660,414. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. May 9, 1949. [May 22, 1948] No. 12326/49. Class 38 (iii). A control system for propulsion motors of electric vehicles comprises switching devices for connecting a motor to power conductors, a multiple step controller for the step-by-step shunting of the field winding of the motor, switching devices for establishing a dynamic braking circuit for the motor, and a relay operative during coasting of the vehicle and responsive to the motor speed during coasting for automatically controlling the said multiplestep controller as well as a plurality of unitytype switching devices for shunting resistors included in the said dynamic braking circuit whereby, during coasting of the vehicle, both the resistance of the dynamic braking circuit and the field shunting resistance are adjusted in accordance with the vehicle speed so as to be ready for prompt dynamic braking action. As shown, the system comprises two series traction motors M1, M2. When controller AC is moved to the "switching position" switches LS1, G, JR, are closed to connect the motors in series, resistors 18, 21, 22, being included in circuit. Movement of the controller to the ''series position" closes switch LS2 to remove resistor 18 from the motor circuit, resistorshunting switches R1-R5 then being sequentially operated under the control of a limit relay CR. Following the close of switch R5, a switch J is closed for transition to parallel connection of the motors. The closure of switch J causes the opening of switch JR and shunts the resistor 20 from the motor circuit. Further operation of the controller closes switch MG for parallel connectionofthemotors, acceleration being continued by the progressive closure of switches R2-R5. A magnet valve SF is then energised to actuate field shunting controllers FC1, FC2, in a step-bystep manner until the accelerating cycle is complete. For coasting operation, the controller AC is returned to the "off" position to open switches LSI, LS2, G, and disconnect the motors from the supply. A switch V1 is closed to establish dynamic braking connections with resistors 19, 21, 22, in circuit and the controllers FC1, FC2, in the position affording weak field excitation so that the braking-current is small. Each field winding is connected across the armature of the other motor. During coasting operation the controllers FC1, FC2, and the resistor-shunting switches are regulated by a speed-responsive relay SR so that the control apparatus is prepared to respond quickly when braking is called for. When the braking-current decreases sufficiently to permit the relay SR to full to its lowermost position, coil FF is energised to actuate the controllers FC1, FC2, toward the full-field position. When the field controller reaches a pre-determined position, a switch B2 is closed to remove a portion of the resistor 19 so increasing the motorcurrent and actuating the relay SR to its uppermost position. This energises the magnetcoil SF and moves the controllers FC1, FC2, towards the weak-field position until the coilcircuit is interrupted at contact segment 31, the field controller then remaining stationary until the motor current decreases and relay SR drops to return the field controller towards the full-excitation position. Upon the field controller reaching another pre-determined position, a switch B3 is closed to remove an additional portion of the resistor 19. Switches B4, B5, B6, may be closed in progression under the control of relay SR in a similar manner. If the vehicle speed continues to decrease during coasting, the switches R1-R5 will close under the control of the relay SR. For dynamic braking operation a controller BC is moved to energise relay BR and transfer the control of the field-shunting controller and resistorshunting switches from the relay SR to the current-responsive relay CR. A rate coil RC on the relay CR is energised through a rheostat 24 which is adjusted in accordance with 'the position of the controller BC. The magnetising coil M and neutralising coil N of a time-delay relay are both energised to actuate the relay to its uppermost position. The coil FF is energised to move the field-shunting controller towards the full-excitation position until the coil circuit is interrupted at contact 33, the coil M being also de-energised at this time. The field controller is held stationary until the time-delay relay drops, thus ensuring that the field-strength is built up before resistorshunting operation begins. Closure of contact TD1 energises coil FF and moves the field controller to the full-excitation position whereupon switches B2-B6 are operated in sequence to remove resistor 19. The sequential operation of the resistor-shunting switches starts from the last switch which was closed during coasting operation, the amount of resistance being related to the speed. With the removal of resistor 19, switches R1-R5 close to shunt resistors 21, 22. Air braking may, follow dynamic braking. The rate coil RC is energised during acceleration, through a resistor 23 which may be adjusted in accordance with the vehicle-load. The calibration of the relay CR is modified during braking by a coil BRC which is connected across a portion of resistor 25 in the motor circuit.