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Project Lead the Way Conference 2010
Electric Vehicle Power Control Comparison
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Dan Cole |
October 1, 2010 was the date of Purdue-South Bend's second annual Project Lead the Way (PLTW) Conference. The faculty and staff at Purdue hosted about 100 students from area high schools. Sessions included hands-on projects in Electrical Engineering Technology (EET), Mechanical Engineering Technology, and Industrial Technology; as well as a careers session and a campus tour.
In keeping with the conference theme of electric vehicle technology, the EET project was a comparison of two different forms of motor control: resistive and pulse-width-modulated (PWM). It was developed by a sophomore EET student, Dan Cole, using the National Instruments simulation package called Multisim.
The resistive control circuit, shown below, used a variable resistor to limit the amount of power applied to the motor. The variable resistor is called a rheostat. Although effective, the problem with the rheostat is that it is very inefficient except when the motor is operated at full power (which is a very small percentage of the time). When the motor is not operated at full power, the rheostat dissipates a large proportion of the batteries' supplied power as heat. This means a lot of the battery power is wasted, dramatically shortening how long/far the electric vehicle can be driven.
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Resistive Control Circuit |
The PWM control circuit, below, uses switched transistors called MOSFETs to control how much power is delivered to the motor. The transistors are switched on and off very rapidly. One complete on-off cycle is called the switching period. The percentage of time the transistor is on each period, called the duty cycle, determines how much of the available power is delivered to the motor. When the transistors are on, their resistance is so low that nearly 100% of the power is delivered to the motor. When the transistors are off, no current flows, no power is lost, and the motor's rotational momentum keeps it moving until the transistors turn on again. This technique is much more efficient than resistive control.
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Pulse-Width-Modulated Control Circuit |
The high school students ran simulations and took data using each technique, then plotted the results. As shown in the plot below, the PWM control results in much higher efficiency during part-throttle driving, which is where most of the time driving occurs.
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Efficiency Comparison Plot |
For more information, see the EET session and project paper at the ASEE web site.
Feedback or questions are welcome. Please direct them to Professor Harding.