16-08-2011, 02:59 PM
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INTRODUCTION
1.1 Purpose:
This 500W Three-Phase Inverter is a project whose purpose is to support the University of Illinois Future Energy Challenge (FEC) team. The purpose of said team is to develop a motor drive system which is both highly efficient and cost effective. Successful implementation of such a design could potentially reduce national overall energy consumption by 10% annually if widely used throughout society (e.g. in household appliances and industry).1 These savings would greatly reduce CO2 released into the atmosphere from the burning of fossil fuels, thus detracting from the greenhouse effect. This three-phase inverter represents the final stage of the FEC motor drive circuit.
1.2 Description and Block Diagram:
The block diagram representing our project is shown in Figure 1. A speed command is given to the controls team, and a digital switching signal is generated. These switching signals are sent to the hex-bridge through a gate driver chip. The hex-bridge takes as input a 200V DC bus voltage and the switching signals to produce a balanced three-phase sinusoidal output which drives the induction machine.
Figure 1: Block Diagram
1.3 Performance and Design Specifications:
The three-phase inverter needs to be able to handle 200 volts DC ± 5V. At this voltage the inverter must meet the following specifications:
• Motor speed control between 150-5000 RPM
• Drive a 50-500W load at nominal speed of 1500 RPM
• Three-phase sinusoidal output current
• At least 70% efficiency under all operating conditions
• MTBF > 10 years1
1.4 Subprojects:
The overall project proceeded smoothly because the design was broken down into several smaller tasks which were implemented by various combinations of team members. These subprojects are:
• Hex-bridge circuitry
• Gate drive circuitry
• Switching signal implementation (waveform generator)
• PCB layout
2 DESIGN DECISIONS
2.1 Hex Bridge:
The standard three-phase inverter has as its genesis, the hex-bridge. The hex-bridge takes a DC bus voltage and uses six switches (MOSFETS) arranged in three phase legs as shown in Figure 2. Each line is then connected from the middle of each phase leg to the motor itself. The waveforms on these lines must be a balanced three-phase sinusoidal waveform in order to drive the induction motor properly. This is achieved by carefully controlling the switching waveforms at the gates of the switches. For each leg, the two switches have a 50% duty cycle ensuring that there will be no DC component in the output signal.
