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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
Start | Power Inverter | Gate Drivers | Control Board | Space Vector Modulation | PID Tuning | Altering Motor Control Parameters | HMI | Protocol | Firmware/Software | Sources & PDF
CONTROL BOARD
The inverter’s control board is primarily a stripped-down variant of the Microchip MC-1 development board (see Photo 2).[6] (An alternate version of Photo 2 with callouts is posted on the Circuit Cellar FTP site.) We did this for code compatibility and to reduce actual development time. Several simple changes were made to the PCB design. The bias resistors on the gate drivers were moved to the output of the logic buffer and given the capacity to function as pull-up, pull-down, or float, depending on the placement of a jumper. Regulators were added to the board, providing stable 5- and 15-V sources. Additional decoupling capacitors were placed in the circuit to reduce the inevitable noise (motor noise and switching noise) that is inherent to the system. The complete control board is shown in Figure 3.
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| Photo 2 —The motor control board is shown here. We based our design on a Microchip Technology dsPIC MC1 motor controller board. |
We revised the control board four times. The first board was tight, with limited room for additional prototyped circuitry. This would be excellent for a production model but a nuisance for development. Further revisions increased the board’s size and spaced out the components. There is now more real estate for a larger prototype area, allowing for circuit changes if required. The final board includes all of the changes. The schematic is available on the Circuit Cellar FTP site.
The most significant difficult aspect of the board design process involved trying to anticipate the needs of future users. The board was given more CAN bus connectors than the standard MC1 board. We included two DE-9 connectors instead of one, as well as four custom sockets for additional CAN bus modules. The final connection between the control board and the inverter is shown in Photo 3.
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| Photo 3 —This shows the interconnections between the motor control board and the gate drivers on the inverter. Shielded cable was used to reduce EMI. |
A daughter board was created to assist in debugging as well as provide an interface to the LabVIEW application used for motor tuning and instrumentation (see Photo 4). (An alternate version of Photo 4 with callouts is posted on the Circuit Cellar FTP site.) The daughter board acts as a bridge between the CAN bus and the USB with robust error checking between the two protocols. The daughter board microcontroller and the CAN bus controller were left with separate clocks due to the timing sensitivity of the CAN bus. With some minor modifications, the module could also control an optional LCD. The module connects to the main board with a four-pin connector that also provides power to the module. Four of these sockets are provided on the main board.
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| Photo 4 —This daughter board was used to interface the HMI to the control board. The control board is connected via a CAN bus and the LabVIEW HMI via USB. |
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