GB762665A - Improvements relating to servo mechanisms - Google Patents

Abstract

762,665. Electric positional control systems. ENGLISH ELECTRIC CO., Ltd. July 23, 1954 [July 24, 1953], No. 20622/53. Class 40 (1). [Also in Group XXXVII] A three - phase induction motor is used for driving the output shaft in a closed loop positional control system and controlled through a magnetic amplifier. As shown, a threephase induction motor 24 drivesan output shaft 10 through gearing 10b in positional correspondence with an input shaft 11. Lack of correspondence between the two shaft positions is detected by magslips 12, 15 driven by the respective shafts. The rotor 12c of receiver magslip 12 is fed from the phase leads 14a, 14b and the error signal developed in the rotor 15c of magslip 15 is converted in a polarized rectifier 16, Fig. 2 (not shown), into a D.C. of corresponding magnitude and polarity. This D.C. is fed to a magnetic pre-amplifier 17 having push-pull output windings 17a, 17b connected via rectifier bridges 18, 19, respectively, across the phase leads 14a, 14b. The D.C. outputs of these bridges 18, 19 which thus vary differentially in accordance with the magnitude and sense of the error signal, are fed respectively through the series connecting control windings 20a-21a-23a-22a and 20b-21b-23b-22b of magnetic amplifiers 20-24, which each have an additional D.C. winding (not shown), arranged to oppose the windings 20a, 23a and assist 21a, 22a. The winding pairs 20a, 21a ... &c. of each amplifier are connected in opposition. The output coils 20c-21c-23c- 22c of these amplifiers are connected bridge fashion across the phase supply leads 14a, 14b and the phase windings 24a, 24b of the motor 24 are connected across the output of this bridge. The third phase winding of the motor is directly connected by a switch 29 to the third supply phase 14c. The magnitude and sense of the energization of the motor phase 24a, 24b thus varies in accordance with that of the error signal so that the motor runs in the follow-up sense. A postion of the bridge output feeding the motor is also fed back negatively to the pre-amplifier input via a polarized rectifier 25, similar to 16, and a transformer 28, to inject a voltage varying according to the rate of change of the input voltage to the motor windings. Further negative feed-back, proportional to the motor speed and derived from an induction generator 26 driven by the motor, is fed back via a further polarized rectifier 27, similar to 16, to the pre-amplifier 17. To reduce the current taken by the motor 24 when stationary, the switch 29 is thrown to its other position when the motor phase winding 24c is series connected with the output of a further magnetic amplifier 30 having a D.C. control winding 30b energized according to the differential of the pre-amplifier outputs. Thus when motor 24 is at rest the winding 30a acts as a high impedance to restrict the current in the motor phase 24c. The magslips and preamplifier 17 may operate on a higher frequency than the motor 27.