Patent GB-765594-A (GB)
Title: Improvements in or relating to electrical control systems for d.c. motors
IPC: B66C13/24
Filed: 1954-09-20
Abstract: 765,594. Exciting motors. CLARKE, CHAPMAN & CO., Ltd., and HOWLETT, F. C. Sept. 14, 1955 [Sept. 20, 1954], No. 27211/54. Class 35. [Also in Group XXXVII] In a system for controlled lowering of an electrically-driven load, a current-limiting resistor 3 is connected in series with the armature 4 of the motor and is shunted by a control field winding 1 and rectifier 2. A potentiometer across the supply comprises starting resistor 5 and auxiliary field winding 6 which is connected in series with the armature for hoisting operation. A switch 8 is adapted to short-circuit the rectifier, and a shunt field winding 9 is regulated by resistor 10. The speed of the motor is controlled by the position of tap 7 which is moved by contactors regulated from a master controller. When the motor takes power from the supply, the rectifier 2 conducts so that field winding 1 strengthens the main field. If the motor regenerates, however, due to the lowering of a heavy load, the rectifier 2 prevents the field winding 1 from opposing the main field as long as the switch 8 is opened, so avoiding further weakening of the field when resistor 10 is in a high-speed position. The switch 8 is closed at the same time as a stronger field is applied for retardation, the regenerated current passing through the control field winding which opposes the main field and reduces the current surge, so providing better commutation. The rectifier 2 and resistor 10 may be short-circuited simultaneously by means of a double-pole switch. In another arrangement, the field winding 1 is connected in a rectifier bridge network arranged so that the control field winding strengthens the main field during both motoring and regenerating operation. With this arrangement, switches may be provided to reverse the current flow in the control field winding during regenerative operation, if so desired, and a resistance inserted in the rectifier bridge circuit may render the control field less effective during regenerating than during motoring, Figs. 3, 3a (not shown). Alternatively, the bridge circuit may comprise a resistor to make the control winding less effective during motoring than during regenerating operation. In a further modification, the control field winding is replaced by two separate windings each in series with a rectifier, these windings being adapted to strengthen the main field during motoring and regenerative operation respectively. The circuit comprises a switch for allowing the current to flow in both directions in one control winding, and to isolate the other control winding completely, Figs. 4, 4a (not shown). With this modification, a double-pole switch may be provided which enables the rectifier in the motoring control field winding circuit to be short-circuited simultaneously with the shunt field regulating resistance, for high-speed operation. Alternatively, a master controller regulates two singlepole switches for short-circuiting the rectifier in the motoring control field winding circuit and the shunt field regulating resistance, respectively. A further single-pole switch enables the regenerative control field winding to be open-circuited, and a fourth relay provides a time delay in the opening of the switch which short-circuits the rectifier in the motoring control winding circuit so as to control the current-surge during the deceleration of the motor from high speed to normal speed.