GB1369034A - Electric driving apparatus - Google Patents

Abstract

1369034 Control of a plurality of DC motors GARRETT CORP 15 Nov 1971 [16 Nov 1970] 52894/71 Heading H2J Electrical driving apparatus for accelerating and driving a mechanical load by means of energy from an electric supply of substantially constant voltage, includes a driving assembly having at least one dynamo-electric machine mechanically coupled to the load to drive it, an energy bank incorporating a dynamo-electric machine capable of accepting and delivering electrical energy at variable voltage, and means for connecting the driving assembly and the energy bank to the supply, so that during one stage the load is accelerated by energy derived from part only of the supply voltage, while the dynamo-electric machine of the energy bank functions as a motor accepting energy derived from a further part of the supply voltage, and during a subsequent stage the energy bank is adjusted to cause its dynamo-electric machine to function as a generator. The embodiment of invention shown in Fig. 5 employs four D.C. traction motors 100, 102, 106, 108 driving a rapid transit car and an energy bank in the form of a D.C. machine 112 coupled to a flywheel 116, the five machines being connected to a substantially constant D.C. supply 132, 134 which also powers a control circuit 150 for controlling the field windings 118, 120, 122, 124, 126 of the machines and the contactors 140, 144, 148, 162, 164, of which the latter two control braking resistors 158, 160. Sensers 166, 168, 170 provide signals representative of the current of machine pairs 104, 110 and of the machine 112, which signals are passed to the control circuit 150 for comparison with command signals 156 derived manually or automatically. All machines are separately excited, but machine 112 may have an additional series winding to provide a compound characteristic. Also this machine may be coupled to an A.C. generator, as well as the flywheel 116, to supply vehicle services such as air conditioning and lighting. A typical operating cycle of the system may be obtained by manual and/or automatic control and comprises the following stages:- (1) Vehicle stopped and ready to start.-Contactor 140 is closed and all others are open. Machine 112 is idling at a speed determined by nearly full excitation of its field winding 126, the machine pairs 104, 110 carrying the armature current of machine 112 and having their field windings unexcited.. (2) Acceleration below base speed.-Full excitation is applied to the field windings of machine pairs 104, 110 and the field winding 126 is controlled to maintain a desired armature current in the machines, all of which function as motors with the machine 112 accelerating to a maximum speed as its field current is reduced, preferably by a ramping action over a time period chosen to produce a smooth start of the vehicle. At about half base speed, the speeds of the machines are such that the voltage of supply 132, 134 is applied entirely to the machine pairs 104, 110, at which point the excitation of the field winding 126 is reversed and the machine 112 acts as a generator driven by the flywheel 116 to assist the supply voltage in accelerating the machines 104, 110 to the base speed. (3) Transition at base speed.-At this speed the voltage across contactors 144, 148 has dropped to zero, allowing them to be closed to connect the machine 112 and machine pairs 104, 110 in parallel across the supply. At the same time the field windings 118, 120, 122, 124 are changed from maximum to variable excitation and the field winding 126 is controlled to raise the speed of the machine 112 to its idling value in stage (1), at which value it is maintained. (4) Acceleration above base speed.-Controlled decrease of the excitation of the field windings 118, 120, 122, 124 raises the vehicle to top speed at the desired value of armature current of stage (2). Machine 112 is maintained at its idling speed. (5) Cruising.-The excitation of field windings 118, 120, 122, 124 is increased to reduce the armature current of machine pairs 104, 110 to maintain the desired speed. (6) Regenerative deceleration above base speed.- The excitation of field windings 118, 120, 122, 124 is increased to reverse the armature current of machine pairs 104, 110 which return power to the supply 132, 134 at a rate corresponding to the desired deceleration. (7) Transition at base speed.-When the field currents reach full maximum value, the armature currents of machines 104, 110, 112 are equalized. Contactors 144, 148 are opened when the voltage across them has fallen to zero, so reconnecting the machines in series. (8) Regenerative deceleration below base speed.- The excitation of field windings 118, 120, 122, 124 is maintained at maximum value and the excitation of the field winding 126 is decreased to allow the machine 112 to speed up and so absorb energy from the machine pairs 104, 110. When half base speed is reached, the excitation of winding 126 is reversed and the machine 112 then assists the machine pairs 104, 110 in their regenerative action. A typical pair of characteristics of flywheel speed, voltage and power against vehicle speed are discussed, Fig. 7 (not shown). If regenerative braking is not possible, the above stages (6), (7) and (8) are modified as follows:- (6a) Deceleration above base speed.-Contactor 140 is opened and contactors 162, 164 are closed. Machine 112 continues to run as a motor (supplied by machines 104, 110) at its idling speed. Braking power of machines 104, 110 is absorbed mainly by resistors 158, 160. The excitation of field windings 118, 120, 122, 124 is increased as necessary to maintain the desired armature current. (7a) Base speed transition.-The operating procedure is the same as for stage (7) already described and additionally contactor 164 is opened to cut out resistor 158 and the speed control of the vehicle is transferred from control of the field windings 118, 120, 122, 124 to control of the field winding 126. (8a) Dynamic deceleration below base speed.- The excitation of field windings 118, 120, 122, 124 is maintained at maximum value and the field winding 126 is controlled to cause the machine 112 to draw power from the system and accelerate until half base speed is reached. At this point its winding excitation is reversed to cause it to restore power to the system until it has fallen to its lowest speed corresponding to the "vehicle stopped" condition, upon which contactor 162 is opened, contactor 140 is closed and the winding 126 is controlled to restore the machine 112 to its idling speed. The braking power available during stages (6a), (7a), (8a) may be doubled by a modification, Fig. 6 (not shown), which is similar to Fig. 5 but has an additional contactor in the motor armature circuit and additional contactors controlling a single tapped braking resistor. The stages of a typical braking operation of this modification are described. The energy bank 112, 116 may be replaced by two mechanically-coupled D.C. machines, the first of which is connected in the same way as the armature of machine 112 and the second of which has its armature in parallel with the supply 132, 134 and its field winding also controlled by the control circuit 150, Fig. 8 (not shown). The operation of this embodiment is described and is similar to another embodiment, Fig. 4 (not shown) having two traction motors and a machine-flywheel energy bank. The operation of the various embodiments may be modified to omit the transition from series to parallel coupling of the machines. Such an operation is described for Fig. 6 (not shown) wherein the flywheel energy is absorbed by the load on the machine 112 or during deceleration of the vehicle.