Patent GB-438690-A (GB)
Title: Improvements in and relating to speed ratio indicating devices
IPC: G01P3/58
Filed: 1935-03-28
Abstract: 438,690. Speed indicators. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London. March 28, 1935, No. 9702. Convention date, March 28, 1934. [Class 106 (iii)] [See also Group XXXVIII] A differential voltmeter device for measuring the ratio of two speeds by indicating the ratio of voltages proportional to these speeds is provided with means for ensuring accurate indications with varying speeds. Means are provided for simultaneously varying the speeds of the two shafts, whilst maintaining constant their speed ratio, and for simultaneously actuating the speed-changing device and the means for maintaining constant the voltage input to the instrument. The device is shown applied to a reversible cold strip steel rolling mill to provide an indication of the " draft " taken, that is to say, the reduction of the strip effected during each pass. Automatic control of the screw-down device of the mill may be provided to regulate the percentage " draft taken by the mill rolls. The strip 10 passes from a reel 12 over a cooling drum 13 to the mill rolls 11, and proceeds with reduced thickness over a cooling drum 14 to a reel 15. Motors 16, 17, 18 supplied from a generator 20 drive the rolls 11 and the reels, 12 15. The field winding 23 of the generator is separately excited from a direct current source 24 and in its circuit is included a variable resistance 25 for varying the voltage of the generator and the speeds of the motors. For measuring the supply and delivery speeds of the strip,10 the drums 14, 13 are made to drive tachometer generators 33, 34 comprising preferably direct current machines with direct current field windings 35, 36. The voltages generated are supplied to coils 38, 40 so as to cause a needle 41 to take up a position indicating the ratio of such voltages, and therefore of the speeds of the drums 14, 13. As the voltages of the generators 33, 34 decrease, due to decrease in mill speed, a contact arm 45a, which is moved simultaneously with, but in an opposite direction to, the contact arm 25a, decreases the resistance of the field circuit of the tachometer generators 33, 34, thus increasing the excitation by an amount proportional to the decrease in speed and maintaining constant the voltages supplied to the coils 38, 40, so that the deflection of the needle 41 remains as before. A differential voltmeter 46 provided with a scale calibrated to read the difference between the delivery and supply speeds of the strip. has two coils 47, 48 energized from the same generators 33, 34 and a needle 50 provided with a spring 51 biassing it to a central position. In the circuit of the coil 48 is a rheostat 52 having a contact arm 52a movable on a scale calibrated in percentage " draft " taken by the rolls, zero deflection of the needle 50 indicating that the rolls are operating as desired. The voltmeter 46 may be provided with contacts 53, 54 in the energizing circuits of reversing contactors 55, 56 which control the energization of the motor 57 for operating the screw-down in the up or down direction, to decrease or increase the " draft " when the needle 50 is deflected to bridge the contacts. A switch 27 is operated to reverse the direction of operation of the mill. In a modification, the field windings of the tachometer generators do not have a compensating rheostat but are directly connected to a source of direct current, and rheostats are included in the circuits of the voltmeter coils to ensure that the voltmeter operates with the same degree of accuracy over the entire range of speed of the mill.