Patent GB-666497-A (GB)
Title: Improvements in speed controlling means for alternating current motors
IPC: B66C13/26
Filed: 1948-10-18
Abstract: 666,497. Control of A.C. motors. HARNISCHFEGER CORPORATION. Oct. 18, 1948. [Oct. 30, 1947] No. 27008/48. Class 38 (iii). [Also in Group XXXV] An alternating-current motor speed-controlling means includes an eddy-current torque resisting means mounted in driven relationship to the shaft of the motor and having windings effective, on excitation, to give rise to eddy currents which create a' magnetic field which resists rotation of the motor, and is characterized by having (a) in circuit with the motor, adjustable impedance means subject to selective adjustment in pre-determined amounts varying from larger to smaller impedance to vary the amount of power accepted by the motor, and (b) in circuit with the winding of the eddycurrent torque resisting means, adjustable excitation current regulating means subject to adjustment in pre-determined amounts varying from a larger to a smaller excitation of said winding, and (c) other means correlating the position of adjustment of said impedance means and said current regulating means so that for each pre-determined amount of impedance in circuit with the motor there is provided for the winding of the eddy-current torque-resisting means a pre-determined substantially constant excitation current which is not subject to any substantial variation by reason of variation in the speed of the motor. Fig. 1 shows an induction motor 1 on the shaft of which is mounted a generator 3 and an eddy-current brake 4 having an exciting winding 5. A friction brake 6 having operating winding 7 is provided, the motor driving a hoist drum 8. A rotor resistance 12, 13, 14, has taps connected to a drum controller 15 in the "off" position of which the coil 5 is moderately excited through transformer 38 and rectifier 39, resistors 58, 53, being placed in series with the coil. When the controller is moved to the first hoisting-position the excitation of the eddycurrent brake is increased to a maximum, a relay 33 being closed simultaneously to energize a hoisting contactor 20. The secondary circuit of the motor is completed in the first hoisting- position of the controller, the rotor resistance being reduced by movement to the higher notches. In the third hoisting-position the excitation of the coil 5 is confined to the circuit through resistor 58 and is thereby reduced. Movement to the fourth position introduces a resistor 83 to reduce the excitation of the eddy-current brake further. Thereafter, energization of the coil 5 is terminated and the relay 33 is de-energized, the contactor 20 being maintained closed through contacts on the controller. Movement of the controller to the first lowering-position furnishes maximum excitation for the coil 5, the lowering contactor 21 being energized. The motor exerts no lowering torque in this position of the controller, however, since the rotor circuit has not been completed. The load descends under the control of the eddy-current member, the brake 6 being released when contactor 21 closes. In succeeding lowering-positions the rotor resistance is connected and then reduced, the rotor exerting an increasing torque in the downward direction for higher covering speeds. In the seventh lowering-position of the controller the excitation of coil 5 is diminished, and in the eighth position it is terminated, contacts on the controller maintaining the contactor 21 energized when relay 33 opens. A standby source of excitation for the winding 5, such as the generator 3, is provided in an emergency. When there is no power available from source 16, 17, 18, contacts 109, 111, and 113 are closed to connect generator 3 across the coil of a power-failure relay 45. Failure of the brake 6 may cause the load to descend and so drive the generator. Relay 45 closes to connect the generator to the winding 5, the eddy-current brake preventing excessive lowering-speed. A battery, which is re-charged through rectifier 39, may be substituted for the generator 3 as a standby source. The generator may supply the eddy-current brake during normal operation, the rectifier being dispensed with. In the modification of Fig. 2 (not shown), a motor controller is provided to regulate electro-magnetic contactors which handle all the load currents. Figs. 3 and 4 (not shown), illustrate the construction of a suitable eddycurrent brake (see Group XXXV).