Patent GB-1054780-A (GB)
IPC: H02M5/27
Abstract: 1,054,780. Converting. PRECISION MECANIQUE LABINAL. Oct. 22. 1963 [Oct. 26, 1962], No. 41698/63. Heading H2F. [Also in Division H3] The Specification, relates to a circuit for converting from a higher frequency to a lower frequency, in which controlled rectifiers are arranged so that the or each load phase may be supplied with current of either polarity from each phase of a three-phase supply (Fig. 1, not shown). The controlled rectifiers may be semi-conductor controlled rectifiers, transistors, thyratrons or mercury arc valves. The control circuit is arranged so that in each half cycle of the or each low frequency wave the low frequency voltage is first raised gradually from zero to a maximum, then held at the maximum and finally decreased gradually to zero (Fig. 2, not shown). According to the invention the control circuit for firing the controlled rectifiers includes means for producing a trapezoidal waveform at the lower frequency, a sawtooth waveform at the higher frequency, a resistance capacitor charging network fed with the higher frequency waveform and a device which continuously compares the instantaneous voltage of the lower frequency waveform with the instantaneous voltage on the charging network and supplies a pulse to fire one of the controlled rectifiers when the instantaneous voltage of the lower frequency signal reaches a predetermined proportion of the voltage on the charging network. Control circuit for ons rectifier.-The sawtooth waveform and trapezoidal waveform (Fig. 4) whose production is described below are fed into the circuit of Fig. 3 at 17 and 16 respectively. The charging circuit comprises resistor 19 and capacitor 20, and the voltage across the capacitor is applied to the emitter of the unijunction transistor 24. The trapezoidal wave is applied, together with a D.C. bias to the second base of the unijunction transistor. The circuit uses the property of this transistor that it will conduct between its first base and its emitter when the voltage between the emitter and first base exceeds a certain proportion of the voltage between its bases. When the ratio of the charging circuit voltage to the inter base voltage exceeds the intrinsic standoff ratio of the transistor the charging circuit is discharged across resistor 27. The charging circuit then recharges so that a further output pulse is obtained and so on until the ratio condition is no longer fulfilled (reference 25 in Fig. 4). The pulse thus obtained triggers controlled rectifier 26 which connects a supply derived through transformer 33 in the main circuit to the primary of the transformer 34. The pulse induced in the secondary of transformer 34 then triggers the main controlled rectifier P. A similar circuit is provided for each other rectifier. The waveform shown in Fig. 4 indicates how this arrangement varies the firing point of one controlled rectifier in successive cycles. The controlled rectifier is turned off either at the end of the high frequency half cycle or when the following controlled rectifier in sequence supplies a higher voltage to the load. High frequency signals.-The high frequency signals are derived either from the high frequency source (which may be an alternator driven by an aircraft engine) or from a tachogenerator on the alternator shaft (Fig. 5, not shown). The sine wave signals thus obtained are clipped to approximately rectangular shape before forming into sawteeth (Fig. 6, not shown). In Fig. 7, which refers only to one phase, the engine shaft 38 drives an alternator 40 and a tachogenerator 10. The tachogenerator output is clipped by Zener diodes 50, 51, and the clipped signal is amplified by transistor 42 to appear across resistor 53. The square pulses so produced charge capacitor 54 linearly through resistor 55, and at the end of each pulse the capacitor 54 discharges rapidly through diode 56 so that a sawtooth waveform is developed at the base of transistor 44 which is connected as an emitter follower. In another arrangement (Fig. 8, not shown) transistor 42, connected as an emitter follower, charges capacitor 54 at constant current while each pulse continues, the capacitor being in series with a diode 59 and with the emitter of transistor 42. When the pulse is not present another diode discharges the capacitor rapidly. The sawtooth waveform across capacitor 54 is amplified as before. Low frequency signals.-In a first arrangement (Figs. 9 and 11, not shown) a sinusoidal oscillator and (in the case of a polyphase output) a phase shifting arrangement provide reference signals. For each output phase there is a unit of the type shown in Fig. 12 in which the reference signal is applied at 64 to a charging circuit, which because of the delay caused thereby distorts the sine wave (Figs. 13, 14, not shown). The distorted signal is amplified and clipped by transistors 77 and 82 to give the desired waveform. In a second arrangement (Figs. 10 and 11, not shown) pulses at a frequency of six times the lower frequency are generated. The pulses for each rectifier are converted by a monostable multivibrator 83 (Figs. 15 and 16, not. shown) into square pulses of a length not greater than half the low frequency period. The square wave is then integrated at 85 giving a symmetrical sawtooth which is clipped at 87 to give the desired trapezoidal waveform.