Patent GB-644281-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: H02P7/292
Filed: 1945-01-30
Abstract: 644,281. Automatic control systems for electric motors. BRITISH THOMSONHOUSTON CO., Ltd. Jan. 30, 1945, No. 2368. Convention date, Feb. 1, 1944. [Class 38 (iv)] [Also in Group XXXVII] Control apparatus for an electric motor comprises an electric valve circuit for supplying a rectified voltage to the armature of the motor, a reversing switch for connecting the armature to the valve circuit for rotation in either direction, a stop control device, means responsive to operation of the stop control device for controlling the switch to reverse the connections of the armature to the valve circuit to provide for inverter operation of the valve apparatus to brake the motor to rest, and means responsive to a pre-determined low speed of the motor for disconnecting the motor from the valve apparatus. The Figures show the application to control of a motor as follows : (1) the motor, when started by pressing button 39 or 40, runs up to speed under automatic current control and then continues under automatic speed control ; (2) the motor, when reversed by pressing the other button, is braked to a standstill by 16 inverter action of valves 15 and 16 and then accelerated in the reverse direction, and (3) the motor is stopped by pressing button 41 and is again braked. by inverter action but is automatically stopped before it can accelerate in the reverse direction. The grids of mercury-vapour valves 15-16 and mercury-vapour valves 13-14 are supplied respectively through transformers 37 and 38, the phase of the grid potentials being controlled by saturable reactors 34 and 36 respectively. Any difference between a part (selected by speed-controlling potentiometer 18) of the fixed reference voltage across glow tube 48 (connected in series with glow tube 47 across the rectified output of transformer 42) and a part (selected by potentiometer ,52b of the voltage across motor armature 10a is applied to the grid of valve 50. On an increase in motor speed and armature voltage, valve 50, controlling the grid of valve 49, becomes more conductive, valve 49 becomes less conductive and the current in D.C. saturating coil 34b of reactor 34 is decreased, thereby decreasing the output voltage of valves 15 and 16 and decreasing motor speed. Similarly any difference between a part of the reference voltage (selected by field potentiometer 19) and the voltage across field winding 10b is applied to the grid of valve 55 controlling the grid of valve 54 which in turn controls the current through saturating winding 36b of reactor 36 to control the voltage applied by valves 13-14 to field winding 10b and thus keep this voltage substantially constant. Potentiometers 18 and 19 are mechanically coupled to turn in the same direction, each one being effective over the range in which the other is ineffective. To limit the armature current, any difference between the reference voltage and a part (selected by potentiometer 60). of the rectified output voltage of the current transformer 27 in the armature circuit, is applied to the grids of control valves 57 and 58. Valve 57 has the same effect as valve 50, so that on excessive current the armature voltage is decreased. Valve 58, makes valve 54, by grid control, more conductive on excess motor current, thereby increasing the current in field winding 10b. The difference between the reference voltage and a part of the armature voltage (across resistances 53 and 66) is applied to the grid of valve 64, when regulators 18 and 19 are moved to increase motor excitation, valve 64, responding to increased armature voltage, acts, through valve 54 on valves 13-14 to prevent considerable increase in, current in field 10b until motor speed has decreased. When switches 30-31 are both open, contacts 30a-31a connect the grid of control valve 57 to the junction of resistors 67-68 connected between one side of the motor armature and one terminal of the rectified output of transformer 42 ; this, by retarding the phase of the grid voltage of valves 15-16, limits the initial armature current on reclosure of one of switches 30-31. At the same time contacts 30d-31d connect braking resistance 79 across the motor armature. Also the contacts 30b-31b connect across the rectified output of transformer 42 the coil of time-lag relay 70 which delays reclosure of switches 30-31. Valve 71 between cathode and grid of valve 57 prevents this grid from becoming excessively negative at light motor loads. Relay 72 when de-energized, closes its contacts 72a to connect the full reference voltage to valve 55 to maintain full field strength. Valve 73, having its cathode connected through resistance 75 to one side and its grid to the other side of the armature is conductive only when, following a reversal by switches 30-31, the motor armature voltage is the reverse of that corresponding to the switch closed and valves 15-16 are acting as inverters ; valve 73 then controls valve 54 to limit the motor excitation. A connection from braking resistance 79 through double diode 78 to the grid of valve 64 prevents the field strength from increasing in response to movement of field regulator 19 while switches 30 and 31 are open. When the supply through transformer 12 is switched on relay 25 closes after a time delay. Field winding 10b is fully energized and, in series therewith, relay 26 which energizes stop relay 85 which maintains itself. Relay 70 is energized from transformer 42. Forward push 39 energizes switch 30. The motor accelerates, under armature current control, until it is up to speed and then it comes under the automatic speed control. Relay 72, energized by contacts 30h, maintains itself and contactor 30 ; relay 70 drops. At a predetermined low armature voltage, relay 88 lifts. If reverse push 40 is then pressed, switch 30 is de-energized and, after time-delay relay 70 has re-closed, switch 31 is energized. Relay 72 remains energized, and excitation is not temporarily strengthened. The motor is braked by inverter action and then accelerated in the reverse direction. Stop push 41 de-energizes relay 85 ; relay 86 or 89 is energized-according to the direction of rotation of the motor- and this de-energizes switch 30 or 31. Reclosure of relay 70 energizes the other reversing switch through contacts on relay 86 or 89. Relay contacts 86e being closed, relay 90 lifts, maintains itself (before relay 70 re-opens) and effects a zero-speed setting of the circuit of valve 50. The motor is braked by inverter action ; should it tend to reverse, the speed control will tend to maintain zero speed, but at low armature voltage relay 88 drops, deenergizes the closed reversing switch and restores normal stand-still conditions. Relay 26 in response to field interruption or thermal relay 32 in response to overload has the same effect as pressing the stop push.