Patent GB-722421-A (GB)
Title: Electrical apparatus for controlling equipment in dependence on the displacement of a part thereof
Inventor: KELSALL RICHARD HOPE
IPC: B23Q15/00
Filed: 1951-09-04
Abstract: 722,421. Control of A.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. May 27, 1952 [Sept. 4, 1951], No. 20865/51. Class 38 (3). Relates to means for operating a relay when the time integral of the voltage of a tachometer generator reaches a predetermined value so that electric motor or like apparatus driving the tachometer generator can be made to operate for a fixed distance. As shown, a three-phase induction motor M drives a tachometer generator TG, a portion of the output voltage of which is applied from potentiometer P1, P2 to the control grid of a pentode valve V1 forming part of an integrating circuit of the Miller type. Energization of an initiating relay closes contacts IC/2 in a D.C. control circuit for a main contactor F whereby the motor M is started. Contacts IC/1 also close to apply a potential to the screen grid of the valve through a relay RL so that the valve at first passes a small anode current which increases due to the speed-responsive voltage derived from the generator TG. The anode potential of the valve V1 falls proportionally to the integral of the voltage derived from the tachometer generator and, when this potential reaches a given low value, the screen grid current rises rapidly to a value sufficient to actuate the relay RL. The contacts RL/1 then open to de-energize the contactor F which disconnects the motor from its A.C. supply and applies D.C. braking. The angular displacement through which the motor has rotated by the time that the screen grid current makes its rapid increase is thus controlled by the setting of the resistors P1, P2, the potentiometer P1 being calibrated linearly in terms of the angular displacement of the motor, whilst the potentiometer P2 sets the ratio of maximum to minimum displacement. The integrating valve and its associated 'relay may be duplicated so as td provide protection against failure of the control circuit. Thus, the valve VI' and relay RL' may be added, the latter having contacts RL'/l connected in series with the contacts RL/1 in the control circuit for the contactor F so that the motor will stop in response to operation of either of the relays RL, RL'. A no-volt relay NVR is de-energized to stop the motor if the relays RL, RL' do not operate closely in step, a capacitor C2 allowing a small time delay in operation of the no-volt relay to allow for slight differences between the two integrating circuits. When the relays RL, RL' operate to stop the motor the screen grid voltages of the valves V1, V1<SP>1</SP> are removed and hold-in circuits for the respective relays are completed. A potentiometer P4 connected across high-tension supply lines and controlling the anode potentials of the valves V1, V1<SP>1</SP> may be used for pie-setting the displacement of the motor. The tachometer generator may have' a field winding energized from the high-tension supply so that variation of the supply voltage changes both the voltage applied to the valves V1, V1<SP>1</SP> and the output of the tachometer generator in such a manner that the operation is not appreciably affected by such variation. Specifications 665,380 and 693,447 also are referred to.