  26. Three-Phase Bridge Inverters (cont’d) The transformer’s secondary phase voltages, vA0, vB0, and vc0 may be expressed as follows: where m is the transformer turns ratio (= Ns/Np). Note that each of these waves is a function of  angle. EE2301-POWER ELECTRONICS

  27. Three-Phase Bridge Inverters (cont’d) The output line voltages are given by: While the component voltage waves va0, vd0, vA0 … etc. all contain triplen harmonics, they are eliminated from the line voltages because they are co-phasal. Thus the line voltages are six-step waveforms with order of harmonics = 6n1 at a phase angle . EE2301-POWER ELECTRONICS

  28. Three-Phase Bridge Inverters (cont’d) The Fourier series for vA0 and vB0 are given by: EE2301-POWER ELECTRONICS

  29. Three-Phase Bridge Inverters (cont’d) The Fourier series for vAB is given by: Note that the triplen harmonics are removed in vAB although they are present in vA0 and vB0. EE2301-POWER ELECTRONICS

  30. PWM Technique While the 3 6-step inverter offers simple control and low switching loss, lower order harmonics are relatively high leading to high distortion of the current wave (unless significant filtering is performed). PWM inverter offers better harmonic control of the output than 6-step inverter. EE2301-POWER ELECTRONICS

  31. PWM Principle The dc input to the inverter is “chopped” by switching devices in the inverter. The amplitude and harmonic content of the ac waveform is controlled by the duty cycle of the switches. The fundamental voltage v1 has max. amplitude = 4Vd/ for a square wave output but by creating notches, the amplitude of v1 is reduced (see next slide). EE2301-POWER ELECTRONICS

  32. PWM Principle (cont’d) EE2301-POWER ELECTRONICS

  33. PWM Techniques Various PWM techniques, include: • Sinusoidal PWM (most common) • Selected Harmonic Elimination (SHE) PWM • Space-Vector PWM • Instantaneous current control PWM • Hysteresis band current control PWM • Sigma-delta modulation EE2301-POWER ELECTRONICS

  34. Sinusoidal PWM The most common PWM approach is sinusoidal PWM. In this method a triangular wave is compared to a sinusoidal wave of the desired frequency and the relative levels of the two waves is used to control the switching of devices in each phase leg of the inverter. EE2301-POWER ELECTRONICS

  35. Sinusoidal PWM (cont’d) Single-Phase (Half-Bridge) Inverter Implementation EE2301-POWER ELECTRONICS

  36. Sinusoidal PWM (cont’d) when va0> vT T+ on; T- off; va0 = ½Vd va0 < vT T- on; T+ off; va0 = -½Vd EE2301-POWER ELECTRONICS

  37. Sinusoidal PWM (cont’d) EE2301-POWER ELECTRONICS

  38. Sinusoidal PWM (cont’d) Definition of terms: Triangle waveform switching freq. = fc (also called carrier freq.) Control signal freq. = f (also called modulation freq.) Amplitude modulation ratio, m= Vp VT Frequency modulation ratio, mf (P)= fc / f Peak amplitude of control signal Peak amplitude of triangle wave EE2301-POWER ELECTRONICS

  39. Multiple Pulse-Width Modulation • In multiple-pulse modulation, all pulses are the same width • Vary the pulse width according to the amplitude of a sine wave evaluated at the center of the same pulse EE2301-POWER ELECTRONICS

  40. Generate the gating signal 2 Reference Signals, vr, -vr EE2301-POWER ELECTRONICS

  41. Comparing the carrier and reference signals • Generate g1 signal by comparison with vr • Generate g4 signal by comparison with -vr EE2301-POWER ELECTRONICS

  42. Comparing the carrier and reference signals EE2301-POWER ELECTRONICS

  43. Potential problem if Q1 and Q4 try to turn ON at the same time! EE2301-POWER ELECTRONICS

  44. If we prevent the problem Output voltage is low when g1 and g4 are both high EE2301-POWER ELECTRONICS

  45. This composite signal is difficult to generate EE2301-POWER ELECTRONICS

  46. Generate the same gate pulses with one sine wave EE2301-POWER ELECTRONICS

  47. Alternate scheme EE2301-POWER ELECTRONICS

  48. rms output voltage • Depends on the modulation index, M Where δm is the width of the mth pulse EE2301-POWER ELECTRONICS

  49. Fourier coefficients of the output voltage EE2301-POWER ELECTRONICS

  50. Harmonic Profile EE2301-POWER ELECTRONICS

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