GB1028371A - Improvements in or relating to electric amplifiers - Google Patents

Abstract

1,028,371. Control of A.C. motors. SMITHS INDUSTRIES Ltd. Feb. 4, 1963 [Feb. 2, 1962], No. 4106/62. Heading H2J. The control phase winding of a two-phase induction servo motor is connected to terminals 11, the motor being controlled in accordance with signals applied to input terminals 16. In the absence of any signal, transistors 21-24 are fully conductive, transistors 1-4 being cut off under this condition. When a train of pulses of alternating polarity is applied to terminals 16, the two halves of the input stage 21-24 are cut off alternately due to a positive pulse being applied to the base of either transistor 21 or transistor 23, provided that the pulse amplitude is sufficient to overcome the negative bias from rectifier 19. When transistor 21 is cut off, for example, transistor 22 is also cut off and transistors 1, 2 are rendered conductive to energize one half of winding 6. A train of widthmodulated pulses applied to terminals 16 thus causes output current pulses of alternating polarity to flow in the secondary winding 8 of the output transformer. The speed of the motor is controlled by varying the pulse-width, and the direction of rotation by reversing the polarity of the pulses. Smoothing capacitor 9 is connected in series with inductance 10. If no signal voltage is applied across terminals 32, transistors 33, 34 are equally conductive so that equal currents flow in the two pairs of control windings 38, 39, and 41, 42, of magnetic amplifier 37 which then has no output current. When signal currents produced by sources 30, 31 are unequal, a voltage is developed across terminal 32 whereby one of the transistors 33, 34 conducts more than the other, the output pulses applied to terminals 16 being widthmodulated in dependence upon the magnitude of the voltage across terminals 32 and the phase of these pulses being determined by the sense of that voltage. The source 30 may be a demand signal source and the source 31 a feedback source driven by the motor. Such feedback source may be a potentiometer for representing the angular position of the motor shaft or a tachometer generator.