Patent GB-684426-A (GB)
Title: Improvements relating to electric motor control systems
Inventor: ASBURY ALLAN
IPC: B23D36/00
Filed: 1950-05-24
Abstract: 684,426. Control of D.C. motors. ENGLISH ELECTRIC CO. Ltd. May 24, 1951 [May 24, 1950; March 20, 1951], Nos. 13013/50 and 6653/51. Class 38 (iii) [Also in Groups XXXV and XL (b)] A voltage shear is driven by a motor M supplied with variable voltage from a constant speed generator G having its field winding energized from exciter E1 which is excited from an electronic amplifier 10. The input circuit 10a of the amplifier is supplied from two tachometer generators TG<SP>1</SP>, TG<SP>2</SP>, producing opposing outputs depending on the speed of the flying shear and that of the billet as it leaves the mill stand, respectively. The input circuit 10b is supplied in accordance with the voltage-drop across resistor 11, and, in combination with input circuit 10a, functions to control the rate of acceleration and deceleration of the motor M in accordance with the square of the billet speed. For this purpose, resistor 11 is connected, through rectifiers 12, 13, and resistor 14, across resistor 16 which carried the armature current of the motor M. Resistor 14 is supplied from exciter E.2 driven from the mill stand and excited from generator TG<SP>2</SP>. A limit switch 17 is provided, together with mechanism 18 which counts the impulses from photo-electric cell 19 associated with toothed wheel 19a, driven by the mill stand, and lamp 19b. Actuation of start button 21 energizes relays 22, 23, so that contacts 23a open to disconnect the device 18. Contacts 22b connect generators TG<SP>1</SP> TG<SP>2</SP>, to the input circuit 10a and the generator G is excited to start the shear motor. If the load current tends to exceed a value determined by the setting of a tap of the resistor 14, rectifier 12 passes current through resistor 11 and the excitation applied to exciter E1 due to the unbalance between the voltages of generators TG<SP>1</SP>, TG<SP>2</SP>, is limited, thus holding the motor accelerating current at the appropriate constant value for the desired rate of acceleration. When the motor M has accelerated so that the speed of the knives corresponds substantially to the linear speed of the billet the input circuit 10b becomes ineffective and the amplifier 10 operates to maintain constant cutting speed of the shear motor. When the shear knives are clear of the billet, the limit switch opens contacts 17a to de-energize relays 22, 23. Contacts 22b revert to the position shown and the motor M decelerates to standstill at a rate controlled by the input circuit 10b in a manner similar to that obtaining during acceleration. Closure of contacts 23a connects the device 18 to cell 19 and, after a pre-determined number of impulses have been counted, relay 24 is energized to re-initiate acceleration of the motor M. The limit switch 17 may be replaced by a single-toothed wheel directly coupled to the crank operating the shear knives and arranged to intercept a beam of light directed on to a photo-cell during the period in which it is desired to open the contacts 17a. In a modification, Fig. 3 (not shown), interruption of light falling on a photocell by the front end of the billet as it approaches the flying shear causes acceleration of the motor from its pre-determined rest position, the photo-cell being positioned so that cut is made at the desired distance from the front end of the billet. When the front end of the cropped billet interrupts the light falling on a second photo-cell, the flying shear is again accelerated, the position'of the cut being determined by that of the second cell. The sequence of operation is then repeated each time the front end of the billet interrupts the light falling upon the second photo-cell. The photo-cells and associated apparatus may be replaced by mechanical switches.