Patent GB-628749-A (GB)
Title: Improvements in or relating to systems for the control of polyphase electric motors
IPC: H02P25/32
Filed: 1946-10-08
Abstract: 628,749. Automatic speed control systems. IGRANIC ELECTRIC CO., Ltd. Oct. 8, 1946, No. 29997. Convention date, Oct. 8, 1945. [Class 38 (iv)] [Also in Groups XXXVII and XXXVIII] A control system for a polyphase induction motor comprises means for adjusting the impedance of the secondary circuit of the motor for obtaining motor speed control at a given desired speed under balanced polyphase conditions of the primary of said motor, space discharge means for regulating the conductivity of the connection of one primary terminal of the motor to the supply circuit, and means for regulating said space discharge means to afford varying unbalanced conditions of the primary of said motor, through a range which may extend from substantially balanced polyphase conditions to a single phase condition, to maintain said given speed of said motor, despite the tendency of the motor, upon load variations within a given range, to depart from said desired speed. The motor is started by closing a switch 11, gas-filled valves 12 and 13 being initially fully conducting so that the motor primary circuit is substantially balanced. A cross-head 25 is at the extreme right-hand position, the whole of the motor secondary resistance being in circuit. A resistor 36 is connected, in series with a resistor 28, across the direct-current bus-bars 31, and 32 and, since a positive potential is applied to the grid of a valve 34 through a contact 36a of the resistor 36, the said valve passes current so that valves 41 and 48 are rendered non-conductive and conductive respectively. A relay 30 is closed, its coil being in the anode circuit of the valve 48, to energize a split phase pilot motor 27 in such direction as to move the member 25 to increase the motor speed. The reduction of the resistor 28, caused by the movement of the contact 28a on the member 25, reduces the potential of the gird of the valve 34 which is so affected, together with the valves 41 and 48, that the current in the coil of the relay 30 is decreased. When the contact 28a reaches a position corresponding to the position of the speed-setting contact 36a, the relay 30 is opened. A positive potential derived from a contact 46a, which is ganged with the contact 36a, is applied to the grid of a valve 50 and is opposed by a negative potential derived from a tachometer generator 45. At starting, the voltage of the generator 45 being zero, the valve 50 conducts to render the valves 53 and 57 non-conducting and conducting respectively. A saturable choke winding 17b, which is in the anode circuit of the valve 57, is energized and the impedance of the choke windings 17a are at a minimum, so that the voltages of the primary and secondary windings 15a, 15b and 15c are in phase with the anode voltages of the valves 12 and 13 which are thereby rendered fully conductive. The current supplied to the motor through valves 12 and 13 may be supplemented by that of a resistor 61. If the motor torque is less than that at full load corresponding to the setting of potentiometers 36 and 46, increase of motor speed increases the voltage of the generator 45 to reduce the current passed by the valves 12 and 13. Subsequent decreases of speed provides reverse operation, the correcting action being effective until the main valves are fully conductive. To reduce the motor speed, the contacts 36a and 46a are moved downwardly to reduce the conductivity of the valve 34 and increase that of a valve 47 to close a relay 29, thereby so energizing the motor 27 that the main motor secondary resistance is increased. Movement of contact 28a to the right eventually so increases the current passed by the valve 34 that the relay 29 is opened, when the contacts 28a and 36a are in correspondence. The primary current through the valves 12 and 13 will be less than maximum during deceleration the voltage of the generator 45 exceeding that provided through the contact 46a. Specification 618,515 is referred to.