Value engineering considerations in electrical drive systems for traction applications

The general requirements for electrical equipment used in traction applications are reviewed. These requirements and conditions include high inertia load, smooth torque control over the full range, high reliability, low maintenance, high efficiency, low mass, extended weak-field range for the motors, high top speed, dynamic or regenerative braking, motor and gearing exposed to heavy shock loading, limited motor diameter and length, and limited space for control circuitry. A systems approach is required to obtain the most effective design for each application. The advantages and disadvantages of different motor types are discussed. It is conluded that the series motor is still the motor against which all other traction motors must be judged. Its disadvantages are all related to the mechanical commutator. Possible replacement by the squirrel-cage induction motor and the brushless synchronous motor is considered. The characteristic features of different converter types are considered, including the three-phase bridge converter/inverter, the pulse-width-modulated (PWM) inverter and the cycloconverter. A current control and voltage control strategy is discussed.