Electronic Design

New Alternator Technology Boosts Power In Future Cars

Researchers at the Massachusetts Institute of Technology's Laboratory for Electromagnetic and Electronic Systems in Cambridge have developed methods to boost the power in upcoming automobiles. Led by David J. Perreault, the scientists have found effective ways to move from the standard 14-V automobile electrical system to the 42-V system of the future.

A 42-V electrical system is necessary to enable the performance of advanced electrical features. Heated windshields, devices for reducing noxious emissions, and advanced suspensions will require additional power. As a result, the team has focused on reconfiguring the alternator, or electrical generator, to enhance vehicle power.

The experimental alternator is based upon mathematical modeling. Its configuration dramatically increases the peak and average power output from a conventional Lundell alternator. The researchers replaced three of the passive elements for directing current, or diodes, in a standard alternator with three active switches. Engineers can then control and optimize energy flow through the vehicle.

Additionally, the experimental alternator configuration incorporates a simple switched-mode rectifier and a load-matching control technique. While the Lundell alternator model optimizes energy only in the idle mode and the fixed output voltage is not matched to the alternator characteristics at other operating points, the MIT alternator attains maximum load-matched power at all speeds.

The new system uses field control and a switched-mode rectifier to achieve higher power than conventional systems. Higher power output is enabled by using the switched-mode rectifier as a second control handle to properly match the constant-voltage load to the alternator. Also, the switched-mode rectifier provides the necessary controlled-voltage transformation to match the constant-voltage load to the alternator as speed varies. Power output in this configuration is a function of the bridge voltage rather than the output voltage. This lets the alternator generate up to its maximum power as speed varies without increasing alternator losses or thermal stress.

Thanks to increased efficiency, the new system dissipates less energy in the form of heat. It also burns significantly less fuel to produce exactly the same amount of power as standard automobiles. The MIT configuration solves the problem of voltage spikes in a 42-V electrical system, too. While a maximum spike in a 14-V system is 80 V, a similar spike in the higher-voltage system can reach 240 V. This design ensures that the voltage spikes up to no more than 60 V during such an episode.

One additional complication of a 42-V electrical system is the inability to jumpstart the vehicle with a 14-V vehicle battery. The design, however, uses the energy stored magnetically in the alternator during current flow to aid in jumpstarting. The stored energy coupled with controls included in the new system make jumpstarting a 42-V vehicle with a 14-V vehicle battery possible.

For more information concerning the MIT alternator experiment, visit http://web.mit.edu.

TAGS: Mobile
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