A controlled current inverter for an electric vehicle.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
The University of 'Canterbury Mark II' electric vehicle has been out of service for several years due to the lack of a suitable inverter to provide a variable frequency AC supply to its traction motors. This thesis describes the design and construction of such an inverter, using high current bipolar junction transistors as the switching elements, so that the car might be returned to service in the near future. The inverter is based on an existing commercial AC motor speed controller. Modifications to this AC motor speed controller were made to suit the low voltage, high current rating of the traction motors. These modifications are described and it is shown that these modifications permitted the inverter to deliver the required increase in current. The inverter differs from most conventional AC motor speed controllers in that it acts to shape load current rather than potential, and uses an asynchronous switching technique to do this. The Thesis describes this technique and the control hardware constructed to implement it. Test results, showing the performance of the combined inverter/motor system are then presented graphically and discussed with reference to standard AC motor theory, giving consideration to the harmonic content of the AC waveforms. Consideration is also given to a suitable closed-loop control system which could be expected to ensure that the inverter's output frequency is controlled in such a manner as to give a safe and predictable response to the brake and accelerator controls.