657,096. Automatic speed control systems. IGRANIC ELECTRIC CO., Ltd. Dec. 23, 1947, No. 33945. Convention date, Dec. 23, 1946. [Class 38 (iv)] [Also in Group XXXVII] A control system for a plurality of similar polyphase induction motors to drive units, e.g. printing presses &c. comprises interconnection between some or all of the motors to restrict them to operate as a group, control means to operate the grouped motors at a required speed, means driven by certain of the grouped motors to produce a voltage variable with the speed thereof and means to effect unbalance of the motor primaries in accordance with the variable voltage to limit the motor speed for slow-speed control of the motors. Fig. 2 shows printing unit motors M1, M2, M3, M4 and folder motors M5, M6. All the motors are axially aligned and coupled through clutches to permit optional grouping of the motors M1, M2, M3, M4 with motors M5, M6 respectively. Each motor has its own control panel which includes two triple-pole relay switches, one to connect lines L1, L2, L3 to the primary windings of its respective motor and the other serves to short circuit the braking rotor resistances r4, r5, r6 of its respective motor. A cross-head type speed regulator 20 is also incorporated in each control panel. The regulator 20 alters its respective rotor resistances r1, r2, r3. The control panels C5, C6 for the folder motors M5, M6 respectively comprise, in addition, an electronic equipment and relay switches 32, 33 of the double-throw type associated with ignition tubes 30, 31. Each speed regulator 20 is geared to a section of a mechanically coupled sectionalized shaft driven by two pilot motors 21, 22, Fig. 1 (not shown), each being controlled by a relay panel. Double-pole switches 50 to 56 provide for different groupings of the driving motors. With switches 50, 56 closed, conductors 60, 61 are gridged by the normally closed contacts of relays 32, 33 of the folder motor panels C5, C6. If the relays 32, 33 are de-energized when the motors are running, the supply line L3 is excluded and the conductors 60, 61 are joined together resulting in providing a single phase supply to the motors which is utilized for braking under which conditions the rotor resistances r4, r5, r6 are now inserted by opening of the switches 28. Fig. 3 shows one of the electronic equipments which comprise two ignition type tubes 30, 31 interconnected to pass alternatively both half waves of an A.C. These tubes have their ignition electrode 30a, 31a under the influence of thyratron tubes 70, 71. The cathode of the tube 31 is connected to one terminal of the primary winding of the M5 through the switches 50, 27. The cathodes and grids of the thyratrons 70, 71 are connected to the two secondary windings of a transformer T, the primary winding of which is connected across a middle tapping of an impedance 76 and a tapping of a circuit comprising a resistance in series with a winding 77 of a saturable reactor 78. This circuit is connected across lines L1, L3 in parallel with the impedance 76. Tubes 81, 82, 85 serve to control the current flowing in winding 79 of the reactor 78. The grid of the tube 85 is connected to the negative pole of a tachometer generator 87 mechanically coupled to the rotor of motor M5. With the control illustrated in Fig. 4, set to afford to the motors of a group a slow speed under the control of the electronic equipment, the required slow speed is achieved by the effect of the tachometer generator 87 which decreases the period of conduction of the tubes 30, 31 with the result that the effective current supplied to one terminal of the primary windings of each of the motors grouped with the motor M5 is similarly decreased. The negative potential applied to the tube 85 decreases its conduction with the result that the conduction of the tubes 82, 81 is increased in the former and decreased in the latter. Consequently, the current in the saturating winding 79 of the reactor 78 is decreased and so is the current in the winding 77 which causes an increase of the retardation of the phase angle of the voltage in the windings of the transformer T with respect to the voltage of the lines L1, L2, L3 and thereby retards the moment of ignition during the respective halfcycles of the tubes 70, 71 and this through the tubes 30, 31. When the contact of the potentiometer 86 is properly adjusted the periods of conduction of these tubes may be so restricted as to hold the motors to a very low speed. Selective predetermined slow speed may be provided in various ways as, for example through the medium of an additional potentiometer 89. The relay switch 88 introduces potentiometers 86, 89 according to its position. Specification 618,515 is referred to.