Lecture on Three-Phase Power Electronics and Inverters

Overview

• Previous Lecture Recap: Focused on diode rectifiers for high power systems, converting AC to DC.
• Current Focus: High-frequency switching power conversion, specifically three-phase inverters.

Replicating Single-Phase for Three-Phase Inverters

• Concept: Use three single-phase inverters to build a three-phase inverter.
• Example: Three-phase machine with open-end windings.
  • Phase A, Phase B, and Phase C each driven by a single-phase inverter.
  • Total 12 devices (4 MOSFETs per phase).
• Real-World Implementation: Used in some MIT megawatt systems.
• Downside: Not common in standard three-phase motors, which are usually wye or delta connected.

Common Wye and Delta Connections

• Wye Connection: Three motor windings connected at a neutral point.
  • A, B, C - Three terminals
• Delta Connection: Closed loop connection of motor windings.
  • A, B, C - Three terminals, no neutral

Driving Three-Phase Loads (Wye or Delta)

• Structure of Three-Phase Inverters: Preferable setup for most applications.
• Configuration:
  • 6 switches instead of 12
  • Midpoint (V_r) as reference voltage
  • Connected to three-phase load- V_a, V_b, V_c
• Advantages:
  • Lower loss, less semiconductor area
  • Increased device utilization

Operation Modes

Six-Step Operation

• Simple Control: Switches follow six-step pattern; minimal switching.
• Waveform Characteristics: Fundamental components dominate; triple harmonics cancel.
• Applications: Suitable for high-output requirements.

PWM Technique

• Enhanced Control: Uses triangle wave comparison (reference to carrier).
• Generated Waveforms: Average output approaches sinusoidal form, reducing harmonics.
• Line to Line and Line to Neutral: Harmonics canceled; clean sinusoidal currents.
• Modulation Index (M):
  • M <= 1: No distortion; clean sine wave
  • M > 1: Harmonic distortion emerges

Third Harmonic Injection

• Purpose: Enhance voltage synthesis without distortion.
• Technique: Injecting a third harmonic to extend the voltage range up to M = 1.15.
• Advantage: Increases the possible fundamental without hitting the voltage ceiling.

Practical Considerations

• High-Speed Machines: May prefer independent phase drive for high-frequency switching.
• Electric Vehicles Example: Prius using series-connected devices with igbts and separate diodes for low loss.
• Voltage Synthesis Limitations: Explained saturation points and how to manipulate duty cycles.

Question & Answer Highlights

• Switch Selection: IGBTs vs. MOSFETs
  • IGBTs for lower forward drop
  • Modern designs using silicon carbide FETs
• Voltage Control: Dynamic control of DC bus voltage in practical systems like EVs.
• Loader Response: Inductive loads, neutral dynamics, etc.

Conclusion

• Summary of how different inverter designs suit particular applications and operating conditions.
• Next lecture will cover a new topic in power electronics.