D-C series motors have historically been used for driving electric vehicles. Solid-state control has made it possible to operate a–c motors from d-c power sources. Controllers for a–c motors must invert, modulate voltage and frequency, and commutate the d-c power. Although d-c motors are larger than a–c motors because commutation devices are self-contained, controllers for d-c motors are much less complex. Thus, d-c drive systems are usually simpler and less costly than a–c systems.
Unlike internal combustion engines, the continuous torque rating of a motor is considerably less than its maximum torque. Several kinds of enclosures are available for d-c motors for matching average torque capability with load requirements and d-c motors are well adapted to highway vehicles. Brush life is no problem; more than 50,000 miles between brush changes is easily obtained.
Regenerative braking generally does not prove economical for passenger type highway vehicles. Dynamic braking is costly to install and control. At the present state-of-the-art, the d-c motor, earliest of all traction motors, is still the top contender for driving highway vehicles.
