Efficiency optimisation at DC drives for small electrical vehicles

Electrical vehicles are well recognised for zero emission operation but have severe limitations in practical application due to the restricted energy capacity of batteries. For long operating distances highest overall efficiency is required. DC machines are still considered for cost saving applications when standard machines can be used for small electrical vehicles, especially when modernising existing DC machine cars without changing the mechanics. An efficiency optimised power conversion system with main emphasis on choppers and the power-optimised operation is described. The sources of losses are DC connection lines, fuses, magnetically operated circuit breakers, the drive converter, the auxiliary supply converter (e.g. for lights), the connection lines from converter to motor, the motor and the mechanical power train. All these losses are defined and calculated. The efforts, expenditures and final results for loss minimisation are presented. The MOSFET chopper is designed for highest efficiency with low on-state and switching losses. Despite the fundamental disadvantages of DC motors (rotor windings cooling, commutator, problematic operation at standstill) the overall system can be competitive with AC drives. The described loss minimisation measures increase efficiency remarkable especially for low battery voltages.