Patent GB-901745-A (GB)
Title: Electric power supply system with controlled output frequency
IPC: H02M5/293
Filed: 1959-08-13
Abstract: 901,745. Frequency changing. SPECIALTIES DEVELOPMENT CORPORATION. Aug. 13, 1959 [Sept. 5, 1958], No. 27687/59. Class 38 (2). [Also in Group XL (c)] An arrangement for providing a polyphase output of constant frequency f (say 400 c/s.) from a polyphase alternator 10 providing a frequency f1 (say 3000 c/s.) includes a squarewave multivibrator 11 oscillating at a frequency f2 (say 3400 c/s.) and a plurality of switch modulators 12-20 fed from the two sources to give a difference frequency f2 - f1, which is kept constant by a feed - back arrangement controlling the square-wave generator. As shown, Fig. 1, the alternator provides three outputs at phase angle 0 degrees to modulators 12, 15, 18, three at angle 120 degrees to modulators 13, 16, 19, and three at 240 degrees to modulators 14, 17, 20. The source 11 provides three outputs at phase angle 0 degrees to modulators 12, 13, 14, three at angle 120 degrees to modulators 15, 16, 17, and three at 240 degrees to 18, 19, 20. Three phases of the output frequency are obtained at terminals 21, 22, 24, each phase being obtained from the series connection of the outputs of a vertical column of modulators, e.g. outputs at C, F and J give the first output phase at K. An output from terminals 24 is fed to a frequency discriminator 25 giving a direct voltage proportional to the output frequency and this voltage controls the frequency of the square-wave generator, so that the output frequency is kept constant and independent of variations in the alternator frequency. The square-wave source 11 is supplied from the alternator 10 through rectifiers 26, and output terminals 21, 22, 24 are also connected to a voltage regulator 27 whose output is fed to the alternator field winding so that the output voltage is kept constant. Each switch modulator is as shown in Fig. 2. The alternator frequency is fed to terminals 34, 35 of a bridge, each arm 29, 30, 31, 32 of which is constituted by a bridge network of diodes 39, 40, 41, 42. Across each bridge is a power transistor Q1, Q2, Q3, Q4, whose base-emitter circuits are inductively coupled to an inductor 49 fed with the square-wave frequency f2, so that conduction through the bridges is controlled by #2. The arrangement is such that the output between terminals 36, 37 is positive when the amplitudes of f1 and f2 are similar, i.e. both negative or both positive, and is negative when they are dissimilar, and the output has two frequency components f2+f1, f2-f1. The summation of the three outputs C, F and J gives an output comprising a large amplitude component of f2 -f1 and a small amplitude component of 5f2 +f1 (20,000 cycles). The frequency discriminator 25 may comprise an m-derived filter giving an output voltage proportional to the input frequency which voltage is rectified and smoothed and fed as bias to source 11. Alternatively a Foster Seeley discriminator may be used. The square-wave source may be a frequency division multivibrator, including a master multivibrator of frequency 6f2 which provides trigger pulses for three multivibrators giving square waves of frequency f2 at 120 degrees phase-displacement. The system may be modified by using a 3- phase, 3-wire alternator and connecting the input supplies to the modulators in parallel (Fig. 4, not shown), and in a further modification (Fig. 5, not shown) a two-phase alternator is used in combination with a 6-phase square-wave generator to give a 3-phase output.