Patent GB-625573-A (GB)
Title: Improvements in or relating to the control of electric motors
IPC: H02P25/32
Filed: 1946-06-05
Abstract: 625,573. Automatic speed control systems. IGRANIC ELECTRIC CO., Ltd. June 5, 1946, No. 17077. Convention date, June 21, 1945. [Class 38 (iv)] [Also in Group XXXVII] A control system for a polyphase electric motor for a hoist or the like comprises space discharge means, having a control element for varying the effective impedance thereof, interposed between the polyphase source and one of the terminals of the motor, means to impress upon said control element a potential variable in response to speed variations of said motor, and means to reverse the sense of said response to thereby control the current passing through said space discharge means, to afford, through potential regulation of a single terminal of said motor, motor speed control extending between approximately 90 and 250 per cent slip. An induction motor 10 is adapted to be energized from the threephase lines L1, L2 and L3, which when energized, close a no-volt relay 38, having a self-holding contact 38b, through contacts of a master controller M. At the first hoisting position of the master controller switches 13 and 11 are closed and two motor primary winding terminals are directly connected to the supply whilst the third is connected to the line L3 through thermionic valves 14 and 15 which are connected in inverse-parallel. The motor primary winding is thus energized with unbalanced polyphase current,. the degree of unbalance being in accordance with the conduction of the valves 14 and 15. A solenoidoperated brake is removed, a relay 20 is closed to short-circuit a portion of the motor secondary resistance, and a relay 54 is closed. The gridcathode circuits of the valves 14 and 15 respectively comprise the secondary windings 39a and 39b of a transformer 39. A phaseshifting network comprises the transformer secondary winding 39c an inductance 40 which is connected across two supply lines, a resistor 41, and a coil 42a of a saturable reactor 42, so that a variation of current in the reactor saturating winding 42b effects a change of phase-displacement between the grid and cathode voltages of the valves 14 and 15, the arrangement being such that when there is no current in the saturating winding the phasedisplacement is at a maximum and the valves are blocked. A variation of motor speed effects a change of rectified output of a tachometer generator 43 which is applied to the grid of a valve 45 in opposition to the voltage across a resistor 56. A variation of the current passed by the valve 45 effects an alteration in that passed by a valve 50, in the opposite sense, and that passed by a valve 53, in the same sense as the valve 45, thereby varying the saturating current of the reactor 42 and affording a low constant hoisting speed with the motor load varying between zero and a maximum determined by the amount of rotor resistance. The grid-cathode circuit of the valve 50 comprises an adjustable resistor 51 which may be set to vary the sensitivity of response to control by the tachometer generator. In the second hoisting position of the master controller the switch 13 is de-energized and a switch 12 is energized to short-circuit the valves 14 and 15 and disconnect them from the line. The rotor resistance is re-inserted and the relay 54 deenergized. In successive positions of the master controller the rotor resistance is gradually removed and is finally short-circuited. When the master controller is moved in the lowering direction the switches 11 and 13 are energized and the switch 12 is de-energized. The switch 20 is de-energized for all lowering positions. The motor primary winding is supplied with unbalanced voltage regulated to provide substantially constant speed as before-described. When the motor controller is further moved in the lowering direction a contact 34 is moved across a resistor 55 to increase the positive bias of the valve 45. The relay 54 riot being energized, the voltage of the anode of the valve 45 is impressed on the grid of the valve 53 so that an increase of current through the valve 45 causes a decrease of current passed by the valve 53 and increased speed of the motor. The motor lowering speed is thus increased by passage of the contact 34 across the resistor 55, the motor speed being held constant, within certain load values, at each setting'of the controller. Means may be provided for commutating the resistor 55 for hoisting, as it is for lowering. In a modification, single-phase excitation of the motor primary winding is provided at the third lowering position of the master controller, either of the motor primary terminals which are supplied from the lines L1 or L2 being connected to that supplied from the line L3 through a resistor and the valves 14 and 15. Specification 618,515 is referred to.