Generator for generating a low frequency alternating current for feeding an electric motor and a servo system with such a generator and an asynchronous motor is fed by the generator

摘要

1,178,935. Frequency changing; control of A.C. motors. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 10 Sept., 1968 [13 Sept., 1967], No. 43008/68. Headings H2F and H2J. In a circuit for producing an output voltage having a frequency variable in dependence on a control signal, and of the type in which a highfrequency A.C. input signal is sampled at a slightly different frequency, the sampling signal is produced by deriving a sawtooth signal having a slope proportional to the control signal amplitude, and comparing it with a fixed sawtooth signal derived from the input signal. The output signal is used to control a servomotor 10. Converter circuit, Fig. 2.-The input signal from generator 11 is fed to unit 12 which modulates its amplitude in accordance with the control signal S in to compensate for the decreased reactance of the windings of motor 10 at low frequencies. The output from 12 is fed to phaseshifter 13 to produce a three-phase output which is then sampled by electronic switches S 1 , S 2 , S 3 (controlled by sampling circuit 15) to produce an output having a frequency which is the difference between the input frequency and the sampling frequency. This signal is then filtered by LP 1 , LP 2 , LP 3 to produce a sinusoidal signal, which after amplification at A 1 , A 2 , A 3 is fed to the motor 10. Sampling circuit. Fig. 5.-The control signal S in is applied directly and via an inverter 24 to the two fixed contacts of a changeover switch 23, the movable contact of which is connected to the input of integrator 25. Whenever the integrator output reaches predetermined limits, unit 26 changes its state, producing a squarewave C, Fig. 6, which actuates switch 23, to produce at B a sawtooth, B, having a slope controlled by the signal S in - The input signal from generator 11 is converted to a constant sawtooth A in unit 20, and the two waveforms A, B are fed to a comparator 21 to produce waveform D, E. After shaping in 27, the resulting signals G, H are fed together with signal C to logic circuit 28 which produces the output I = G.C + H.C to actuate the switching elements S 1 , S 2 , S 3 in the main converter.