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Issue #217 August 2008
Subcategory Winner - Microchip 2007 Design Contest
INTELLIGENT ENERGY SOLUTIONS
Electric Vehicle Inverter Design
Build A System For Powering AC Induction Motors
by Dan Hall, Tristan Kasmer, Doug Krahn, Adam McIntyre, and Dena Ponech
The Electric Vehicle (EV) Inverter project is a starting point for the conversion of a vehicle with a gas engine to one with an electric motor. The inverter was built using a control board and software based on Microchip Technology’s MC-1 development board. The finished design can power a wide range of AC induction motors.
Start | Power Inverter | Gate Drivers | Control Board | Space Vector Modulation | PID Tuning | Altering Motor Control Parameters | HMI | Protocol | Firmware/Software | Sources & PDF
The world is in the process of advancing alternative sources of energy, and the concept of electric vehicle (EV) conversion is becoming more and more popular. But anyone interested in this concept will discover that the process of converting a gas-powered vehicle to electric power is fairly cost-prohibitive. This challenge led instructors in the Electronics Engineering Technology program at Camosun College in Victoria, BC, to pitch the idea of an “open-source” EV conversion kit to our class to take on as our final project. We accepted and the EV Drive Team was formed.
The open-source concept of an EV design has numerous benefits, such as accessibility, cost, and advancement. You should be able to easily obtain all of the inverter’s components (if not purchase an assembled system), and once constructed, it will be able to power a wide range of AC induction motors (ACIMs). Obtaining a typical three-phase high-power inverter for driving an ACIM can cost between $8,000 and $25,000. In this article, we will present a cost-effective method of applying an alternative source of energy. Any technically minded person should be able to complete this project for around $2,500.
Our inverter was constructed with a control board and software based on Microchip Technology’s MC-1 development board and three dual-IGBT modules with gate driver boards from Powerex. We used these development tools to build a basic 100-kW three-phase inverter in an open-source process, which can be continually improved on.
All of the inverter’s components of the inverter design are “off-the-shelf” units readily available at several electronics distributors. The parts are subject to change if the open-source design is modified to suit a user’s needs or improved by advancing technology. This aspect of the inverter’s design provides for the non-restrictive application of the system. It gives you the freedom to choose a motor that was not originally designed for a vehicle. For example, we used an ACIM that was intended to drive an elevator.
The ACIM is the simplest and most rugged electric motor. It consists of two basic electrical assemblies: the wound stator and the rotor assembly. The outer stationary “stator” consists of coils that are supplied with an AC current to produce a rotating magnetic field. The inner non-stationary “rotor” revolves as a result of the torque that is created by the rotating magnetic field. The induction AC motor derives its name from currents flowing in the rotor that are induced by alternating currents flowing in the stator.
You can control the speed of an ACIM by varying the frequency and amplitude of the drive voltage. Early configurations of drives used SCRs fired at the appropriate times to create an unsophisticated sinusoidal input waveform. As the semiconductor industry evolved, SCRs were replaced with MOSFET or IGBT devices, which are more efficient than the SCR, and could be switched at higher frequencies to continuously generate variable-drive voltages and currents that closely resemble a sinusoidal waveform.
The AC current that is supplied to the stator, which produces the rotating magnetic field, can be controlled by a PWM algorithm that directly influences the speed of the rotating field and the output RPM of the rotor. We used a PWM technique known as space vector modulation.
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