Patent GB-703990-A (GB)
Title: Improvements in or relating to control systems for electric motors
IPC: H02P7/26
Filed: 1950-09-18
Abstract: 703,990. Control of D.C. motors. BROWN, W. J. Oct. 18, 1951 [Sept. 18, 1950], No. 22872/50. Class 38(3) [Also in Group XXXV] In an automatic speed control system of the kind in which the armature of a separately excited D.C. motor is supplied from an A.C. source through a grid controlled rectifier, the rectifier being controlled by the difference between the armature voltage and an adjustable reference voltage, and in which a dynamic braking resistor is connected across the armature by a relay when the reference voltage is reduced below the armature voltage; compensation is provided for the voltage drop due to the braking current in the armature so that the relay is not released until the motor speed has dropped to the true required value. It is also stated that the grid controlled rectifier may be replaced by a magnetic amplifier, that the motor may have additional series field windings, that the output of a tachometer may be used instead of the armature voltage and that the braking relay may be polarized. In the first embodiment, Fig. 1, the armature 1 is supplied from a grid-controlled mercury-arc rectifier 2 controlled in accordance with armature voltage by a winding 15 controlling a phase shifting network as disclosed in Specification 656,816, [Group XXXV] and the I.R. drop in the armature is compensated by including a part of a series resistor 53 in the control circuit, as disclosed in Specification 656,778. The voltage across this resistor is also compared with a reference voltage tapped off a potentiometer 56 and applied to an auxiliary control winding 106 to effect load current limiting as disclosed in Specification 686,799, [Group XXXV]. The dynamic braking contactor 5 is controlled by a relay 100, controlled by a relay 23 connected in series with the control winding 15 and a rectifier 22 between the armature voltage tapping and the reference voltage tapping. Three reference voltages tapped off potentiometers 92, 93, 94 may be selected by a switch 80. When the reference voltage, is greater than the portion of the armature voltage tapped off potentiometer 82, relay 23 is energized, contactor 5 is energized, and the dynamic braking resistor 8 is disconnected from the armature and the armature is connected to the rectifier 2. If the reference voltage is reduced, so as to reduce the motor speed, rectifier 22 blocks, winding 15, relay 23, and contactor 5 are all deenergized, the rectifier output falls to zero, and the armature is disconnected from the rectifier and connected across the braking resistor until the armature voltage has fallen below the new reference voltage. The reference voltages (stabilized by a barretter 96), the field excitation, and the current limiter are all supplied from auxiliary electrodes 48, 49, on the main rectifier. The dynamic braking resistor 8 is connected to a tapping on the series resistor 53, and it is shown that this connection compensates for the I.R. drop in the armature due to the braking current and so ensures that the braking relay does not open until the motor speed has been reduced to the correct value. A second embodiment, Fig. 4, is similar to Fig. 1, but the I.R. drop compensation under motoring conditions is obtained by a current transformer 149, the output of one winding 148 of which is rectified at 143 and included in the control circuit in series with the control winding 15, rectifier 22 and braking relay coil 141. Under braking conditions, the I.R. drop compensation is obtained by a winding 154 on the braking relay, energized by a voltage tapped off part of the braking resistor 8 so as to bias the relay to remain closed. Since this causes the braking resistor to remain connected until an appreciable signal current is flowing in winding 15 tending to raise the rectifier output voltage, this output is reduced by an opposing control winding 161 connected across the braking resistor 8. Load current limiting is effected by rectifying at 171 the output of winding 169 of the current transformer 149, comparing this with a reference voltage 163 and applying the resultant via rectifier 174 to a rectifier control winding 175. In a third embodiment, Fig. 5 (not shown), the speed controlling signal is applied to the grid of a valve controlling the main rectifier, as disclosed in Specifications 656,817 and 656,778, the braking relay and rectifier being connected in parallel with the grid-cathode circuit of the valve. Compensation for I.R. drop is obtained by feeding the armature through a part of the braking resistor and tapping off a part of this voltage and injecting it into the control circuit. In a fourth embodiment, Fig. 6 (not shown), the braking relay is included in the anode circuit of the valve controlled as above.