Patent GB-780423-A (GB)
Title: Improvements in and relating to electric motor equipments for hoists, winders and the like
Inventor: EVANS OWEN THOMAS
IPC: B66B1/12
Filed: 1953-12-30
Abstract: 780,423. Control of D.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Dec. 30, 1954 [Dec. 30, 1953], No. 36242/53. Class 38 (3). [Also in Group XXXV] In a control system for a hoist, wherein the output of a grid-controlled rectifier to the armature of a D.C. winding motor is varied according to deviations of motor speed from a variable manually set reference value, variable dynamic braking is manually initiated by connecting a tapped load resistance across the armature. The armature 1, Fig. 1, of the separately excited motor is connected through contactors F, R for forward or reverse running respectively to the output of the mercury discharge rectifier Z whose anodes are supplied from the secondary 3 of a three-phase transformer. The rectifier control grids (not shown) are supplied from a transformer winding 6 through a peaking circuit 14, Fig. 2, and the output windings OW of the three-phase transductors T1A, T1B, T1C. The latter have seriesconnected control windings OW energized by the rectified outputs of the speed and limiting transductors T3, T4 respectively over an amplifying transductor T2. A speed reference potentiometer SPR (Figs. 2, 3) energized over closed contacts SC1 is manually set to give an input, over change-over contacts CO3, CO4 and power contacts PC, to the control winding CW of transductor T3 for comparison with D.C. speed and speed rate signals derived from potentiometers 32, 35, respectively, the latter having a rectified input from a tachometer TG associated with the armature. Rectifiers 30, 31 ensure that the net control current in winding CW is of constant polarity independent of the direction of wind. Motoring.-Movement of a lever into the forward range closes contacts MC1, Fig. 3, to energize the coil OCF of the forward contactor F over closed contacts RSC1, R1, SC2, F1C1, thereby closing contactor F to connect the rectifier output to the armature, and closing the power contacts PC, Fig. 2, by energization of their operating coil OCPC, Fig. 3. The leverset output of SPR is now applied to transductor T3 for accelerating the motor, and the increasing speed signal from the tachometer then limits the output of T3 as the reference speed is approached. Forward rotation of the armature closes the forward contacts 37 of a directional switch SPD, Figs, 1, 3, to energize the coil OCFSC, Fig. 3, of the forward selector contactor, thus opening its contacts FSC1 and closing its contacts FSC2. Dynamic braking.-If braking is desired, the lever is moved into the reverse range, thus opening contacts MC1 to de-energize the forward contactor F, and closing contacts MC2; the reverse contactor is not closed, however, since its coil OCR is not energized, contact FSC1 being maintained open by forward running of the motor. Closure of contacts MC2 establishes the braking circuits, since the braking contactor coil OCB is energized, over now-closed contacts F2, to close contacts B, Fig. 1. The coils OCBR1-OCBR4 of the braking-resistance contactors can now be energized, by closure of contacts MC4-M37 corresponding to four lever settings, to close their respective contacts BR1-BR4, thereby short-circuiting the resistance steps LR1-LR4 in series with the armature. The coils OCBR3, OCBR4 are energized over normally open contacts TD1, TD2 of time delay relays, and normally closed contacts CLR1, CLR2 of a current-limiting relay. The former contacts have coils OCTD1, OCTD2 in series with normally closed contacts BR22, BR32 of the first and second braking-resistance contactors, so that contactors BR3, BR4 cannot close until a certain interval after closure of contacts BR2, BR3; and the latter contacts have a coil OCLR1, Fig. 1, energized by an excessive armature current. A similar sequence of operation applies for reverse running and braking in the forward range; in this case reverse contacts 38 of switch SPB are closed to actuate the reverse selector contactor OCRSC. Limiting circuits.-The control winding CW of transductor T4 has a biasing input from a potentiometer 100, a stabilizing feedback input from a potentiometer 101 depending on the rate of change of input to transductors T1A, T1B, T1C, and control inputs across a resistor 103 depending on armature current and acceleration exceeding certain values. The latter inputs are derived by comparing current and acceleration signals, from potentiometers 113, 108 respectively, with reference values set on potentiometers 116, 110. Voltage surges on the armature are prevented by an ignitron I, Fig. 1, which is adapted to connect resistance steps LR1-LR4 across the armature when fired by a signal exceeding a reference set on potentiometer 52, contactor IC being closed during motoring only by energization of its coil OCIC over closed contacts B4, ITC. A reverse current relay coil RCR closes its contacts when a reverse surge flows from armature to rectifier, thereby energizing a coil OC21C to close contacts 21C, Fig. 2, and thus energize coil OCA1C of an auxiliary contactor. Contacts A2C are then closed to energize coil OC1SC, thereby closing contacts ISC, Fig. 1, and ISC1, Fig. 3, the latter maintaining coil OCAIC energized when contacts 2IC re-open. In operation, firing of ignitron I ceases when contacts ISC close, the latter remaining closed until the surge ceases. Details, Fig. 3.-Safety contacts SC1, SC2 are closed when a wind is initiated, and field interposing contacts FIC1 are closed by energization of their coil OCFIC over closed contacts MFR, TFR when the motor and tachometer are excited. When motoring or braking circuits are established, closure of one of contacts F4, R4, FSC4, RSC4 energizes a coil OCFWC to close contacts FWC, thereby shorting a motor field limiting resistor FSR. An emergency brake is biased " on," and is released by energization of solenoid EBS on closure of contacts SC3, FIC2. In emergency, change-over contacts CO1, CO2, C03, CO4 are actuated to connect potentiometer SPR directly to transductors TIA, TIB, TIC. Specification 662,249 is referred to.