Only the drive train components of a front bogie are shown. Lower wheels (green) of the front bogie are coupled to a motor (orange) through a differential and axle housing assembly (dark green), a drive shaft (violet), sprockets and a chain within a transfer case (blue) and a four-speed transmission (silver).
The bogie frame is not shown but directly supports the motor, transmission and transfer case. A pair of arms, not shown, have rearward ends pivotally connected to rearward ends of the bogie frame and have forward ends connected to the axle housings near the wheels. Portions of the bogie frame are resiliently supported through spring and shock absorber units from forward ends of such arms. Any resulting relative movement between the transfer case and the differential that may result is accommodated by conventional U-joints at the opposite ends of the drive shaft.
The illustrated transmission includes four planetary gear units supported on a frame (red) which is secured to the bogie frame. Sun gears of all four units are coupled to the shaft of the motor. Planet gear carriers of all four units are coupled to a sprocket within the transfer case. The ring gears are secured to four discs that can be braked by four disc brakes (yellow) on the transmission frame. The gear units provide four progressively higher output/input gear ratios, each being obtainable by use of one of the disc brakes to bring the associated ring gear to a stop, while allowing the other three ring gears to freely rotate.
The motor is preferably a three phase induction motor of a standard type that is available at relatively low cost and that is rugged and highly reliable in operation. The motor is directly connectable to utility power lines through conductors along the guideway that are engaged by shoes carried by the bogies. The motor is normally operated continuously at a speed close to a synchronous speed determined by the supply frequency and the number of its poles. With a 60 Hz supply and two poles per phase, the motor will operate at a speed close to 3600 RPM.
To control the disc brakes of the transmision, servo valves of an hydraulic system are electrically controlled. They can be used to obtain progressively higher speed ratios and to accelerate the vehicle to a speed at which the ring gear of the gear unit having the highest gear ratio is stopped, the vehicle being then efficiently driven at a substantially constant speed. In urban or other regions in which a lower speed may be desirable, a gear unit having a lower gear ratio may be used. In any case it is generally desirable to operate as much as possible at a speed at which the ring gear of one of the units is stopped, so as to minimize loss of energy in the disc brakes. One advantage of the arrangement is high efficiency. Another is that the servo valves may be operated at electrical power levels which are very low in relation that which would be required to control the electrical power supplied to a variable speed motor. The result can be increased reliability along with easier and simpler control.
The servo valves can also be used in decelerating the vehicle through operation of gear units having progressible lower output/input gear ratios, causing the induction motor to operate as as a generator and supply power back to the utility power lines, using the kinetic energy of the vehicle that was developed during acceleration of the vehicle. Thus, a further advantage of the arrangment is that regenerative braking can be obtained.
Electrially controlled servo valves of the same hydraulic system may be used to operate disc brakes for the four lower wheels to bring the vehicle to a complete stop when desired and to apply increased braking in any circumstances in which insufficient regenerative braking is available. One of such disc brakes (yellow) is shown for the left lower wheel and is coupled to the frame of the bogie through connections that are not shown.
The drive arrangement is designed to be highly reliable, using disc brakes, servo valves and other types of standard hydraulic components, and using electrical control at low power levels. The arrangement is also highly efficient. The induction motor can efficiently convert the electrical energy from the utility into mechanical energy and the drive train efficiently transmits such energy to the wheels, operating most of the time at a constant speed. The disc brakes only dissipate energy during acceleration and deceleration and, with the four speed transmission, the energy then lost in the brakes is only a small fraction of the total energy that is transmitted from or to the utility power lines and that is supplied to or from the vehicle as mechanical energy.
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