Patent GB-1409601-A (GB)
Title: Adjustable voltage thyristor-controlled hoist control for a dc motor
IPC: H02P7/28
Filed: 1972-09-04
Abstract: 1409601 Control of D.C. motors SQUARE D CO 4 Sept 1972 [17 Sept 1971] 40880/72 Heading H2J A control system for a D.C. series motor 11A, 11F used to hoist or lower a load 14 comprises an A.C.-D.C. converter 44 which supplies the armature and field during hoisting, an A.C.- D.C. converter 49 which supplies the field during lowering with the converter 44 supplying the armature, a teaser field resistor 37 which is automatically disconnected when the load being hoisted is sufficient to prevent overspeeding of the motor, the disconnection being dependent on the motor current, a master switch 58 of known type whose contacts 67, 69 ... 74 control the winding 75w of an undervoltage relay 75, the winding 41w of a dynamic lowering contactor 41, the winding 21w of a first hoisting contactor 21, the winding 39w of a second hoisting contactor 39, the winding 36w of a teaser field contactor 36, the winding 28w of a lowering contactor 28, the winding 32w of a dynamic braking contactor 32, and the winding 24w of a main contactor 24, a signal converter 79 supplied by differential transformers 62, 64 which are controlled by the switch 58 to produce a converter D.C. output of adjustable magnitude and selected polarity according to the hoisting or lowering setting of the switch, a reference control circuit 81 which receives the D.C. output and controls the firing circuits 88, 91 of the converters 44, 49 of conventional thyristor type, a load sensing module 82 receiving the output of circuit 81, and current sensing resistors 26, 52 which respectively supply the module 82 and the circuit 81, the resistor 26 supplying the current signal determining the disconnection of the teaser field resistor 37 and the resistor 52 providing a feedback signal during energization of the field 11F by the converter 49. Also provided are an overload relay 46, a limit switch relay 30, a braking resistor 31, an isolation resistor 22, a spring- urged, electromagnetically-released brake 16, an overload relay 25 and an optional over-hoist limit switch 15 having contacts 15a ... 15d and being associated with a resistor 29. The circuits 81, 82 are described, Figs. 2 and 3 (not shown), and comprise amplifiers, resistors, diodes, potentiometers, switching transistors and capacitors, the circuit 81 also being controlled by contacts of members 28, 30, 32 and the module 82 controlling the energization of first and second load sensing relays (76, 77) whose contacts 76a, 77a respectively control the teaser field contactor 36 and the braking contactor 32. Control during hoisting.-The speed of the motor is determined by the setting of the master switch 58 and the resistor 37 is connected or disconnected according to whether the load being hoisted is less than or greater than a predetermined value. Typical operations of the module 82 are described in which the first load sensing relay (76) is energized after a time delay, is maintained energized upon decrease of motor current following disconnection of resistor 37 and is de-energized when the load decreases, for example, upon load release or breakage of the cable. Control during lowering.-The speed of the motor is determined by the setting of the master switch 58 which simultaneously controls the converters 44, 49. The value of isolation resistor 22 is high enough to permit the separate control of the armature 11A and field 11F by converters 44, 49, but maintains a connection between 11A, 11F to provide a dynamic lowering loop 11A, 15d, 22, 11F, 40 and 41a which produces emergency dynamic braking upon power failure. The braking resistor 31 is connected during slow lowering of heavy loads, but is disconnected to obtain high speed lowering leaving only the dynamic lowering loop in circuit. To prevent a change in speed consequent upon disconnection of the resistor 31, the values of resistors 22, 31 are suitably chosen, the resistor 31 is disconnected at a predetermined value of motor current and the outputs of the converters 44, 49 are adjusted to compensate for the change of speed that would otherwise occur. The current sensing resistor 26 again provides the signal to module 82 to cause energization of the second load sensing relay (77) at a predetermined current value. A typical operation of circuits 81, 82 is described, in which the resistor 31 is disconnected and the outputs of converters 44, 49 are adjusted to respectively decrease the armature current and increase the field current, all three operations occurring simultaneously. Resistors 22, 31 are chosen to ensure that the motor speed ranges with and without the resistor 31 in circuit overlap regardless of the motor load. Operation for various loads is also discussed, Figs. 4 to 9 (not shown), including the possibility of variation of only the armature current to prevent speed change. The Specification also describes disconnection of the resistor 31 without compensatory control of converters 44, 49, operation of the limit switch 15 during hoisting and the subsequent lowering with switch 15 operated, during which the resistor 31 cannot be disconnected and the motor can only be operated in its low speed range regardless of the output of the converter 79. If the converter 44 is provided with a current limiting circuit, its sensing resistor may replace the resistor 26 and may provide an output which is combined with that of the resistor 52 to supply the module 82.