Voltage Source Inverter (VSI) Fed Indexer Motor Drive

Introduction

• VSI stands for Voltage Source Inverter.
• Converts DC Supply to Variable Frequency AC Supply.

Key Components & Devices

• Transistors: Six transistors (TR1, TR4, TR6, etc.) used in the inverter.
• Self-Commutated Devices: Can be used at various power levels:
  • Low Power: Mode switch
  • Medium Power: IGBT and Power Transistors
  • High Power: GTO (Gate Turn-Off Thyristor) and IGCT (Integrated Gate Commutated Thyristor)

Operating Modes

• Step Wave Inverter:
  • Switch transistors with a time difference of T/6.
  • Each transistor should be on for a duration of T/2.
  • Operation independent of rectifier or Chopper.
  • Uses large electrolytic filter capacitor to filter out harmonics.
• Pulse Width Modulation (PWM) Inverter:
  • Transistors switch to control output voltage and frequency precisely.
  • Involves more complex control compared to step wave.

Circuit Components and Functions

• Chopper: Converts fixed voltage DC supply to variable voltage DC supply.
• Capacitor (C): Large electrolytic filter capacitor connected to DC link.
  • Makes inverter operation independent of upstream components (rectifier/chopper).
  • Filters out DC link voltage harmonics.
• Controlled Rectifier: Converts AC to DC when the supply is AC.
  • Provides DC input for the inverter.

Inverter Operation

• DC Supply: (Step Inverter) Use Chopper to convert to variable voltage DC.
• AC Supply: Use Controlled Rectifier to convert to DC.
• DC Supply: (PWM Inverter) Direct input without additional conversion.
• AC Supply: Use Diode Bridge Rectifier.

Important Formulas

• Inverter Output Line and Phase Voltage: (Fourier Series Representation)

V_{AB} = \frac{2\sqrt{3}}{\pi} V_D [ \sin(\omega t) - \frac{1}{5} \sin(5 \omega t) - \frac{1}{7} \sin(7 \omega t) + \frac{1}{11} \sin(11 \omega t) + ... ]

• VD is the DC input voltage.