Patent GB-687891-A (GB)
Title: Improvements in control systems for electric motors
IPC: B66C13/24
Filed: 1950-12-14
Abstract: 687,891. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 14, 1950 [Dec. 30, 1949], No. 30534/50. Class 38 (iii) An electric motor circuit, particularly for hoisting equipment, includes an armature and a shunt field winding, and a series field winding of the motor is connected through a rectifying means to the armature circuit so that the direction of the field of this winding is maintained when the armature current is reversed. As shown, a direct-current hoist motor has an armature 10a, a series field winding 10b, and a shunt field winding 10c, the series field winding being connected to a bridge rectifier 13 so that the direction of current through the winding 10b remains unchanged despite reversals of current through the armature. The motor has a master controller 18 which, in the first hoisting position, energises the motor through a circuit comprising contactor 19, resistors R1-R4, contactor 27, switch 23, and constructor 20. At the same time a contactor 22 is closed to shunt the motor by resistor R6. To increase the motor-speed the controller is 'moved to the second hoisting- position wherein resistors R1, R2, are removed. In the third hoisting-position the shunt resistor R6 is disconnected, and the fourth and fifth hoisting- positions of the controller provide for the removal of resistors R4, R3, in 'succession. Over-travel of the hoist operates limit switch 23 to disconnect the motor from the fine, establishes a dynamic braking circuit through resistor R8, and applies electro-magnetic brake 15 so that upward movement of the hoist is quickly arrested. If the hoist should then start downwardly the current through the motor armature is reversed but that through the series field winding remains in the same direction so that dynamic braking-action is also provided in the downward direction. For lowering operation the controller is moved through the "off" position, to the first lowering notch in which the motor is connected to the supply through contactor 19, resistor R1, contactor 31, resistor R6, and contactor 22. The resistor R2 is shunted across the armature on the first lowering- position of the controller, and, on the second lowering notch, the resistor R3 is inserted in series with resistor R2 in this shunt circuit. An additional resistor R4 is inserted in the motor armature shunt circuit on the third lowering notch. In the fourth controllerposition resistor R2 is removed from and resistor R5 is added to the shunt circuit which is now connected across the armature and resistor R2 in series, resistor R1 being removed from the line. The highest, lowering speed is provided by the connection of resistors R2, R7, in series with the armature, the whole being shunted by resistors R3, R4, R5. Fig. 4 (not shown) is a graphical illustration of the speedtorque curves obtained when hoisting, and under lowering conditions with the load either driven or over-hauling. During regenerative operation a rising or drooping speed-torque characteristic may be afforded by a suitable choice of circuit-resistance. The shunt field winding of the motor may be additionally supplied through a bridge rectifier so as to maintain excitation thereof in the event of supply failure, Fig. 5 (not shown).