| Summary: | M.Ing. (Electrical And Electronic) === Because of the high costs of available wheelchair drive systems, an investigation into an alternative drive system was undertaken. Three phase cage rotor induction machines were selected as the most suitable replacement for the current DC machines, having less maintenance requirements and costing less to manufacture. The 24V DCsupplied by the on board battery is converted to three phase alternating current with variable amplitude and frequency by means of a MOSFET inverter. A simple control algorithm, a differential algorithm and a PWM algorithm are pro.grammed into a INTEL 8097 micro regulator. The control algorithm ramps up the stator frequencies of the machines in a fixed time interval to a value determined by the operator. This results in a constant relation between the two frequencies, complying to the differential requirement. During an overcurrent situation, the control ramps the machine frequencies to a value which will tend to reduce the stator currents while still complying to the differential requirement. The airgap flux of the machines is kept constant by keeping the relation of the stator frequency to the stator voltage constant, except in the low speed area, where the stator voltage is altered to compensate for the voltage drop over the stator resistance. Because of the operator closing some sort of a speed feedback loop, no attempt was made to reduce the speed error of the drive caused by the slip-nature of induction machines. In order to test the drive under different operating conditions, a test setup was developed, employing DC machines connected mechanically to the traction machines.
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