GB1335373A - Tower cranes - Google Patents

Abstract

1335373 Control of A.C. motors RICHIER 3 Nov 1970 [3 Nov 1969] 52327/70 Heading H2J The jib of a tower crane is rotated by a wound rotor induction motor 11, Fig. 2, working against an eddy current brake 13 to produce the torque speed curve DEFGHIJKLM, Fig. 3. The motor is supplied via reversing contactors 21, and is controlled from 4-position speed controller 19, which via speed detector 22 sensing rotor voltage controls device 23 which alters the motor torque-speed characteristic by inserting resistors or a thyristor circuit, in its rotor circuit. For motoring notches 1-4 of controller 19, the excitation voltage of the brake 13 is controlled via voltage control contactors 24 and adjustable variable voltage regulator 25 containing thyristors, and for notch 0 via adjustable regulator 28 comprising a network of resistors and capacitors which provides a continuous rate of change of brake excitation voltage, of ramp form to give the braking characteristic C. Actuation of a mechanical locking brake (14), Fig. 1 (not shown) disconnects the excitation from brake (13). Notching up.-For notches 1-4 respectively, in either direction the torque speed characteristic DEFGHIJKLM is obtained as follows: Motor characteristic 1 is combined with brake characteristic A to give operating characteristic 1a; Motor characteristic 1 is combined with brake characteristic B to give operating characteristic 1b; Motor characteristic 1 alone; Motor characteristics 2, 3 with transition from characteristic 2 to 3, performed automatically by the predetermined speed detector 22, or a time delay relay, which removes resistors or thyristors from the rotor circuit. For fast notching up, the brake excitation does not take place, giving characteristic DIJKLM. Notching down reverses the above sequence except that, upon notching from 4 to 3, the motor characteristic changes directly from 3 to 1. Notch 0 gives a ramp excitation of brake 13 via regulator 28 with no motor supply. The mounting of the system in the tower crane is displayed in Fig. 1 (not shown). More than three motor operating characteristics may be used.