Our first contract involved using an unmodified series wound motor. We used contactors to steer field current for forward and reverse. Our power electronics based on fluid cooled Infineon IGBTs and microprocessor controller worked brilliantly. We tested the bus to 14 tonnes on the steepest Wellington hills by filling it with bags of cement.
Our second contract involved converting the motor for separate excitation whereby the field coils were re-wound to operate at 25 A instead of near 500 A. This enabled us to efficiently control them via IGBTs rather than having to use contactors. A very interesting aspect to this part of the project was the simulation to assist us in developing a field control system capable of good armature voltage control to track trolley bus overhead voltage supply fluctuations.
The normal approach in these situations is to use a compound field winding design where a proportion, maybe 15% of the field excitation comes directly from armature current (series wound). This assists with armature voltage regulation. However, with the steep Wellington hills, we did not want to reduce the motor maximum torque which would have been the inevitable result of a compound winding, especially in reverse!
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