Patent GB-588973-A (GB)
Title: Improvements in a cyclical control system for direct current motors operating doubleskip hoists
IPC: B66B1/06
Filed: 1944-02-25
Abstract: 588,973. Control of D.C. motors. FREYN ENGINEERING CO. Feb. 25, 1944, No. 3541. Convention date, Dec. 11, 1942. [Class 38 (iii)] In a control system for direct current motors operating a double skip hoist having a winding drum which is driven by two motors each provided with a shunt winding, the motors are first connected in series for starting and slow speed operation, and in parallel for high speed operation, the speed of the motors being controlled by simultaneously strengthening or weakening the shunt fields, such control of the motors being effected by means dependent upon the position of the skips to afford motor operation in a predetermined sequence. Each motor has a series field winding which is connected in circuit for parallel operation and removed therefrom for series operation, the transition from series to parallel operation, and vice versa, being effected without opening the motor circuits. The system can be modified for separate motor control, and in such case the speed of the controlled motor is governed through its shunt winding. A control transfer switch is provided which enables the hoist to be operated so that each motor can be controlled from each of two positions or both motors may be controlled from one of these positions. The placing of the control transfer switch in any one of five alternative positions effects the operation of a multiplicity of switches in a large number of circuits and accomplishes the required re-arrangement of these circuits. The system comprises a limit switch coupled to the winding drum, adapted to move in accordance with the movement of the skips and provided with segments adapted to close contacts in the control circuits so as to govern the speed of the skips at various points in their travel. Push button control is used for normal automatic operation but a manually-operated master switch is provided, in a position remote from the said control means, with six positions to the right or left for determining the speed of upward movement of either skip. For normal twomotor operation a push-button is actuated to close a contactor S1 and connect the armatures in series. The resistors R1-R2 and R2-R3 are in this circuit and the resistors C-D, D-E, and F-G are in circuit while the motors are at rest and at the commencement of the starting operation. Relays are energized in timed sequence to shortcircuit resistors R1-R2 and R2-R3 and a further increase in speed is obtained by introducing the resistor C-D into circuit. A pair of contacts are then closed by the limit switch to place the armatures in parallel directly across the mains with intermediate field strength. The resistors D-E and F-G are introduced into the shunt field circuit under the influence of timed sequence relays to accelerate the motors to their full running speed. At the first slow down position, the limit switch un-bridges a pair of contacts to de-energize a relay coil and introduces resistors 1R3-1R2 1R2-1R1, and 2R3-2R2, 2R2-2R1, into their respective circuits and short-circuits the resistors D-E and F-G. After a time delay the motor armatures are connected in series with resistors R1-R2, R2-R3 and after a further time delay these resistors are short-circuited. At a further check-point of the limit switch, means are provided to stop the hoist if transition from parallel operation has not taken place and if the shunt fields of the motors have not been strengthened. The resistor C-D is then shortcircuited and the resistor R2-R3 introduced into the series armature circuit. Near the limit of travel limit-switch contacts are bridged to introduce resistors R10-R11 and R12-R14 into shunt with the armatures. At the limit of travel the motor armatures are given an electrical braking influence and the brakes are set. For single-motor operation from a position such as a stock-house, assuming that No. 1 motor is to be operated, switch IKS will be closed and switch 2KS opened. Switch 3KS thrown upwardly and switches 4KS and FT2 downwardly. The control transfer switch (Fig. 3, not shown) will be moved to the appropriate position. The hoist is started by a push-button and operations continue in the same general manner as with two motors. For single-motor control from a position such as a hoist-house a master switch (see Fig. 2, part E) is provided: This type of operation applies only to a single motor, either motor may be employed. Increasing speed is obtained by moving the master switch away from its central position, but the limit switch operates to control the speed at the various check-points. Means are provided for reducing the speed of the two motors in case of a tendency to overlap or in case of an overload.