Patent GB-1434723-A (GB)
Title: Hoist control system
IPC: H02P25/26
Filed: 1972-04-04
Abstract: 1434723 Control of A.C. motors SQUARE D CO 4 April 1973 [4 April 1972] 15528/72 Heading H2J A hoist control system for overhauling loads, using a polyphase wound rotor induction motor having a primary and a secondary, comprises switching means for connecting and disconnecting the primary to and from a source of power, a star-connected resistor bank connected to the secondary and having a star point, a first electromagnetic contactor having contacts which are closed when the contactor is unenergized arranged to at least partially complete the star point of the resistor bank when closed, and a voltage sensitive relay having its operating means connected to the secondary and responsive to the voltage of the secondary for controlling the operation of the contactor. In the described embodiment, the motor IM drives a crane hoist and has its secondary connected to a resistor bank RB controlled by electromagnetic contactors 1A ... 7A whose operating coils are controlled by a master switch MS having contacts a ... p, the control system further including relays R1, R2, R3, SR responsive to the voltage of the secondary through a rectifier D1 and calibrating resistor CR, electromagnetic contactors H, L, M controlling the motor primary, an electromagnetically-controlled friction brake B and an undervoltage relay UV, the usual overload relays, over-speed switch and over-hoist limit switch being present but not shown. Contactors 1A, 2A, 3A have spring- closed main contacts which are controlled by coils energized through the normally-closed contacts of relays R1, R2, R3 energized in succession by the increasing secondary voltage to de-energize these coils. When power is first applied to the motor, the relay UV is energized and held so for a predetermined time to allow relay SR to pick-up. If this does not occur, due to failure of the supply to R1, R2, R3, the relay UV is de-energized to switch off the motor and apply brake B. In a typical lowering operation, movement of the switch MS to any of its first four positions causes energization of the contactors H, BR, M, successive operation of the relays R1, R2, R3 at each position according to the value of the load and the calibrating resistor CR, and the consequent cycling of the contactors 1A, 2A, 3A to open and close the star point and vary the resistance of the bank. Movement to the fifth position opens contactor H, closes contactor L to apply lowering power to the motor and closes contactor 4A, incidental closure of contactors 1A, 2A, 3A also occurring. Contactor 5A then closes after a predetermined time delay. Movement from the fifth to the OFF position changes over contactors H, L to effect electrical braking after a time delay, applies the friction brake B and maintains contactors 1A, 2A, 3A closed. Contactor H is de-energized by energization of the contactor 2A when the secondary voltage has fallen to the drop-out value of the relay R2, back-up being provided by a normally-closed auxiliary contact of contactor 1A in the event of mal-operation of the contactor 2A. Another (normally-closed) auxiliary contact is associated with contactor 4A to prevent a mal-operation of the contactor L, when switch MS is in the fifth position, producing sudden electrical braking. When hoisting in the first position of the switch MS, the operation is generally similar to that for the first four lowering positions with the relays R1, R2, R3 operating in reverse order to cause resistance to be progressively inserted into the secondary. Movement to the remaining hoist positions successively energizes the contactors 4A, 5A, 6A and 7A with predetermined time delays.