Lecture on Power Electronics with Three-Phase Elements

Importance of Diode Rectifiers & High-Frequency Switching

• Diode Rectifiers: Essential for converting AC to DC in high-power systems.
  • Example: Used in the front end of high-power systems.
• High-Frequency Switching Power Conversion: Needed for various applications.
  • Single-phase inverters were discussed previously.
• Three-Phase Inverters: Different approaches compared to single-phase.

Building a Three-Phase Inverter

Replicating Single-Phase Systems

• Open-Ended Windings: Can build three-phase inverters by constructing three single-phase inverters.
  • DC bus BDC feeds into single-phase inverters for phase A, B, and C.
  • Each single-phase inverter: 4 power MOSFETs + diodes.
  • Total: 12 devices for the entire three-phase system.
  • Example: MIT's megawatt system using sets of single-phase inverters for custom machines.

Common Three-Phase Motor Connections

• Y-Connected: Three windings connected together at a neutral point (A, B, C terminals).
• Delta-Connected: Windings connected in a closed loop (A, B, C terminals with no neutral).
• In these configurations, simple replication of single-phase inverters can cause short circuits and system failure.

Three-Phase Bridge Inverter

• Structure: Uses a DC bus with active switches instead of diodes.
  • Six devices (instead of 12), reducing complexity and losses.
  • Active control in all three-phase applications such as DC to AC converters.
• Diode Integration: Devices with reverse diodes or MOSFETs with built-in diodes ensure path for current when switches are off.

Components in Modern Inverters

• Examples: Older Prius inverter with IGBTs and separate diodes, modern inverters using silicon carbide FETs.

Operations of Three-Phase Inverter

Potential States

• Eight States: Each bridge leg either connects to the top or bottom of the DC bus.
• Six-Step Operation: Mimics diode rectifier pattern, creating square waves for each phase.
  • Line-line voltages are harmonic-free due to phase differences.
  • Neutral voltage contains third harmonic and other triple n components.

Advanced Waveform Synthesis

• PWM Techniques: Generating sinusoidal waveforms or other desired waveforms.
  • Triangle Intercept PWM: Using high-frequency switching to synthesize desired voltages.
  • Modulation index (M): Ratio of desired voltage amplitude to maximum possible without distortion.

Minimizing Harmonic Content

• Third Harmonic Injection Trick: Adding third harmonic component allows higher modulation indices (up to 1.15) without distortion.
• Pure Sinusoidal Outputs: Achieved using PWM; critical for reducing low-frequency harmonic content.

Speed and Voltage Control in Electric Machines

• Modulating V_subm: To control motor speed and power, increasing voltage amplitude as needed.
• Maximum Voltage Synthesis:
  • Without distortion: Up to VDC/2.
  • With third harmonic injection: Up to 1.15 x VDC/2.
  • With full six-step (square wave) operation: Higher but with undesirable harmonics.

Control Challenges & Solutions

• Distortion at High Voltages: Solved by controlling fundamental and harmonic contents.
• Increasing DC Bus Voltage: Use of additional boost stages (e.g., in electric vehicles) to manage voltage requirements.