Patent GB-816456-A (GB)
Title: Improvements in or relating to electric motor control apparatus
IPC: H02P7/292
Filed: 1957-04-15
Abstract: 816,456. Automatic speed-control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. April 15, 1957 [April 27, 1956], No. 12249/57. Class 38(4). [Also in Group XXXVII] The armature of an electric motor driving a machine-tool is supplied from a Scott-connected transformer through a plurality of thyratrons each having a high-voltage A.C. control component upon which is superimposed a D.C. component, an error signal, representing the I.R.-drop across the armature. These components co-operate to shift the firing angles of the thyratrons and compensate for the changing I.R.-drop as the load on the motor changes. The error signal is amplified, the amplifier being of the D.C. type with negative feedback to prevent instability. A field winding of the motor is also supplied through discharge devices, one of which is controlled, and, to prevent commutation of the armature thyratrons from causing surges of field current through the common supply transformer, a filter network is provided which also serves as a phase-shift network between the anode and grid of the field rectifier. The motor is started by closing a selected push-button controlling a forward or reverse contactor, a relay being actuated so that contact 173 closes to connect the adjustable arm 205 of a speedsetting resistor to the grid G 14 of a double triode valve AT5. The control potential on grid G14 gradually increases from the value set by another resistor which was connected to the grid during stand-by operation, the rate of increase being determined by the discharge of a capacitor originally in circuit with the stand-by resistor, Fig. 2D (not shown). The current passed by the triode section A14-K14, and by the armature thyratrons, is thus incresed, and the motor rotates at a rate determined by the initial speed-setting. The motor is further accelerated by adjusting the arm 205 to increase the potential across resistor RO1 and the corresponding potential applied to the control circuit of the error signal amplifier. The resultant increase in armature current is modified in accordance with the voltage of a network N1 which is connected across the armature of the motor. The voltage-drop across resistor RO1 eventually reaches a constant magnitude due to saturation of the section A14-K14, increase of armature voltage is correspondingly prevented regardless of the setting of the arm 205. When the armature potential reaches a predetermined value, a relay 5CR is actuated to connect arm 205 to grid G13, the motor now being controlled by the triode-section A13-K13. The output resistor RO2 of this section is connected to control thyratron T6 in the field supply unit. As the potential across this resistor is increased by further increase in the speed-setting, the field is weakened and the motor is further accelerated. To prevent damage to the motor if the field strength is suddenly increased with the motor operating at high speed, a relay 7CR responds to a predetermined value of armature voltage whereby contact 163 opens to disconnect the arm 205 from the grid G13, and contact 167 closes to connect the grid to arm 207 of a maximum-speed resistor. The field is instantaneously reduced to its minimum strength, the armature voltage being likewise reduced so that relay 7CR drops out. The relay thus changesover from its actuated to its unactuated condition until the speed of the motor is reduced to the set value. When the arm 205 is moved from a high-speed to a low-speed position, the speed reference signal across resistor RO1 is substantially reduced so that the armature thyratrons pass less current. Under these circumstances, thyratrons AT3, AT4 are rendered conductive and relays 10CR, 12CR, are actuated. This causes a braking resistor to be placed across the motor for reducing its speed in a relatively short period. If the motor is over-speeded, due to failure of a circuit component leading to an excessive error signal, thyratron AT3 is rendered conductive but thyratron AT4 fails to conduct. Relay 10CR is actuated but relay 12CR remains unactuated, so that the motor is disconnected from its supply and the braking resistance is connected across its armature. A low voltage is made available for the armature of the motor during stand-by operation, and, when the machine is to be inched, this low voltage is impressed immediately on the armature so that rotation takes place instantaneously after the armature contactor is closed. For this purpose, the control circuit of the grid G14 comprises a stand-by resistor and a jog resistor, together with a capacitor to regulate the rate of increase of armature voltage during the inching operation.