Downloaded from SAE International by University of New South Wales, Monday, August 20, 2018660199 D-C Electric Drive for Off-Highway Vehicles R. W. Volpe Locomotive & Car Equipment Dept., General Electric Co. DC ELECTRIC DRIVE systems have demonstrated to users of off-highway vehicles unmatched performance and have pro­ vided materials haulage at unprecedented savings in total cost per ton moved. It is appropriate to discuss these drive systems and establish the parameters that make them of such unique value to the vehicle user. BASIC WHEEL DRIVE SYSTEMS The designer of a wheeled vehicle may choose to drive the wheels in one of many ways, which may be categorized in three basic classes: 1. Dependent (rigid) wheel drive. 2. Semi-independent wheel drive. 3. Independent wheel drive. Dependent wheel drives were common on steel wheeled locomotives where the drive wheels were rigidly connected together on one side of the vehicle so that all wheels main­ tained the same speed. This system is not common on mod­ ern maneuverable rubber tired vehicles. Semi-independent wheel drive is obtained by placing a gear type of differential between the wheels. Properly de­ signed units can deliver torque to each driven wheel, roughly in proportion to the adhesion (ability to accept torque with­ out slipping) available. The GE-769-C1 traction motor in Fig. 1 is an example of a motor specifically designed to power the standard dif­ferential of off-highway trucks. This motor and associated gen­ erator and control gear have been applied to 65 ton rear dump trucks manufactured by LeTourneau-Westinghouse Co., KW-Dart Truck Co., and Clark Equipment Co. As vehicles grow in size and power requirements, the abil­ ity to transmit full engine torque through the single gear tooth mesh in a differential becomes a limiting parameter. The differential also limits the amount of dynamic braking, Fig. 1 - GE-769-C1 traction motor •ABSTRACT This paper points up the advantages of d-c electric drive systems for off-highway vehicles and compares their per­ formance with diesel or gas turbine powered vehicles using conventional differential drives. It is shown that the in­dependent wheel drive system provided by the electric drive develops torque at each wheel that allows the wheel to min­ imize slipping or sliding. This is accomplished by the mo­ torized wheel described herein. Downloaded from SAE International by University of New South Wales, Monday, August 20, 2018Fig. 2 - GE motorized wheel Fig. 3 - GE GT-603 generator either electric or hydraulic, which can safely be used. As the braking level is increased, the point is reached where one wheel driving through the differential forces the other wheel on the axle to rotate in the opposite direction. The problems inherent with differential input drives are eliminated by independent wheel drives. The ideal inde­ pendent wheel drive system develops torque (tractive effort) at each wheel within the limits of the wheel/roadway co­ efficient of friction and allows each wheel in a turn to run at the speed necessary to eliminate slipping or sliding with­ out external system controls. A system using d-c series motors provides these charac­ teristics. The motors, to reduce their size and weight, are usually relatively high speed machines, driving the wheels through two or more gear reductions. One arrangement, pi­ oneered in 1908 by a manufacturer in Great Britain for use on fire engines, placed a planetary gear set within the wheel hub and mounted the motor to the gearbox (inboard of the wheel) between two I beams that served as the vehicle axle. Present applications of this principle substitute a tube for the beams or use the motor frames, per se, for the tube. With the beams for an axle, the motors are completely available for inspection and maintenance; with the tube for an axle, access to the motors is limited by the size and number of the inspection ports that ca