ABSTRACT

It is generally recognized that there is a need for new methods of affordable, non-polluting, personal transportation. A literature review shows that bicycles are very efficient, well understood machines, but that electric versions have had little commercial success over the past century. By applying modern power semiconductor technology, a standard separately-excited DC motor is now used to efficiently propel a bicycle.

The practicality of such a vehicle was demonstrated by entering an early version in the 1992 American Tour de Sol. Compared to the other electric vehicles in the event, this electric bicycle consumed a similar amount of power per mile per unit weight, but used the least power per mile, due to its small size. A final prototype demonstrates the full implementation of both mechanical and electrical components.

To better analyze the motor, a non-linear model is used. It is shown that maximum efficiency is achieved by maintaining a fixed ratio between the armature and field currents. This ratio is found from the motor characteristics, determined by lab-bench testing. By combining the motor model with a model for the batteries and the physics of the bicycle itself, a computer simulation adequately predicts the system's performance.

Design issues raised when using power MOSFET's, which should not be considered as ideal switches at high current levels, are discussed in detail. The resulting power unit is driven with discrete logic based on digital counters to form a pulse-width modulation controller. Two current feedback loops provide complete control of both the armature and field. The power unit is also appropriate for micro-computer control applications.

Jim Coate, jcoate@alumni.tufts.edu