Patent GB-721084-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P25/26
Filed: 1953-02-10
Abstract: 721,084. Control of A.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Feb. 10, 1953 [March 21, 1952], No. 3674/53. Class 38 (3) An induction motor HM for driving a hoist is connected to saturable reactors MR, BR, adapted to impose a controlled phase unbalance on the polyphase motor terminal voltage. The control circuit for the saturable reactors comprises a source of voltage dependent upon the motor speed, represented by a motor-driven pilot generator PG, as well as a source of variable voltage I dependent upon the effective motor torque, so that the phase unbalance of the motor terminal voltage depends upon both the motor torque and speed. The main reactor winding 68 is connectable between supply terminal L2 and motor terminal 13, the balancing reactor winding 75 being connected, in series with a transformer secondary winding W2, between the motor terminals 13, 66, so that the potential at terminal L4 is in phase opposition to that at terminal L2. By varying the relative reactancevalues of the windings 68, 75, the electrical position of the terminal 13 with reference to terminals L4, L2, may be regulated throughout a wide range, a vernier rheostat VR allowing such adjustment to be made in fine steps. Thus, the excitation of the reactor control winding 29 is a function of the setting of the vernier rheostat VR, the output voltage of the device I, and the output of the pilot generator PG, an additional constant control voltage being provided for the winding by means of a transformer secondary winding W3 and rectifier R3. The torque-responsive voltage source I comprises a spring-biased crank arm 117 which is rotated about axis 118 in accordance with the load on the hook and is coupled to the rotor of the device I. The output at terminals 40, 41 is sensitive to the relative positions of the rotor and stator of the device, being zero when there is no load on the hook. When a master controller MS is moved to its first hoisting notch, leads L1, L3 are connected to motor terminals 62, 66, whilst the circuit to terminal 13 is completed through winding 68. The pilot generator PG is heavily excited so that it provides a relatively high voltage acting in opposition to the voltage derived from rectifier R3. In the second hoisting position of the controller, the excitation of the pilot generator is reduced, and, in the third position, the coil 68 is short-circuited so that the motor torque and speed are determined by time limit control of the rotor resistance sections 81 ... 86. When the controller is returned to its neutral position, the pilot generator PG is connected to motor terminals 13, 62 to effect dynamic braking, and the brake 73 sets to stop the load. In the event of power failure, rectifier 105 prevents de-energization of field winding 27 so that the conditions are generally the same as for stopping. For the lowering operation, power is applied to leads 60, 64 through contacts 59, 63, the motor terminal 13 being supplied through main reactor MR. The speed of lowering is increased by the progressive movement of the master controller through the lowering notches to decrease the field excitation of the generator PG in steps, and likewise to increase the resistance in circuit with the winding 29. When the controller reaches its fifth lowering position, the contactor H drops out and contactor L connects the motor terminals directly to the alternating-current leads. If an attempt is made to lower or hoist a load greater than the rated load, the load-sensitive device closes contacts 228 whereupon relay 2CR is energized and an alarm device 229 is actuated. The operation of relay 2CR causes the removal of resistor sections 84, 85, 86 and opens the contact 16. This provides a desirable speed torque characteristic.