Patent GB-1493875-A (GB)
Title: Electrical system including dynamoelectric machine operable as starter/generator
IPC: F02N11/04
Filed: 1975-02-07
Abstract: 1493875 Starting prime movers; control of brushless motors; converting GENERAL ELECTRIC CO 7 Feb 1975 [7 Feb 1974] 5298/75 Headings H2F H2H and H2J A starter-generator system for a jet engine of an aircraft comprises a dynamoelectric machine 10 which is operated as a brushless D.C. motor to run the engine up to idling speed and subsequently driven by the engine to operate as an A.C. generator producing a constant frequency output, the machine being connected to cycloconverters 21, 22, 23 which are controlled, during motor operation, by signals from a rotor position sensing arrangement consisting of members 14-17, 20, from a network 40 ensuring that there exists a correct phase polarity of a high frequency supply 31 for the machine, and from a current control network 47, the members 14-17, 20 and networks 40, 47 being disconnected, together with the supply 31, by a speed responsive signal 41 when idling speed is reached, after which the converters 21, 22, 23 are controlled by a reference wave generator 42, whose signal is compared with the integral of the line-to-line voltage of the machine to vary the firing points of silicon controlled rectifiers of the converters to produce a constant frequency output to supply a load 32. During motor operation, the field windings of the machine 10 are supplied with D.C. from the rectified output of an exciter generator 12 operating as a rotating transformer and mounted on the same shaft as the machine 10, the engine and a permanent magnet generator 14 which controls Hall elements 15, 16, 17 in accordance with the rotor position of machine 10. The exciter 12 has armature windings on its rotor and field windings on its stator, the latter being star connected during motor operation of the machine and supplied with A.C. controlled by networks 13, 47, 50, 51, 52, the changeover to generator operation causing the field windings to be connected in series and supplied with D.C. from the output of generator 14, with the output of the armature windings again being rectified to supply the field windings of machine 10. Control of the exciter field windings is described in co-pending application 5299/75. At low motor speeds of machine 10, the network 47 is responsive to a starting current control network 44 sensing the input current from supply 31, but the network 44 is overrridden at a predetermined speed (sensed by network 49) by a network 48 sensing the current in the armature windings of machine 10. Current control by memebers 47, 48 continues until the back EMF of machine 10 equals the supply voltage, whereupon the network 50 permits operation of a hitherto disabled field weakening circuit 51, to control network 13, 52 to reduce the field current of exciter 12 and so allow continued motor operation of the machine 10. Previous to this field weakening action, the networks 13, 52 control the field current to a constant value with increasing speed, to prevent excess current to the field windings of machine 10 and its consequent overheading. Networks 44, 47, 48, 49, 50, 51, 52 comprise operational amplifiers, reference potentiometers, resistor, capacitor and diode circuits and interconnecting diodes, Fig. 2 (not shown). Control of the cycloconverters and machine 10.- Each machine winding W1-W2 is connected to the corresponding SCRs 1-6 in oppositelypoled banks as shown in Fig. 3. During motor operation appropriate SCRs are fired to control current into and out of winding pairs 1, 4; 3, 6 and 2, 5 all three supply phases A, B and C feeding respective pairs in turn at low machine speeds and each phase feeding a plurality of pairs as the speed increases to that producing a back EMF equal to the supply voltage. Fig. 4 shows the circuits 93-98 producing the firing signals for the SCRs of Fig. 3, each circuit 93-98 having AND gates (e.g. 101-106) receiving signals from the rotor position logic network 20 (Fig. 1) via terminals 114 and a respective enabling signal AP, AN, BP, BN, CP, CN produced by the circuit shown in Fig. 6. The signals at terminals 114 are produced by feeding the outputs of the Hall elements 15, 16, 17 to respective operational amplifiers and thence to interconnected NAND gates, Fig. 9 (not shown), the arrangement being such that the logic position signals selectively enable individual SCRs in the various banks whenever the Hall voltage indicates that the flux density adjacent a particular winding is maximum, but only if the flux density adjacent the winding which is to have current transferred to it next is low. Operation of the position logic circuit is described, Fig. 8 (not shown) with reference to a Truth Table relating the conditions of the Hall elements to Positive and Negative bank SCRs which may be fired. The circuits 93-98 also include AND gates (e.g. 100) enabled by the speed signal 41 and feeding respective modulators M1-M6 of the type described in USA Specification 3,400,321, whose outputs provide the firing signal 1AP to 6AP, 1AN to 6AN, 1CN to 6CN. Each enabling signal AP, BP, CP occurs when that supply phase (e.g. phase A) is negative with respect to the preceding phase (e.g. phase C). Similarly the signals AN, BN, CN occur when the supply phase is positive with respect to the proceeding phase. The durations of the signals AP, AN, ... CN are also dependent on the current error signal produced by the network 47 of Fig. 1. Thus in Fig. 6 this error signal is fed to operational amplifiers 136-141 with signals from amplifiers 129, 130, 131 proportional to the integrals of the line-to-line voltages of the supply phases ABC, the latter signals also being fed to amplifiers 133, 134, 135 to subtract these pairs of integrals and obtain signals indicative of the correct phase polarity corresponding to signals AN, BN, CN. Inversion in NAND gates 152, 153, 154 gives the signals for correct phase polarity corresponding to signals AP, BP, CP. NAND gates 143-148 and 155-160 combine the signals of the aforementioned gates and amplifiers and a start signal to produce the signals AN, AP, BN, BP, CN, CP. Typical signal waveforms are described, Figs. 5 and 7 (not shown). For generator operation of the machine 10, the modulators M 1 -M 6  of Fig. 4 receive only signals 107 obtained by comparing the integral of the line-to-line machine voltage (obtained at terminals 92 in Fig. 3) with the reference wave voltage from network 42 of Fig. 1, thereby obtaining a constant frequency output, for example, as described in U.S.A. Specifications 3,400,321, 3,431,483 and 3,593,106.