Aircraft Electrical Systems

that if other systems could not be operated either by air, hydraulic oil, cables, numerous mechanical link· ages or petrol, then they were quite unnecessary! A majority of the individuals were mechanics, and the ground engineers as they were then known, and un· doubtedly, when "elcctrickery" began proving itself as a system operating media, it came as a pleasant relief to leave all relevant work to that odd character, the electrician, who speaking in some strange jargon and by means of diagrams containing numerous mystic lines and symbols, seemed better able to cope with it all! With the continued development of the various types of aircraft, the sources of electrical power have also varied from the simple battery and wind-driven generator, through to the inost complex multiple a.c. generating systems. Similarly, the application of power sourc es have varied and In conjunction with dev elop- ments in electronics, has spread into the areas of other systems to the extent of performing not onl y a controlling function but, as is now so often the case, the entire operating function of a system. As a result, the work of the electrician ass umed greater importan ce and has become highly specialized, while other main- tenance specialists found , and continue to find it increasingly nece ssary to broaden their knowledge of the su bje ct; indeed it is incumbent on them to do so in order to carry out their important duties. This also applies to pilots in order that t he y ma y meet the technical knowle dge requirements appropriate to their duties and to the types of aircraft they fly. Fundamental electrical principles are describ ed in many standard text books, and in preparing the material for th.is book it was in no way intended that it shou ld supplant their educational ro lo , However, it has been consid ere d convenient to briefly re view cer- tain relevant principles in the chapters on generation and conversion of power supplies, to "lead-in" to the subject and, it is hoped, to convey more clearly how they a rc appUed to the systems described. In keeping with the introductory nature of the book, and perhaps more important, to keep within certain size limitations, it obviously has not been possible to cover aU types of aircraft systems. However , in drawing co mparison s it is found that applications do have quite a lot In common, and so the exam ples finally chosen may be considered sufficiently representative to provid e a useful foundation for further specialized study. The details given embrace relevant sections of the various sy llabus es es tablished for the technical exam- inati on of maintenance engineers and pilots by official organizations, training schools and professional soc!· eties. In this connection, therefore, it is also hoped that the book will provide a useful source of reference. viii A selec tion of questions are provided for each chapter and the author is indebted to the Society of licensed Aircraft Engineers and Technologists for permission to reproduce questions selected from examination papers . Valuable assistance has been given by a number of organi zations in supplying technical data, and in granting permission to reproduce many of the illustra- tions, grateful acknowledgement is hereby made to the following - Amphenol Ltd . Auto Diesels Braby Lld, Aviquipo <ilfB ritain Ltd. Belling & Lee Ltd . B.l.C .C. British Aircraft Corporation (Operating) Ltd. Britten -Norman Ltd. Cannon Electric (G.B .) Ltd. Davall. Dowty El ec trics Ltd. Graviner (Colnbrook) Ltd. Hawker Siddeley Aviation Ud. Honeywell Ltd. international Rectifier Co. (G.B .) Ltd . Lu cas Aerospace Ltd. Nrwton Brothers (Derby) Ltd . Normalair-Garrett Ltd . Plcssey Co., Ltd . SAFT (United Kingdom) Ltd . Sangamo Weston Ltd. Shell Aviation News. Smiths Industries Ltd. Standard Telephones & Cables Ltd. Thom Bendix . Varley Dry Accumulators Ltd . Finally, thanks are also due to the publishers for having padently awaited the completion of sections of manuscript and also for having accepted a number of changes of subject. Copthome, Sussex E.P. CHAPTER ONE Direct Current Power Supplies INTRODUCTION Depending on the type of aircraft, and the extent to whi ch electrical power is to be utilized for the operation of its systems and components, the primary supply of such power may either be direct current (d.c.) or alternating current (:Le .). This chapter deals with the first of the se supplies and how it is produced by both generators and batteries. Examples of some typical aircraft systems are described in Append.ix 7. Fundamental Pr inciples of Generators A generator is a machine that converts mechanical energy into electrical energy by the process of electro- magnetic induction. In both d.c. and a.c. types of generator, the voltage Induced is alternating; the major difference between them being in the method by which the ele ctrical energy is collected and applied to the circuit externally connected to the generator. Figure 1.l(a) illustrates a generator in its simplest form , i.e. a single loop of wire "AB'' arranged to rotate between the pole pieces of a magnet . The ends of the wire are brought together to form a circuit via slip ring;, brushes and the externally connected load. When llie plane of the loop lies at right angles to the magnetic field (position 1, Fig . I.I (b )) no voltage is induced ln the loop. As the loop rotates through 90 degrees the wires cut the lines of force at right angles until at position 2 the induced voltage is at a maximum . As the loop approaches the vertical position again the voltage decreases since the rate at which lines of force are cut dimlnishes. At position 3 the induced voltage Is zero. If rotation is contin ued , the number of lines cut gradually increases, until at 270 degrees (position 4) it is once again maximum, but as the cutting is in the opposite direction there is also a reversal of the direction of induced voltage. As rotation continues, the number of lines cut decreases and the induced voltage reduces to zero as the loop returns to position 1. Plotting of the induced voltage tluoughout the full cycle produces the alternating or sine curve shown. tol <D e 1, A + 0 (b) Sli prinQ\ ® G> @ (D @ A B '-I I' I' A:'.78 - A)6 8 A 8 A Fig I.I {a) Simple form of generator (b) Induced vo ltage To convert the a.c. produced into unidirectional or d.c., it is necessary to repla ce the slip rings by a collecting devlct! referred to as a commutator. 1hls is shown In Fig. 1.2 (a) and as will be noted it consists of 2 two segments insulated from each other and connected to the ends of the loop. The brush es are se t so that each segment moves out of contact with one brush and into con tact with the other al the point where the loop passes through the positions at which induced voltage is minimum . In other words, a pulsating current increasing to maximum in one direction only is produced as shown by the curve in Fig. l .2(b ). (a) (b) Fig 1.2 Conver sion of a.c. to d.c. (a) Use of commutator (b) Current wave-form In order to smoo th out the pulsations and to pro · duce a more constant output, additional wire loops and commutator segmen ts are provided. They are so interconnected and spaced about the axis of rotation, that several are always in a position of maximum action, and the pulsating output is reduced to a ripple as indicated in Fig. 1.3. Generator Classifications Generators are classified according to the method by which their magnetic circuits are energized, and the following tluee classes are normally recognized - (l) Permanent magnet generators. !1 0 > Zero 180° 'h Fig 1.'.l 270° 360° 3/4 One revolution Efrec1 on output using sove ral coils (2) Separately-excited generators, in which electro· magnets are excited by current obt ained from a separate source of d.c. (3) Self-excited generators, in which elec_tro· magnets are excited by current produced by the machines themselves. These generators are fur- l her classified by the manner in whi ch the fixed windings, I.e. the el ec tromagne1i c field and armature windings, are interconnected. In aircraft d.c . power sup ply systems , self-excited shunt-wound generators are employed and the fo llow- ing details are therefore related only to this type. Fixed Winding Arrangement Figure 1.4 illustrates the arrangement of the fixed windings of a basic four-pole machine suitable for use as a self-excited generator. Th e fixed portion of the armature circuit co nsis ts of the four brushes , the lin ks connecting together bru shes of like polarity and the Terminal box Commutator Pig J.4 Fixed winding arrangements Pole cables connecting the linked brushes to the terminals indicated A and A1• The four field coils are of high resistance and connected in series to form the field winding . They are wound and connected in such a way that they produce alternate North and South polarities. The ends of the windin~ are brought out to the terminals indicated as Zand Z 1 • Generator Characteristics The characteristics of a generator refe r to the relation- ship between voltage and the current flowing in the external c!rouit connected to a generator, i.e. the load current, and there are two which may be closely defined. These are: the external characteristic or relationship between terminal voltage and load current; and the internal charar,teristic or relationship between the actual electromagnetic force (e.m.f.) generated in the annature windings and load current. These relationships are generally shown in the form of graphs, with the graph drawn for one particular speed of the generator. Self-excited Shunt-wound Generators Shunt-wound generators are one of three types in the self.excited class of machine and as already noted are used in air craft d.c. power supply systems, The term "shunt-wound" is derived fr om the fact that the high-re sistance field winding is co nnected across or in parallel with the armature as shown in Fig . 1.5. The Fig 1.5 Term,nol voltage I Connection of shunt-field winding armature current divides into two branches, one forme 'd by the field winding , the other by the external circuit. Since the field winding is of high resistance, the advantage is gained of having maximum current flow through the external circuit and expendi- ture of unnecess ary electrical energy within the gener- ator is avoided. 3 Operat i ng Principle and Characteristic When the armature is rotated the conduators cut the weak magnetic field which Is due to residual magnet- ism in the electromagnet system. A small ~.m .f. Is induced in the armature winding and is applied to the field winding, causing current to flow through it and so increasing the magnetic flux. This, in turn, causes a progre ss ive increase in the induced e.m.f. and field current until the induced e. m.f. and terminal voltage rea ch the steady open-circuit maximum. The characteristic for this type of generator is shown in Fig , 1.6 and it will be observed that the terminal l!l 0 > 0 Fig 1.6 ...... ' 1 ..... , .,\ ' \ \ 1 )1 / ,,,, I ,,/ I / ,;/ Characteris 1i c of sclf~xcited shunt-wound generator voltage tends to fall with Increasing load current. Th.is is due to the voltage drop (IR drop) in the armature winding and also to a weakening of the main fl~ by armature rea ctio n. The fall in terminal voltage reduces the field current, the main flux is further weakened and therefore a further fall in terminal voltage is pro- duced. If the p rocess of increasing the load Is continued after the full working load condition has been reached, the termfoal voltage will fall at an in creasi ng rate until it can no longer sustain th~ load current and they both fall to zero. With reduced excitation the external cha racteristic of a shunt-wound generator falls mu ch more rapidly so that the point at which voltage collapse occ ur s will be reached with a much smaller load cur- rent. In practice, field current is adjusted to maintain constant vo ltage under all load conditions, by a voltage regulator the operation of whi ch will be des cribed later. 4 Sometimes a generator will lose its residual magnet- ism or become incorrectly polarized because of heat, shock, or a momentary current in the wrong direction. Th.is can be corrected by momentarily passing current through the field from the positive terminal to the negative terminal; a procedure known as "flashing the field". Generator Construction A typical self,exciled shunt-wound four-pole genera- tor, whJch is employed in a current type of turbo-prop civil transport aircraft, is illustrated in Fig. 1.7. lt is designed to provide an output of 9 kilowatts at a con - tinuous current of 300 amperes (A) over the speed range of 4,500 to 8,500 rev/nun. In its basic form the construction follows the pattern conventionally adopted and consists of five principal assemblies: namely, the yoke, armature, two end frames and brush-gear assembly. THE YOKE The yoke forms the main housing of the generator, and is designed to carry the electromagnet system made up of the four field windings and pole pieces . It also provides for the attachment of the end frame assemblies. The windings are pre-formed coils of the required ampere-turns, wound and connected in series in such a manner that when mounted on the pole pieces, the polarity of the field produced at the poles by the coil current is alternately North and South {see Fig . 1.4). The field windings are suitably insulated and are a close fit on the pole pieces which are bolted to the yoke. The faces of the pole pieces are subjected to varying magnetic fields caused by rotation of the armature, giving rise to induced e.m.f. which in tum produces eddy currents through the pole pieces causing local heating and power wastage. To minimize these effects the pole pieces are of laminated co nstruction; the thin soft iron laminations being oxidized to insulate and to offer high electrical resistance to the induced e.m.f. INTERPOLE AND COMPENSATING WINDINGS During operation on load, the current flowing through the armature winding of a generator creates a magnetic field which is superimposed on the main field pro- duced by field-winding curreht. Since lines of force cannot intersect, the armature field distorts the main field by an amount which varies with the load; such distorting effect Is termed armatur11 reaction. If uncorrected, armature reaction produces two addition. undesirable effects: (i) it causes a shift of the Magnetic Neutral Axis, i.e . the axis passing through two points at which no e.m.f. is induced in a coil, setting up re- active sparking at the commutator, and (il) it weakens the main field causing a reduction in generated e.m.f. The position of the brushes can be altered to mini- mlze these effec~s under varying load conditions, but a mor e effective method is to provide additional wjnd. ings in the electromagnet sys tem, such windings being referred to as interpole and compensating windings. lnterpole windings a re wound on narrow-faced auxiliary pole pieces lo cated midway between the main pole s, and are connected in series with the armature, The windings are such that an interpole has the same polarity as the next main pole in the directio of rotation, and as the fluxes are opposite in dire ction to the armature flux, they can be equalized at all load! by having the requisite number of turns. ln order to provide true correction of armature rea ction, the effects produced by interpoles must be supplemented, since alone they cannot entirely elim· inate all distortion occurring at the main pole faces. Compensating windings are therefore connected in series with the lnterpole and armature windings, and located In slots cut in the faces of the main pole shoes The sides of the coils thus lie parallel with the sides of the armature coils. The ampere.turns of the winding are equal to those of the armature winding, while the flux due to it is opposite in direction to the armature flux . . AUXILIARY TNTERPOLES The effectiveness of interpoles in minimizi ng reactant spa rking Is limited by armature speed, and their appli1 ation as individual com ponents of a field-winding system is, therefore, restricted to generators operatlni over a narrow speed range, e.g. the designed range of the generator illu strated in Ffg. l. 7. ln the case of generators designed for operation over a wide range, e.g. 285 0 rev/min up to I 0,000 rev/min, the use of interpoles alone would produce a side effect resulting in reactance sparking as the generator speed is reduce from maximum to minimum. To counteract this, and for a given load on the generator, it is necessary to reduce the rnagnetomoUve force (m .m.f.) of the interpoles. The desired effect may be obtalned by winding auxiliary coils over the interpole coils and connecting them in series with the generator shu1 field winding in such a way that each coil, when energized by shunt field circuit current, produces an o amp Wi ndow str op Anti-drive eod head End caver Retaining~ I{~~ cap Fig l.7 Sectioned view of a gen era.t ot Temii no l Te rrmnol <XWef Sealed bo l ibeo nng Shaft ond p late a sse1n bly Beori ng suppoc1 heocl Yoke ond field coils V, 6 m,m.f. of opposite polarity to that produced by the interpolc coil on the same pole shoe. An exact balance between rcactance e.m.f. and commutation e.m.f. is maintained over the full working range of generator speed to assist in producin g sparkless commutation. ARMATURE ASSEMBLY The armature assembly comprises the main shaft (which may be solid or hollow) core and main winding, commutator and bearings; the whole assembly being statically and dynamically balanced. Jn the generator shown, the shaft is hollow and internally splin ed to mate with splines of a drive shaft which passes through the entire length of the armature shaft, Armature windings are made up of a number of individual identical coils which fit into slots at the outer edges of st ee l laminations which form lhc core of Lh e armature. The coils are made from copper strip and as security against displacement by centri· fugal force, steel wire (in some cases steel strip) is bound round the circumferen ce of the armature. The ends of each coil are brought out to the commutator and silver brazed to sepa rate segments, the finish of one coil being connected to the same segme nt as the beginning of another coil. The complete winding thus forms a closed circuit. The windings are invariably va cuum -impragnated with silicone varnish to main· tain insulation resistance under all conditions. In common with most aircraft generators, the commutator is of small diameter to ntinimize centri· fugal stress in g, and is built up of long, narrow copper segments corresponding in number to that of the field coils (a typical figure is 51 coils). The segment surfaces are swept by brushes which are narrow and mounted in pairs (usually four pairs) to maintain the brush contact area per segmen t - an essential pre- requisite for effective commutation. The armatures of all aircraft generators arc support ed in !ugh efficiency ball or roller bearings, or in combinati ons of these two types. Where combina· tions are used.in a single genera tor it will be found that the ball bearing is invariably fitted at the drive en d of the armatur e shaft, and the roller bearing at the commutator end. This arrangement perm.its lateral expansion of the armature shaft, arising from temperature increa ses in the generator, without expos· ing the be arings to risk of damage. Bearings are lubri ca- ted either with a specified high-melting-point grea se or lubricating oil and may be of the sealed or non· sealed types. Sealed grease-lubricated bearings are pre-packed by the manufacturer and require no further lubri cation during the life of the bearing. Non-sealed grease-lubricated bearings are assembled with suffic· i ent lubri cant to last for the period of the generator servicing cycle , In general the lubricant for oil. lubricated bearings is introduced into the bearing through the medium of oil -impregnated felt pads. Seals are provided to prevent oil escaping into the interior of the generator. END FRAME ASSEMBLIES These assemblies are bolted one al eac h end of the yoke and house the armature shaft bearings. The drive end frame provides for Lhe attachment of the genera- tor to the mounting pad of the engine or gear-box drive (see also p. 8) and the commutator and frame provides a mounting for the brush-gear assem- bly and, in.the majority of cases, also provides for the attaclunent of a cooling air duct. In spection and replacement of brushes is accomplished by removing a strap which normally covers apertures In the commu- tator end frame . BRUSH -GEAR ASSEMBLY The brush·gear assembly is comprised of Lhe brushes and tlte holding equipment necessary for retaining the brushes in the correct position, and at the correct angle with respect to the magnetic neutral axis. Brushes used in aircraft generators are of the electro· graphitic type made from artificial graphite, The graphite is produ ce d by taking several forms of natural carbons, grinding them into fine powder, blending them together and consolidating the mi xture into Lhc desired solid sh ape by mechanical press ure followed by exposure to very high temperatu re in an electric furnace. These brushes possess both the robustness of carbon and Lhc lubricating properties of graphite. In addition they are very resi stant to burn- ing by sparking, they cause little comm ut ator wear and their heat conductivity enables them to with - stand overloads. As stated earlier, an essentia l prerequisite for effective conunutation is that brush contact area per commuta tor segment should be maintained. Th.is is accomplished by mounting several pairs of brushes in brush holders; in the gen erator illustrated in Fig. I. 7 four pairs of brushes are employed. The holders take the fo rm of open-ended boxes whose inside surfaces are machined to the size of a brush, plus a slight clear- ance enabling a brush to slide freely without tilting or rocking. Contact between brushes and commutator is maintained by the pressur e exer ted by the free ends of adjustable springs anchored to posts on the brush holders. Springs are adversely affected by current passing through them; it is usual , the re fore, to nt an insulating pad or roller at the end of the spring where it bears on the top surface of the brush. The brush holders are secured either by bolting them to a support ring (usually called a brush rocker) which is, in turn, bolted to the commutator end frame, or as in the case of the generator illustrated, bolted directly to the end fran1e. ln order to achieve the best possible commutation a support ring, or end frame, as appropriate, can be rotated through a few degrees to alter the position of the brushes relative to the magnetic neutral axis. Marks are provided on each generator to indic ate the normal operating position. When four or more brush holders are provided, they are located diametrically opposite and their brushes arc alternately positive and negative, those of similar polarity being connected together by bar and flexible wire type links. The brushes are fitted with short leads or "pigtails" of flexible copper braid moulded into the brush dur- ing manufacture. The free ends of the pigtails termin· ate in spade or plate type terminals which are con• nected to the appropriate main terminals of the genera· tor via the brush holde rs and connecting links . TERMINAL BLOCKS The leads from brush-gear assernblies and field windings are connected to terminal posts secured to a block mounted on the commulator end frame or, in some generators, on the yoke assembly (see Fig. 1.7). The tenninals and block are enclosed in a box-like cov er also secured to the end frame. Entry for the output sup.ply cables of the distribution system (refer to Chapter 5) is through rubb er clamps. The rotation of a generator armatu re is sp ecified in a direction, norm· ally anti-clockwise, when viewed from the drive end assembly . A movable link ls fitted between two of the terminals which ca n be co nnected in an alternative position should it be necessar y for the gene ra tor to be driven In the reverse direction. SPARK SUPPRESSION Sparl<.ing at the brushes of a generator, no matter how slight, results in th e pr o pagation of electromagnetic waves which interfere with the rece ption of radio sig- nals. The interference originating in generators may be eliminated quHe effectively by screening and suppres· sion, Scr eeni ng involves the enclosure of a generator in a con tinuous metallic casing and the sheathing of 7 output supply cables in continuous metallic tubing or conduit to prevent direct radiation. To prevent inter· ference being conducted along the di stribution cable system, the screened output supply cables are termin· ated in filter or suppressor units. These units consist of chokes and capacitors of suitable electrical rating built into metal cases located as clo se to a generator as possible. Independent suppressor units arc rather cumbersome and quite heavy I and it is therefore the practice in the design of current types of generator to incorporate internal suppression systems, These sys tems do not normally contain choke s, but consist simply of suitably rated capacitors (see Fig, I. 7) which are connected between generator casing (earth) and terminals. The use of internal suppression systems eliminates the necessity for sc reene d output supply cables and conduits thereby malting for a considerable saving in the overall weight of a generator installation. Rectified Power Supplies In many of the smaller types of single-engined and twin.engined aircraft, the primary d.c. power ls supplied in a manner similar to that of automobiles, i.e. It is a rectified output from a frequency.wild alternating current generator. Its operating frequ en cy is abo ut l 00 Hz at idling speed of the engine and in crea ses with speed to 1200 Hz or higher. The generator or alternator as it is more gene rally called, consists of a rotor, stator, slip ring and brush assembly and end frame s. ln addition six silicon diodes are carried in an end fram e and are connected as a bridge re ctifier (seep. 57) to provide the d.c. for the aircraft's system. The principal constructional features are illustrated in Fig . 1.8. The rotor Is formed by two extruded steel pole pieces which are press-fitted on to the rotor shaft to Fig 1.8 Alternntor supplying a rectified output 8 sandwich a fie ld coil and thus form the core of the electromagnet, Each pole piece has six "fingers" which in position, mesh but do not touch each other. Excitation current is fed to a field coil on the rotor via brushes, and slip-rings whlch are press-fitted onto the rotor shaft. The stator is made up of a number of steel stamp· ings riveted together to form the core around which the three star-connected phase coils are wound . One end of each winding is connected to the bridge rectifier assembly while the other ends are joined to fonn what is termed the neutral point. The stator assembly is clamped between the end frames. Figure 1.9 illustrates the circuit diagram of the alternator. Unlike a conventional d.c. generator; the alternator has no residual magnetism and so its field must be excited initially by d.c. from the aircraft's battery or an external power supply. When d.c. is switched on to the generator; the rotor field coil ls energized and the pole piece "fingers" become alternately north and south magnetic poles . As the rotor rotates, the field induces a three-phase altemating current within the stator which is fed to the diodes for rectification, and then to the aircraft's system. As will be noted from Fig . 1.9, All ernotor when the alternator is supplying the busbar, it will also supply Its own field excitation current to sustain the regulated output. The level of voltage is regulated by a solid -state type of voltage regulator (seep. 14). Generator/Engine Coupling Depending on the type and application, a generator may be driven by an engine either from an accessories gear box, or by a pulley and bell. The generator already shown ln Fig . 1.7, ls an example of one driven through gearing which forms parl of an accessories gear-box. Depending on the rated output of a generator and on the load require- ments of the electrical system of a particular aircraft , there is a specific gear drive ratio. The drive from the gear-box is by means of a quill shaft with either male or female serrations or splines at one or both ends. The serrations or splines mate with corresponding formations on the generator armature shaft (see Fig. 1.7) to transmit the torque delivered by the driving gear. One of the requirements to be satisfied by a quill drive js that lt must effect· lvely Interrupt transmission of the driving torque in Fig 1.9 Circuit diagram of alternator the event that the generator armature seizes up. This is done by designing the drive shaft so that at one section its diameter is smaller than the remaining sections; thus providing a weak spot at which the shaft will shear under the effect of an excessive torque. Quill drives are usually short and rigid, but in some cases a long drive with one end mating with serrations formed deep in a hollow armature shaft may be speci· fled, This arrangement enables the drive to absorb much of the mechanical vibration which is otherwise passed to a generator from an accessories gear-box . The method of securing a generator to an accessor- ies gear-box varies, but in general it Is either one utlllzing a mounting flange or one requiring a manacle ring. In the mounting flange method, the end frame at the drive end of a generator ls usually extended to a larger diameter than the yoke, thus forming a pro - jecting flange . Holes in the flange line up with and accept studs which are located in the mounting pad of the engine or gear-box, and the generator is finally secured by nuts, locking washers, etc. An alternative fonn of flange mounting is based on a generator end frame having two diameters. The larger diameter is no greater than that of the yoke and abuts on the mounting pad while the reduced diameter provides a channel or "gutter", between the yoke and the larger diameter of the end frame, into which the mounting studs project. Another variation of this form of mounting is employed in the generator shown in Fig. 1.7. In the manacle-ring method of mounting the generator drive end frame has an extension with a recess in the mounting face of the driving unit. When the generator extension is fully engaged with the recess, a flange on the end frame abuts on a matching flange formed on the driving unit mounting face. The two flanges are then clamped together by a manacle ring which, after being placed over them, is firmly closed by a tensioning screw, A spigot arrangement is u sually incorporated to provide locat,ion of the generator to the drive unit, and to absorb torque reaction when the generator is operating. The pulley and belt drive is commonly adopted for driving altemators of the tYPe shown in Fig. 1.8, and as may be seen from Fig. 1.10, it_is similttr in many respects to the one adopted in automobiles. The alternator ls sec ured to two mounting brackets one of which is slotted, so that when the corresponding securing bolt Is slackened, the alter- nator may be positioned about the ot]J.er bolt for 9 the purpose of adjusting belt tension. The required drive ratio is, of course, detennined by the diameters of the engine and altomator pulleys. Fig l.10 Pulley and belt drive Cooling of Generators The maximum output of a generator, assuming no limit to input mechanical power , Is largely detennined by the ease with which heat (arising from hysteresis, thermal effect of current in windings, etc.) can be dissipated. With large-bulk generators of relatively low output the natural processes of heat radiation from the extensive surfaces of the machine carcase may well provide sufficient cooling, but such "natural" cooling is inadequate for the smaller high-output generators used for the supply of elec.trical power to aircraft, and must, therefore, be supplemented by forced cooling . The most commonly accepted method of cooling is that which utilizes the n1m or bla st effect resulting from either the slipstream of a propeller or the air- stream due to the aircraft's movemenL A typical cool- ing system is shown in a basic form In Fig. 1.11 . The air is forced at high speed into an intake and is led through light-alloy ducts to a collector at the commu· tator end of the generator. The air discharges over the brush-gear and commutator to cool this natural area of high temperature, and then passes through the length of the machine to exhaust through apertures, surrounded by a perforated strap, at the drive end. In order to assist in ram-air cooling and also to pro".ide some cooling when the aircraft ls on the ground, 10 many types of generator have a fan fitted at the drive end of the armature shaft. Eng,ne nace lle Filter ;creen Air $COOP Commutotor end of gene1otor Romo,, flow Fig . l.11 1·ypical cooling ~ystem Cooling of the alternator shown in Fig. 1.8 Is provided by a fan at the drMng end and by air passing through slotted vents in the slip ring end frame, Heat at the slllcon diodes Is dissipated by mounting them on steel plates kn ow n as "heat sinks". Brush Wear The carbon from which electro-graphllic brushes are made is extremely porous and some of the pores are so very fine that ca rbon ha s an exceptional ability to absorb other substances into its structure, and to retain them. Moisture is one of these substances and it pjays an important part in the functioning of a bru sh co ntact by affording a sub stantial degree of lubrication. The moisture is trapped under the inevitable irregular, itles of the contact fa ces of the brushes and forms an outside fi..lm on the com mutator and it is with th.is mm that the brushes mak e contact. °Just how vital a par t moisture docs play was, however, not fully realized until aircraft began operating at high altitudes and the problem arose of brushes wearing out very rapidly under these cond itions. Inve stigations inl o the problem showed that the fundamental difficulty was the extreme dryness of the atmosp here, this , in its tum , producing th ree second ary e ffects : (I) friction between brushes and co mmutator becau se the lubricating film cannot form, (ii) contact re sistance becomes negligible giving rise to heavy reactive spa rki ng and accelerated brush erosion and (iii) static electrical char ge s due to fri ction, producing molecular broakdown of the brushes. These effects have been largely eliminated by usin1 brushes which have a chemical additive as a means of replacing the function which atmospheric moisture plays in surfa ce skin formation. Two distinct categor· ies are in general use: brushes of one ca tegory form a constant-resistance semi-lubricating film on the corn· mutator, while those in the other category are, in effect, self-lubricating brushes which do not form a film. The composition of the film-f o rming brushe s includes chemi ca ls (e.g. barium fluoride) to build Up progressively a constant-resistance semi-)ubricatin film on the commutator surfaces. Brushes of this category do not wear abnormally at altitudes up to 60 ,000 feet providing that generators to which they are fitted have been previously ''bench run" for sorm hours to allow the formation of the protective film. This film. once formed, is ve ry d ark in colour and rn : often give the impression of a dirty commutator. Brushe s of the non -film-forming category contain lubricating ingredient such as molybdenum disulphid which is often packed in cores running longitudinally through the brushes. Since the brush is se!f-lubricall11 it is unnecessary for generators fitted with this ty pe to be run for hours prior to entering service. Howeve they do have the clisadvantage of appreciably shorter life, due to somewhat more rapid wear, when com- pared with film . forming brushes. Vo ltage Regulation The efficie nt operation of aircraft electrical equipm , requiring d.c. depend s on the fundamental rcq uirern that the generator voltage at the di stribution bu sbar system be maintained constant under all conditions load and at varying s peeds, within the limits·of a pn sc ribed range. It is necess ary , the refo re, to provide a device that will regulate the output vullage of a gen, ator at the designed value and within a specified to!, ance. There are a number of factors which , either sepa or in combination, affect the output voltage of a d., generator, and of th ese the o ne whlch ca.n most con veniently be controlled is the fi eld circuit curr ent, which in its turn controls the nux density. This con trol can be effe cted by incorporating a variable resi: in series with the field winding as shown in Fig . J. l Adjustments to this re'sistor would vary the resistan of the fi eld winding , and the field current and outp1 voltage would also vary and be brought to the requi co nt rolling value. The application of th e resistor in manner indicated is, however, limited since it is essen · tial to incorporate a regulating devic e which will auto- matically respond to changes of load and speed, and also, automatically make the necessary adjustments to the generator field current. Three of the regulation methods conunon ly adopted are: the vibrating contact method; the one based on the pressure/ resfatance characteristics of carbon, namely, the carbon pile method, and the one based on solid- state circuit principles. Shunt field winding ' j Oec . Fig 1.12 Control of f1eld circuit cutrent Load 11 Vibrating Contact Regulator Vibrating contact regu· Iat ors are used in several types of small aircraft employ Ing comparatively low d.c. output generators and a typical circuit for the regulation of both voltage and current of a single generator system is shown in basi c form in Fig. 1.13. Although the coil windings of each r egu lator arc interconnected, the circuit arrangement is such that either the voltage regulator only or the current regulator only can operate at any one time. A third unit, called a reverse current cut-out relay, also forms part of some types of regulat or, and since tl1e relay has a circuit protection function, a description of its cons truction and operation will be given in Chapter 7. Voltage Regulator Th.is unit consists of two windings assembled on a common core . The shunt winding con- sists of many turns of fine gauge wire and is connected in series with the current regulator winding and in parallel with the generator. The series winding, on the other hand, consists of a few turns of heavy gauge wire and is connected in series with the generator shunt- field winding when the contacts of both regulators are closed, i.e. under static condition of the generator system . The contact assembly is comp rised of a f'ixed contact and a movable con ta ct secured to a flex.ibly- To (11;lrib~tion .------------------------.------1---system + VollOQG rsqv lotor R Fig l.l3 Cu1,ent 1eq u101 0r Vibrating contact regulator principle l\nnotufe 12 hinged armature. Movement of the armature and, therefore, the point at which contact opening and closing takes place is controlled by a spring whlch is pre-adjusted to the required voltage setting, When the generator starts operating, the contacts of both regulators remain closed so that a positive sup ply can flow through the generator shunt·field winding to provide the necessary excitation for .raising the generator output. At the same time current passes through the shunt winding of the voltage regulator and, in conjunction with the series winding, it Increases the regulator's electromagnetic field . As soon as the generator output voltage reaches the pre,adjusted regulator setting, the electromagnetic field becomes strong enough to oppose the tension of the am1ature spring thereby opening the contacts. In this equilibrium position, the circuit to the series winding Is opened causing its field to collapse. At the same time, the supply to the generator field winding passes through a resistance (R) which reduces the excitation current and, therefore, the generator output voltage. The reduced output in turn reduces the magnetic strength of the regulator shunt wincling so that spring tension closes the contacts again to restore the generator out· put voltage to its regulated value and to cause the foregoing operating cycle to be repeated. The frequency of operation depends on the electrical load carried by the generator; a typical range is between SO to 200 times a seco nd. In regulators designed for use with twin·generator systems , a third coil is also wound on the electro· magnet core for paralleling purposes (seep. 16) and is connected to sepatate paralleling relays , Cun-ent Regulator Thi s unit linuts generator current output In exactly the same way as the voltage regulator controls voltage output, i.e. by controlling generator field~xcitation current. Its construction differs only by virtue of having a single winding of a few turns of heavy wire. When electrical load demands are heavy, the voltage output value of the generator may not increase suffic. iently to cause the voltage regulator to open its con· tact s. Conseqi•P.ntly, the output will continue to increase until It reaches rated maximum current, this being the value for which the current regulator is set. At this setting, the current flowing through the regu · lator winding establishes a strong enough electro- magnetic field to attract the armature and so open the contacts. Thu s, it is the current regulator which now inserts resistance R in the generator shunt-field circuit to reduce generator output. As soon as there is sufficien drop in output the field produced by the regulator winding is overcome by spring tension, the contacts close and the cycle again repeated at a frequen cy similar to that of the voltage regulator, Carbon Pi_/e Regulator Carbon has a granular surface and the contact re sista nce between two carbon faces that are held together depends not onl y on the actual area of contact, but also on the pressure with which the two faces are held together. if. therefore, a number of carbon discs or washers are arranged in the form of a pile and connected in series with the shunt field of a generator (see Fig. 1.14) the field circuit resis tance can be varied by increasing or decreasing the pressure applied to the ends of the pile and changes in genera- tor output voltage therefore counteracted. Since this method eliminates the use of vibrating co n tacts, it is applied to generators c-apa blc of high current output, and requiring higher field excitation current. The ne ce ssary variation of pile pressure or compression under varying conditions of generator speed and load, is made through the medium of an electromagnet and spring-controlled armature which operate in a similar manner to those of a vibrating contact regulator. Under static conditions of the generator system, the carbon pile is fuUy co mpressed and since there is no magnetic ''puU" on th e armature, the resis tanc e in th e generator shunt. field circuit is minimum ai1d the air gap between the regu lator armature and ele ctro· magnet core is maximum. As th e generator starts operating, th e progre ssive ly increasing output voltage is applied to the regulator coil and the resulting field establishes an increasing "puU " on the arma ture. During the initial "r un-up " stages, the co mb i nat io n of low voltage applied to the regulat or coil, and the maximum air gap betw ee n arma ture an d core, results in a very weak force of attraction being exerted on the armature. This force is far smaller than that of the spring control, henc e the armature maintains its original position and continues to hold the carb on pile in th e fully compressed co ndition; the shunt-field circuit re sis tance is thus maintained at minimum value during run-up to allow generator output voltage to build up as rapidly as possible. This condition continues un- altered until the voltage ha s ri se n to the regulated value, and at which equilibrium is established between magnetic force and spring-control force. The armature ls free to move towards the electromagnet co re if the force of magnetic attraction is increased as a result of any increase in generator speed within the effective 13 ~fiLPilo- pressure Sl\unl field wln.dlno Arma re I.OO<I Spnng oonlrol Fig 1.14 Carbon pile voltage regulation speed range. Ln these circumstances pil e compression is further reduced so that there is more air.space between discs to increase resistance and so check a rise in generator output voltage; it also increases the spring loading that holds the armature away from the core. Thus, a condition of equilibrium is re-established with the armature in some new position, but with the output voltage still at the required regulated value. Any reduction of generator speed, within the effective speed range, produces 11 reduction in gene ra- tor output voltage thus disturbing regula tor armature equilibrium in such a manner that the spring-con trol force predominates and the armature moves away from the electromagnet core. The carbon pile is re· comp r essed by this movement to reduce the generator shunt-fie ld circuit resistance and thereby increase generator output voltage, until the regulated output ls again brought fo a state of equilib rium. When progressive reduction of genera tor speed results In a co nclitfon of maximum pile compression, control of generator output vijlt age Is lost; any further reduction of generator speed, below tho lower limit of the effective range, resulting in proportional decrease in output voltage . When a generator has been run up and connected to its distribution busbar system, the switching on of various requisite consume r services, will Impose loads which disturb the equilibrium of the regulator armature. The effect is, in fact, the same as if the generator speed had bee n reduced, and the regulator automatically takes the appropriate corrective action until the output voltage is stabilized at the critical value, Conversely, a perceptible decrease in load, assuming generator speed to be constant and the regulator armature to be in equilibrium, results in the regulator taking the same action as in the case of an increase in generator speed. Construction The pile unit is housed within a ceramic tube which, in turn, is enclosed in a solid casing, or more generally, a finned casing for dissipating the heat generated by the pile. The numb er, diameter, and thickness of the washers which make up the pile, vari es according to the specific role of the regula tor. Contact at each end of the pile is made by carbon inserts, or in some types of regulator by silver contacts within ca rbon inserts. The initial pressure of the pile is set by a compression screw acting through the pile on the armature and plate-type control spring 14 whlch is supported on a bi-metal washer. The washer compensates for temperature effects on voltage coil resistan ce and on any expansion characteristics of the regulator, thus maintaining consta nt pile compression. The electromagnet assembly comprises a cylindrical yoke in which is housed the voltage coil, a detachable end -plate and an adjustable soft-iron core . A lo cking device, usually in the form of screws, is provided to retain th e core in a pre-set position. Depending on the design of generating sys tem, voltage regulators may be of the single-unit type, shown in Fig. 1.1 5, which operates in conjunction with separate reverse current cut-outs, voltage differential sensing relays _and paralleling relays, or integrated with these components to fonn special control units or panels. 5 Fig 1, 15 Typlc::111 slng!e, unll type regulator I . Armature stop sc rew 2. Magnet c use 3. Ilea\ dlssipator 4. Tcrrninal blo cks 5. Chassis Fig. 1.16 shows the circuit afrangement of a typica solid-state voltage regulator as employed with the type of alternator shown in Fig , 1,8. Before going into it s operation,.however, it will be helpful at this stage, to briefly review the primary function and fundamental characteristics of the device known as the transistor. The primary function of a transistor is to " transfe r resistance" within itself and depending on its con- nection within a circuit it can turn current "on'' and "off' and can increase output signal c.onditions; In other words, it can act as an automatic switching device or as an amplifier. It has llO moving parts and is mad e up of three regio ns of a certain material, usually germanium, known as a semiconductor (see also p. 53 ; and arranged to be In contact with each other in some definite conducting se qu ence . Some typical transistor contact arrangements are shown In Fig . 1.17 together with the symbols used. The letters "p'' and "n" refer to the co nductiv ity chara cteristic of the germani um and signify positive-type and negative-type respectively. A transistor has three external co nne c. lions corresponding to the thiee regions or elements known as the emitter whlch injects the curren t carrien at one end, the collector which collects the current at the other end, and the base which controls the amoun of current flow. The three elements are axranged to contact each other in sandwich form and in th e sequence of either n•p·n or p-n -p. When connected in a circuit the emitter is always forward-biased in order to propel the charged current carriers toward.s the collector, which is always reverse-bia sed in order to coll ect the carriers . Thus, the emi tter of an n·p·n tran- sistor has a negative voltage applied to it (with re spect to base) so as to repel negative electrons in the forwar , direction, while a positive voltage is applied to the emitter of a p-n -p tr an sis tor so as to repel positively charged "holes" in a forward direction. Sin ce reverse bias is always applied to coll ecto rs then the collec t or of an n·p ·n tJansistor is made positive with respect to the emit ter In order to attract negative electrons. Similarly, the colle ctor of a p-n-p tramlstor Is made negative with re spe ct to the emitter so as to attract positively charged "holes". The conventional current flow is, of cou.rse, opposi to the electron flow and passes through a transistor and tho circuit external to it , from emitter to coll ec to1 and through the base. Th1s is inclicated on the symbofa adopted for both transistor arrangements, by arrows on the emitter (see Fig. 1.17). Any input voltage th at in creases the forward bias of the emitter, with respect ,iI Alternolor .------ -I + -- - N Em,ll er l ! - Volloge regulotOI ~-- I + -- z - Reverse currant - tn th is section blo ck eel by ' z' unl1l breakdown occurs TRI = Bos e Fig 1.16 Solld~iaie voltage regulator - -+ Boitery cu rrent - ~ectifi ed curre!lt = Re v erse current To services ! l l l t I I 15 .Alternator switch - • - Elee1,on l low G,' i10 1es' - --cur renl ll ow to the base, increases the em itter•to-collector current flow, and conversely, the current flow is decreased when an input voltage decreases forward bias . The characteristics of transistors are such that small changes-in the emitter-base circuit current result in relatively large changes in collector curren t thereby making transistors efficient amplifying devices. By alternately connecting and disc on necting the base circuit to and from a forward biasing voltage , or similarly, by alternately applying a fo rward and reverse voltage, base current and th~ collec tor current, can be ca used to flow and to cease Oowing . In this manner, a transistor can thereby also function as a switc hing device. •-II - •- l··t •I I I t I ln the regulator circuit shown in Fig. 1.16 , the three transistors (TR 1, TR 1 and TR 3) are connected Ern ,tter b1 os Colleelor b,os Em111cr b•OS Colleclor b,os in the n-p-n arra11gement. When the system co ntrol switch is "on", excitation current flows initiall y from the battery to the bas e of TR2 and through a voltage dividing network made μa of resistan ces R1, R, and RV 1. The purp ose of this network In conjunction with the Zener diode ''Z" (see also p. 55) is to establish P ig 1.17 Transistor contact a rrangoments the system-operating voltage. With power applied lo the base ofT R1 , the transistor is switched on and 16 battery current flows to the collector and emitter junction. The amplified output in the emitter circuit nows lo the base of TR3 thereby switching it on so that the batt ery cu rrent supplied to the field winding can be conducted to ground via the colle ctor-emitter junction of TR), When the generator is running, the rotating magnetic field induces an alternating current in the stator and this is rectified and supplied to the d.c . power system of the aircraft. When the alternator output voltage reaches the pro. set operating value, the current flowing in the reverse direction through the Zener diode causes it to break down and to al.low the cu rr en t to flow to the base of TR 1 thus switching it on. The collector-emitter junc· lion ofTR 1 now conducb, thereby diverting current away from the base of TRz and switching it off. This action, in turn, switches offTR3 and so excitation current to the alternator field winding is cut off. The rectifie.r across the field winding (D 1) provides a path so that field current can fall at a slower rate and thus prevent generation of a high voltage at TR) each time it is switched off, When the alternator output voltage falls to a value which permits the Zener diode to cease co ndu ction, TR 1 will again conduct to restore excitation current to the Oeld winding , This sequence of operation is repeated and the alternator output voltage is thereby maintained al the preset operating value. Paralleling and Load-Sharing In m ult i-engined aircraft, it is generally desirable that the genera to rs driven by each engine should operate in parallel the reby ensuring that in the event of an engino or generator failure, there is no interruption of primary power supply. Parallel operation requires that generators carry equal shares of the sys tem load, and so their output voltages must be as near equal as possi ble under a11 operating conditions. As we have already learned, generators are provid ed with a volt age reg ulator which exercises independent control over voltage output, but as variations in output and elec trical loads can occur, it is essen t ial to provide additional voltage regu lation circ uits having the function of maintaining balanced outputs and load sharing , The method most commonly adopted for this purpose Is that which employs a ''load -equalizing circ1,1i t" "to control generator output via the voltage regulators, The principle as applied to a twin- ge nerator system is illustrated in much-simplified form by Fig. 1.18. The generators arc interconnected on their negative sides, via a series "load-sharing" or ''equalizing" Joop co ntain ing equalizing coils (Ce) ea coil forming part of the Individual voltage regulator circuits, R, Col I ''j (b l Equolizinq current r0 - Fig 1.1 8 l'rindplc of load·sharing .I L in13 i;,QO IDCI Of "> - I I I ~ !11 1rq , aJo ys Fig 1.19 =>- Voltage cool curr _, Eovoli zir,g cu rrc ~ VO lloq;i ccal turr cn1 IIIIIIQo EquQI if'ln(] c;urr Loa d sh ari ng (c31hon pile regulators) The resis t ances R 1 and R1 represe nt th e res istan ces of the nega tiv e seclions (lnterpolc windings) of the generators, and under balanced loa d- shari ng co ndilic the vo lts drop acros s each section will he the same , Busbar No 1 Vo lt age regulator No. 2 Voltage r egulator · ---- ---- ·----- I IC - I I i:t ..AAA/\,, I I -=- I • : II , .!Jr r,~ t-i - - ---- -+--+--- Paralleling relay un it 1--- ----- --- , Field FI I A excitation~ current '--- Ai I f Field ~excitation current -=- Fig l.20 Load sharing {vibrating co.nlact regulators} --.) 18 i.e . V1 .. 11 R1 and Y2 - 12 R1 • Thus, the net volts drop will be zero and so no current will now through the equal i zing coils . Let us now assume that generator No. I lends to take a somewhat larger share of the total load than generator No. 2. In this condition the volts drop V1 will now be greater than Yi and so the negative section of generator No. I will be at a lower potential. As a re-sull, a current le will flow through the equaliz- ing coils which are connected in such a manner tlrnt the effect of 10 is to raise the outpu t voltage of genera- tor No. 2 and reduce that of No . I, thereby effect- ively reducing the unbalance in load sharing . Figure 1.19 illustrates the principle as applied to an equalizing circuit which approximates to that of a practical generating system utilizing carbon pile volt- age regulators. The equalizing coils are wound on the same magnetic co res as the vo l tage coils of the regula- tors, thus , assuming the same unbalan ce d conditions as before, the current le flows in a direction opposite to that flowing through the No . 2 generator vo lt age regulator coil, but in the same direction as the voltage coil current in No . I regulator. The magnetic effect of the No. 2 regulator voltage coil wiU therefore be weakened resulting in a decrease in carbon pile resis- tance and an increase in the output ·or No. 2 generator (see also p. 12), enabling it to take more of the load. The magnetic effect of the No. I regulator voltage coil on the other hand, is strengt hened, thereby increasing carbon pile resis t ance and causing No . .1 generator to decrease its output and t'o shed some of its load. The variations in output of each generator continues until the balanced load . sharing co ndition is once ag ain restored, whereby the equaliz ing- circuit loop ceases to carry current. The principle of paralleling as applied to a twin d.c. generator system utilizing vibra ting contact regulato rs is shown in rig. 1.20. In this case, the equalizing or paralleling circuit comp ri ses an additional coil "Eq'' in the voltage regulation sections "A" of each regulator, and a paralleling relay unit. When both generators are in operation and su pply- ing the requisite regulated voltage , the co ntacts in th e voltage and current (''B") regulation sec ti o ns of each regulator are closed. The contacts of lh e rever se current relays "C" are also closed thereby connecting both generators to the .bu sbar. The ovtputs from eac h generator are also supp li ed to the coils of the paralleling re lay unit and so the contacts of its rela ys are closed. Thus, together with each of the coils ''Eq", the equalizing or paralleling circuit is fom1ed between the generator outputs. Under load-sharing conditions, the current flowing through th e coils "Ecf' is in the same direction as that through the voltage coi ls of the voltage regulating sections of each regulator, but in equal and opposite directions at the contacts of the paralleling relay unit. If the voltage output of o ne or other generator, e. g. number l. should rise, there will be a greater voltage input to the voltage regulatLng section of the number I voltage regulator compared to the input at th e corresponding section of the nu11lber 2 regulator . Ther e will therefore, be an unbalanced Oow of current through the equalizing circuit such that the increase in current through the coll ''Ee( of the number I voltage regulator will now assist the magnetic effect of the voltage coil "D" causin~ the relay contacts to open. The resistance thereby inserted in the {lc ld circuit of number J ge nera tor redu ces its excitat ion current and its voltage output. Because of the unbalanced condition, the Increased current in the equalizing circuit will also flow across the paralleling relay unit contacts to the coil "Eq '' in the number 2 voltage regulator so that it opposes the magnetic effect of its associated coil "D" . In paralleled alternator systems using solid -s tate voltage regulators, any unbalanced condition is detected and adjusted by interconnecting the regulators via tw o addition al paraUeling transistors, o ne in each regu lat or. Batteries In almost all aircraft electrical systems a battery has the following principal functions - (i) To help maintain the d.c. system voltage under t ransient con ditions. The starting of la.1ge d.c . motor- driven ac cesso ries , such as inverters and pumps, requires high input current which would lower the busbar volt. age momentarily unle ss the battery was available to assume a share of the load . A similar condition exists sho uld a short circuit develop in a circuit protected by a heavy duty circuit breaker or current limiter. This function possibly applies to a lesser degree on aircraft wh ere the electrical system is predominantly a.c., but the baste principle 'still holds true. (ii) To ,upply power for short term heavy loads when generator or ground power is not availabl e, e.g. internal starting of an engine. (ill) Under emergency conditions, a battery is intended to supply limited amounts of power. Under these conditions the battery could be the sole remain - ing source of power to operate essential flight instru- ments, radio comm unication equipment, etc., for as long as the capacity of the battery allows. A battery Is a device which converts chemical energy into electrtcal energy and is made up of a num- ber of cells which, depending on battery utilization, may be of the primary type or secondary type. Both types of cell operate on the same fundamental principle, i.e . the exchange of electrons due to the chemical action of an electrolyte and electrode materials. The essential differences between the two lies in the action that occurs during dJschargc . In the primary cell this action destroys the active materials of the cell, thus limiting its effective life to a single discharge operation, whereas in the secondary cell the discharge action co n- verts the active material into other forms, from which they can subsequently be electrically reconverted, into the original materials, Thus, a secondary cell can have a life of numerous discharge actions. followed by the action of re -c onversion more commonly known as charging. The batteries selected for use in aircraft therefore employ secon dary cells and are either of the lead-acid or ni ckel-cadmium type. Lead -Acid Secondary Cell The basic construction of a typical cell is shown in Fig. 1.2 1. lt consists essentially of a posillve electr ode and a negative electrode, eac h of wh_ich is , in turn , made up of a group of lead-antimony alloy grid plates; ~!~?ri; :~~~~ ~~~ ne gative plot! qr0up1 1nterloc~ed . Plgte ;cporators ore nohhown Po61t lve plalo Q~ Fig 1.21 Typical lea d-a ci d seco ndary ce ll t,/e9ot iw pJgte QrOup 19 the spaces of the plates are packed with pastes of acUve lead materials. The two plate groups are inter- leaved so that both sides of every positive plate face a negative surface. The plates a.re prevented from corn- ing into contact with one another by means of separators (not shown) made from materials having high insulating qualities and ability to permit un - obstructed circulation of the electrolyte at the plate surfaces. Each group of positive plates and negative plates is connected through a strap to a terminal post at Lhe top and on opposite sides of the cell , TI1e inter- nal resistance of a cell varies immensely with the dis- tance between the positive and negative electrode sur· faces; therefor e, to obtain the lowest possible resis tan ce th e gap between the plates of each group is made as small. as is practicable. A cell contains an odd numb er of plates, the outermost ones belonging to the negative plate group. The reason for this arrangement is that unlike a positive plate a negative plate will not distort when the electromechanical action is restricted to one side only. The pl ate assem blies of a cell are supported in an acid-proof container. CHEMICAL ACTION Each positive plate of a fully-charged coll consists of the lead-an timony alloy grid into which lead peroxide pa ste (PbO~) has been forced under pre ss ure. The negative plates are of similar basic structure, but with pure spongy lea d (Pb) forced in to Lhe grid . The electro· Jyt e consists of two cons tituent s, sulphuric acid (HiS04 ) and water, which are mixed in such prop- ortions that the relative den sity is generally about 1-25 to l ·27. During discharge of the ce ll, that is, when an ex ternal circ uit is co mpleted between the pos itive and negative plates, elec trons are transferred through the circuit from lead to lead peroxide and the net result of the chemica) reaction is that lead su lph ate (PbS04) forms on both plates. At lh e same time molecules of water are formed, thu s weakening the electrolyte. For all prac t ic al purposes, the ce ll is considered to be dis- cha rged when both pl ates are covered with lead sul- phate and the el ec trolyte has become qui te we:lk . The cell may be recharged by connecting the positive and nega tive plates, respectively, to the positive and negative terminals of a d.c. source of slightly higher vo lt age than the cell . All the fore - going reactions are th en rev ersed ; th e lead sulphate on the po sitive plate being restored to lead peroxide, th e negative plate restored to spongy lead , and the electro- lyt e res t ored to its origina l relative densit y. 20 TYPICAL LEAD -ACID BATTERIES Two types of lead-acid battery may be found in general uso; in one the electrolyte is a free liquid while in the other it is completely absorbed into the plates and separators. An example of the former type of battery is illustrated in Fig. 1.22. The unit has a 24· volts output and consists of two 12-volt cell blocks moulded in high-impact plastic material and housed in iUl acid-proofed aluminium container. The links inter- connecting the cells and cell blocks are sealed and suitably insulated to prevent contact with the container. A plastic tray is fitted on to the top edges of the con- tainer and is sealed around the cell vent plugs by rubber pads, and plastic sealing rings. Tho tray forms the base of a chamber for tho ventilation of acid vapours. A plastic lid combined with an acid-proofed V8nt pl,.,q Qnd wD"'10r 0 Fig 1.22 Lead-ac id baltery (free liquid type) aluminium alloy hold-down frame completely encloses the chamber, Connections are provided at each end of the chamber for co upling the pipes from the aircraft's battery compartment ventilation system (seep. 24). The battery illustrated in Fig. 1.23 utilizes a more specialized form of cell constru~llon than that just described. The plates, aclive mat erials and separators are assembled together and arc compressed to form a solid block. The active material is an infusorlal earth, known as kieselguhr, and is very porous and absor- bent. Thus, when the electrolyte is added, instead of remaining free as in the conventional types of battery, it Is completely absorbed by the active material. This has a number of advantages; notably improved electro- mechanical activity, no di sintegration or shedding of active material, thus preventing internal short-circuits caused by "sludge", low internal resistance and a higher capacity/weight ratio than a conventional battery of comparable capacity. The cells are assembled as two 12-volt unit~ in monobloc containers made of shock-resistanl poly- styrene and these are, in turn, housed in a polyester· bonded fibreglass outer container whlch also supports the main terminal box. A cover of the same material as the case is secured by four bolts on the end flanges of the case. Nickel-Cadmium Secondary Cell ln this type of ce ll the positive plates are composed of nickel hydroxide, Ni(OHh, the negative plat es of cadmi um hydroxide Cd(OHh and the electrolyte is a solution of di st illed water an d potass ium hydro xide (KOH) with a relative dens ily of from l •24 to l ·3 0. Batteries made up of tht:s e ce ll s have a numb er of advantages over the lead-acid type, the mo st notable bein g their ability to maintain a relatively steady volt - age when being dis charged at high cu(rents such as during engine starting. The plates are generally mad e up by a sintering process and the aclive materials are impregnated into the pl ates by chemical deposition. This type of co n. struction a1J ows the maximum amount of active -material to be emplo ye d in the el ec tr oc hemical action After impregnation with the active materials, the plates are stamped out to th e requisite size and are built up Into positive and negative plate groups, in te r· leaved and connected to terminal posts in a manner somewhat similar to th e le ad- acid type of cell. Connon .,. pluq T@rm 1nol 1nsu loto, COM1111 roee otocl e/ i,ner'-", nq 2 eon roc t ~ioove1l 21 Conne<: tor bd r / Containet Fig 1,23 Lca.d11cid battery (11bsorbed liquid type) Insulation is done by means of a fabric-base separator in the form of a continuous strip wound between the plates, The complete plate group is mounted in a sealed plastic container. CHEMICAL ACTlON During charging, the negative plates lose oxygen and become metallic cadmium . The posi t ive plates Bie brought to a higher state of oxidation by the charging current until both materials are completely converted; i.e . all the oxygen is driven out of the negative plates and only cadmium remains, the positive plates pick up the oxygen Lo form ni cke l oxides. The cell emi ts gas towards the end of the charging proc ess, and du ring overcharging; the gas being caused by decomposition of the wa l er component of t he electrolyte into h ydroge n at the negative plat es and oxygen at the positive plate s. A slight amo unt of gassing is necessary to co mpletely charge the cell and so it therefore loses a certain amount of water. The reverse chemical action takes place during di s- chargi ng, the negative plates. gradually gaJ.ning back the oxygen as t he positive pla t os lose it. Due to this interchange there is no gas sing on a normal llischarge. In this way, the chemical energy of the plates is con . verted into electrical energy, and the electrolyte is absorbed by the plates to a poin t wh ere it is not visible from the top of the cell. The electrolyte docs not play an active part in the chemical reaction; it Is used only to provide a path for current flow. The chemical reaction of a nickel-cadmium cell is summarized in Table l .l and may be compared with th at t aki ng place in a lea d-acid battery cell, TYPICAL NICKE1,.CADMIUM BATIERY The const ru c tion of a typical battery currently in use is shown in Fig . 1.24. All the cells are linked and con- tained as a rigid as sembly in the case, A space above the cells provides a ventilation chamber whjch is completely enclosed by a lid held in position b.y a pair of bolts anchored to the aircraft battery com· partment. Acid vapours are drawn out from the chamber via t he vents in the battery case and the interconnecting pipes of the wrcraft's battery com· partment ventilation system. Capacity of Ba tteries The capacity of a battery, or the total amount of energy available, depends upon th e size and number of plate s. More strictly it Is related to the amount of material available for chemical action. The capacity rating is measured in ampere-hours and is based on the·maxirnum current, in amps, which it will deliver for a krtown time period, until it is dis- 22 Corr yino ho~dle VPnl n•po ' Batt11n' Type Sta te of charge ( chaiged Lead- Ac id dischar ged ( C)laJ"ged Nl ckel-Cadm.lum dis charged Fig. 1, :2.4 Nick el-c admium type battery Table 1.1 Chemical Reactions of Batteri es Po si tive Plate Ncga tiv o Pia te PbO , Pb (L ea d Dioxide) (Lead) PbSO, PbS0 4 (Lead Sulphate) (Lead Sulphate) Ni,O, 11n d Nl,0 1 Cd (Nickel Ox1des) (Cadmium) Cd(OH), Nl(OHh {Nickol Hydrol<idc) (Cadmium Hydtoxlde) ) Mo•n bo t lery ' 1;.onnet tor Electrolyte tt,so. Coni;entrnted Sulphuxic Ad tt~so. Weak Sulphuric Acid KOH (Pota S! lum hydr oxl< unaffected by state of cha charged lo a permissible minimum voltage of each cell . The lime taken to discharge is called the discharge rat e and the rated capacity of the battery is the pro- duct of this rate and the duration of discharge (in hour s). Thus , a battery which discharges 7 A for S hours is rated at 35 ampere-ht>urs capacity. Some typical discharge rates of lead-acid and nickel· cadmium batteries are shown in Fig. 1.25. Ampere· hour5 Fig 1.25 Typical d.isc harge rates of lcad •acid and nickel -cadmium batteries STATE OF CHARGE All ballcri es display ce rtain Indications of their state of cha rge, and these are of practical help in maintain- ing operating conditions. When a lead-acid battery is in the fully .charged condition each cell displays three distinct Indicati ons: the terminal voltage reaches its maximum val ue and remains steady; the relative de nsity of lhe electrolyte cease s to rise and remains constant; the plates gas freely. The relative densit y is the so le reliable gui de to the ele ct rical co ndition of the cell of a battery which is neither fully charged nor yet co mpletely dis cha rged. If the relative density is midw ay betw ee n the norm al maximum and minimum values then a cell is ap proxi- mately half discharged . Checks on the relative densily of batteries which do not co ntain free electrolyte cannot be made; the state of charge being assessed o nl y fr om voltage indications. As we have already learned (see p. 21), the electrolyte in the ce ll s of a ni cke l- cadmiu m battery does not chemically react with the plate s as the electrolyte does In a lead-acid battery. Consequently, the plates do no t deteriorate, nor docs the relative de nsity of the electrolyte appreciably chan~e. For this rea so n, it is not possible to detennine the sta le of 23 charge by checking the relative density. Neither can the charge be determined by a voltage test becau se of the inherent characteristic that the voltage remains constant over a major part of the discharge cycle, The only possible check that a battery Is f\Jlly charged is the battery voltage when "on-chargeu: add itio nall y, the electrolyte should be at maximum level under thes e conditions. Fom1 alion of white crystals of potassium carbonate on a prope rly serviced nick el-cadm ium battery installed in an aircraft may indicate that the battery is being overcharged. The crystals form as a result of the reaction of expelled ele ctrolyte vapour with carbo11 dioxide. THERMAL RUNAWAY Batteries are capable of performing to the ir rated cap acities when the temperature conditions and charg- ing rat es are within the value s specified. In the eve nt that the se are ex cee ded "thermal runaway" ca n occur, co ndition which ca uses vio lent gassing, boi ling of the electrolyte and finally melting of the plates and casing, with consequent danger to th e aircraft structure and jeopardy of the electrical system , Since batteries have low thexmal capacity heat can be di ss ipated and this resu l ts in lowering of the effecl· ive internal resistance. Thu s, when associated wi th co nstant vo ltage ch arging, a battery will draw a higher charging current and thereby se t up the "runaway" co nditi on of ever-increasing charging currents and tem pe ra tu res. · · ln some ai rcraft, parlicul arly those employing ni cke l- cadmi um batteries, temperature -s ensing dev ices are located within the batteries to provide a warning of high ba ttery te mpe ratures and to prevent overcharging by disconnecting the batter ie s from the charging sou rce at a predetermined temp era ture (see also p. 29). LOC ATION OF BATTERIE S IN AN AIRCRAFT Depending on the size of aircraft and on the powe r requ ir ements for the operation of essential se rvices un der emergency co nd it jons, a single battery or several batterie s may be provided. When several batteries are emp loyed they are, most often, co n- nected in parallel alth ou gh in some types of aircraft a series co nn ect ion is used, o.g. tw o 14-volt batteries in series, while in others a switching arr angement is 24 (o) Fig. 1,26 Typical battery installations (bl in corpo rated fo r changing from one method of con· ne ction to the other. Batteries are installed in individual compartments specially d esi gned and lo ca ted to provide ad equate heat dissipation, ventilation of gases and protection of ai rframe struc ture against corrosive elements. At the same time batteries should be lo cated es nea r to th e main and battery busbars as physically p ossibl e in order to avoid the use of Lon g leads and con se quent high resistance. Batteries are normally mounted on, and clamped to, a tray secure d to the aircraft structure. Th e tray forms a catchment for any acid which may escape from the battery. Trays may be of any material which is acid -p roo f, non-absorbent and resi stant to 1easo nable impacts. Many reinfor ce d pla stics are suit - able but metal trays are, on the whole, undesirable. Where metal t rays ar e unav oid able they are treated with an anti-co rro sive pai nt or, in some cases , spraye d or coated with p.v. c. The structure under and around the battery area is also tr ea ted to avoid co rr osive att ack by acid fumes and spray. Batteries are secure ly clamped and anchored to their structure to prevent th eir being torn loose in th e even t of a crash landin g, thus minimi zing the risk of fire. Two typic al batt ery in stallations are illustrated In Fig . 1.26 . Venting of batteri es and battery compar tm en ts may t ake various forms si nce il depends lar ge ly on th e installation re quired for a particular ty pe of air craft. Rubber or other non •c orrosive pipes are usually employed as vent lines which t erm inate at ports in the fuselage skin so that the airflow over it draw s air th.rough the pipes by a venturi acti on. In some cases, acid traps, in the form of polythene bottles, are in serted in the lines to prevent acid spra y being ejected on to the outer-skin of the aircraft. In the installation shown in Fig . 1.26(b) fumes and gases generated by the batte ry are extracted by the difference of pressure existing across the aircraft. burlng normal light air tapped fr om the cabin pr ess urization system enters the batt ery ventilation ch amb er and continues through to t he outside of the air craft . On the ground, when no pressu re different ial exists, a non -return valve fitted in the air inlet pre vents fumes and gase s from esc aping into the air craft. Th ese typic al ven ting arr angemen ts are illu strated schemati c· ally in Fig. 1.27 . F ig l .Z7 Battery venting arrangeme nts BATTERY CONNECTIONS The method of connecting batteries to their respective busbars or power distribution points, depends largely on the type of battery employed, and on the aircraft's electrical system. In some cases, usually on the smaller types of aircraft, the connecting leads are provided With forked lugs which fit on to the appropriate battery terminals. However , the method most conunonly employed is the plug and socket type connector shown in Fi g. 1.28. It provides better con- nec tion and, furthermore, shields the battery te rminals and cable terminations. ~1 s 1,,01etywu; ~ ~olu 0 .12~ d 111 ~Coble B851 1of ry Fig. 1.28 Battery plug connector The socket comprises a plastic housing, incorporated as an integral part of the battery, two shrouded plug pins and the female threaded portion of a quick-start duead lead -screw , The plu g consists of a pl astic housing incorporating two sruouded spr ing.loaded sockets and terminals for the connection of battery leads , and the male half of the mating lea d-screw operated by a handwheel. The two halv es, on being engaged, are pulled into position by the lead- screw which th ereafter acts as a lo ck. Reverse rotation of the handwheel separa tes the connector sm oothly with very little effort. In fhis way high contact pre ss ur es and low resistance connections are possible and are consistently maintained. 25 TYPICAL BATTERY SYSTEMS Figure 1.29 shows the circuit arrangement for a battery system which is employed in a current type of turboprop airliner ; the circuit serves as a general guide to the methods adopted. Four batterie s, in parallel are directly connected to a battery busbar which, in the event of an emergen cy, supplies power for a limited period to essential consumer services, i.e. radio, fire - warning and extinguishing systems, a compass system, etc. Direct connections are made to ensure that battery po wer is avallable at the busbar at all times . The batteries also require to be connected to ensure that they are maintained in a charged condition. In the example illustrated this is ac co mplished by connecting the batteries to the main d.c. busbar via a battery relay, power selector switch and a reverse current circuit breaker. Under normal operating conditions of the d.c. supply system, the power selector switch is set to the "battery" position (In some aircraft this may be termed the "flight" position) and, as will be noted, current flows from the batteries through the coil of the battery relay, the switch, and t hen to ground via the reverse cunent circuit breaker contacts. The current flow through the relay coil energizes H, causing the contacts to close thereby connecti ng th e batt er ies to the main busbar via the coil and second se t of contacts of the reverse current circu it breaker. The d.c. se rvic es connected to the mai.n busbar are supplied by the generators and so the batteries will al so be supplied with charging current from this source. Under emer gency con ditions, e.g. a failure of the. generator supply or main bu sbar occurs, the batteries mu st be isolated from the main busbar sin ce their total capac ity is not sufficient to keep all services in operation. The power selector switch must therefore be put to the "ofr' position, thus de -energi z ing the battery rela y. The batteries then supply the essential services for the lime period pre-calculated on th e basis of battery capacity and current consumption of the esse ntial services. Th e reverse current circuit breaker in the sys tem shown is of the electromagnetic type and its purpose is to protect the batteries against heavy current flow from the main busbar. Should thi s happen the current reverses the magnetic field ca using the normally closed cont act s to open and thereby interrupt the circuit between the batt eries and main bu sbar, and the battery relay co il circuit. 26 Mainbusbor To oe11ert1tor ,y,lem , o'>do ll dc.>trvicH -...--...- Toe~nri1'°1 9ttV1Cfi _.__Battery bulOP• Mo ;•~ul 4 llo ll 1,ybui \o'o llmeter selfctor sw1 tcti V Revene current C/ B ·-~~.: : - -~ ,...,.~ To l..(1etnol ----0 ~, BoJrery i• J power Cireuil - - - -; I and ground Ofl po ...1r p1u9 L----_, ---- Wfrtnt flo,.,r ftQm boller1e~ • • · - C"1:0 t9•rtQ c;:ummt flow from qenerolor1 r- _.. "'\ , - - - , r- -- , ' T' ' -r-1 1 ; 1 : . : l l : : 1 : I I I I : I I I ' : l : j : : : ; : ~ -l.; ~ -1 ~~ --r-.J 1 7 Bo11tt•H r- - , I I j i : I I l 1 t : I I ...l- : L .. J - ...A Fig 1.29 Typlc.il batter y sys te m circ uit The battery system in some types of turboprop powered aircraft is so designed that the batteries may be switched from a parallel configuration to a series co nfiguration for the purpose of starting an engine from the batterie s. The circuit arrangement of one such sys tem using two 24-volt nickel-cadmium batteri es is shown in simplified form in Fig. 1".30. Under normal parallel operating conditions, battery l is connec ted to the battery busbar via its own battery relay , and also contacts 1a-1b of a battery switching relay. Battery 2 is directly co nnected to t he busbar via its relay . When it is ne cessary to use the batteries for starting an engine, i.e . to make an ''internal" start, both batteries are first connected ~o the ba ttery busbar in the normal way, and the 24-volt supply is fed to the starter circuit switch from the busbar. Closing of the starter switch energizes the correspond- ing starter relay, and at the same time the 24-volt supply is fed via th~ starting circuit, to the coil of the battery switching relay thereby energizing it. Contacts 1a-1 b of the relay are now opened to interrupt the direct connection between battery 1 and the busbar. Contacts 3a·3b are also opened to interrupt the grounded side of battery 2. However , since contacts 2a-2b of the switching relay are simultaneously moved to the closed position, they connect both batteries in series so that 48 volts is supplied to the busbar and to the starter motor. After the engine has started and reached self. sustaining speed, the starter relay automatically de-energizes and the battery switching relay coil circuil is interrupted to return the batteries to their nonnal parallel circuit configuration. The power selector switches are left in the "battery" position so that when the engine-driven generator is switched onto the busbar, charging current can flow to the batteries . Banerv busbar eanel\l I Re lev 1 j Power selector iWllch E~lernal o Power Bauerv . -1 I I I 1 , I Bollery I : , ~ 1 , ....i... I L I-' 11 -- -I Bauery , - - J Relay 2 L -- - Power aoioclor ;witch o b°1ornal Power r T- 1 8auerv I • : I l ' ' ' - -- - ~ll1··1~·~ 1 I _ _ ~ __ H- Do twnwltc~ln~ , ,. i., :t• 1 raley L . ·-.1 ___ -.J "=" To ~iiolna SUitt ing Sy!;tQm Pig 1.30 Parnllel /se ries con ne ction of batt eries Battery Chargin g from External Power [n some single-engined aircraft sys tems, the battery may be charged when an external power unit is plugged into the aircraft. This is achieve d by a battery relay closing circuit connec ted across the main contacts of the relay as shown in Fig. 1.31 , With the ex ternal power connected and swi tched :in, power is ;ivailab le to the battery relay output tenninal via the closed contacts of the external power relay. At the same time, power is applied to the battery relay closing circuit via ils diode and resistor which reduces the voltage to the input side )fthe battery relay's main contacts and co il. When the battery master swftch Is selected to ·'on", sufficient current Oow s through the coil of :he battery relay to ene rgize it. The closed con ta cts ~f the re lay then allow full voltage fr om the external JOwer unit to 11ow to the battery for the p.urpose 27 of charging it. The purpose of the fuse in the closing circuit is to interrupt the charge in the event of a "shorted" battery. When the battery is being charged in this manner, the voltage and current output from the external power unit must be properly regulated . ,,....... i +-11,I I ! I II I \ . _,/ Closing circuit To rnain busbar o-r------.....-- -- -+ I I Battery relay ----- 1 1- 1 Battery I I master 'i__j switch 1 External I power 1 relay '-- -. -' Fig _l.31 Battery charging from external power "On -board" Battery Charger Units In most types of turbojet transport aircraft currently in service, the battery system incorporates a separate unit for maintaining the batteries in a st age of charge. Temperature-sensing elemen ts are also normally provided in order to automatically iso late the charging On 0 D.C. lrom mo,n T.RU .-----<>1, to--- 1~' ~~ : 6, : O.C.lr,ommo,111.R.U. 1a,: ~ ~~-><>--'---=--'-------- -----------, 1 11 °'-o----~~ Olf Emerg. po,,,-e< swi t <i'1 v, 1 I : 'Batt " ~ ~ - ~'°+ • ~ 2 A3. A2 1 0 A1 ~.-~~~~~~~~-i (); rec I from IX>tti!<ies Sotte,y !;ensirig re lays ~w, tdh A! 3~!~ {--:---11 3\PRe-te™! airren1 CIB Tror.sk><mc, rec: #dtef ""'' Bottl!f)'d'IIJ"ger 3,p Ou lpul o.c. o.c. ffe9.Jotor ).--------1 ?JWl!r s.,,ppiy Charging ame~t Temperoture 1-------------' citl-olf •II I I ~--- - -11 I I I Fig. 1. 32 ln-5itu battery charging system + AC.Relay ~<p A.C. I,--0 .._L";="' ·ou· I e, : <>;-} ~ -=-~ Tobo11 bus 0 .C.R~oy ·~ I Cho«~er I IX>' t <eloy " ery N CX> circuit whenever there is a tendency for battery over. heating to occur . The circuits of "on.board" charger units as they are generally termed, vary between aircraft types, and space lim.its description of all of them. We may however, consider two examples which highlight some of the variations to be found. The much simplified circuit shown in Fig. 1.32 is based on the system adopted for the McDonpeU Douglas DC· IO . In this particular application, the required output of 28 volts is achieved by connecting two 14.yolt batteries in series . Furthermore, and unlike the system shown in Fig . 1.29, the batteries are only connected to the battery busbar whenever the normal d,c, supply (in this case from transformer/ rectifier units) is not available. Connection to the busbar and to the charger unit Is done automatically by means of a "charger/battery" relay and by sensing relays. When power is available from the main generating system, d,c, is supplied to the battery busbar from a transformer/rectifier unit and, at Ute same time, to the coils of the sensing relays . With the relays ener- gized, lhe circuit through contacts A2·A3 is inter- ropted while the circuits through contacts Bl -B2 are made. The battery switch, whlch controls the operation of the charger/battery relay , is closed to the "batt" position wlien the main electrical power is available, and the emergency power switch is closed in the " off' position. The charger/battery relay is of th e dual type, one relay being a.c, opera ted and the other d.c. operated, The a.c. relay coil is supplied with power from one phase of the main three-phase supply te> the battery charger , and as will be noted from the diagram , the relay is energized by current passing to ground via the contacts Bl . B2 of the sensin g relays, the battery switch and the emergency switch. Energizing of the relay closes the upper set of contacts (Al-A 2) to connect the d.c, positive output from the battery char ger to the batt eries, ther eby supplyin g th em with charging current In the event of main power failure, the battery charger will beco me inoperative, the a.c. charger relay will de-energize to the centre off position, and the two sensing relays will also de-energize, thereby opening the contacts Bl ·82 and closing the contacts A2·A 3. The closin g of contacts A2 · A3 now permits a positive supply to flow direct from the battery to the coil of the d.c. battery relay, which on being energized also actuates the a.c. rel ay, the reby closing contacts B1-B2 which connect th e batteries direct to the battery 29 busbar. The function of the battery relay contacts is to connect a supply from the battery busbar to the relays of an emergency warning light circuit. The charg· ing unit converts the main three.phase supply of 115/ 200 volts a. c. into a controlled d.c. output at constant current and voltage , via a transformer and a full-wave rectifying bridge circuit made up of silicon rectifiers and silicon controlled rectifiers (see also p. 57). The charging current is limited to approximately 65 A, and in order to monitor this and the output voltage as a function of battery temperature and voltage , temper- ature-sensing elements within the batteries are connected to the S.C. R. "gates" via a temperature and reference voltage control circuit, and a logi c circuit. Thus, any tendency for overcharging and overheating to occur is checked by such a value of gate circuit current as will cause the S.C . R. to switch off the charging current supply . The second example shown in Figs. 1.33 and 1.34, is based on that used in the Boeing 737. The charger operates on 115 volt 3-phase a.c . power supplied from a ''ground service" busbar, which in turn, is normally powered from the number I generator busbar, and/or from an external power source (see page 27). Thus, the aircraft's battery is maintained in a state of charge both in flight and on the ground . In fUght the a.c . supply is routed to the charger through the relaxed contacts of a battery charger transfer relay and an APU start Interlock relay . The d.c. supply for battery charging is obtained from a t ransfo m1er-rectil1 er unit within the charger, and it maintai11S cell voltage levels in two modes of operation: high and low. Und er normal operating conditions of the aircraft's power generation sy stem , the charging level is in the high mode since as will be noted from the diagram, the mode con trol relay within the ch arger is energi:zed by a rectified output tluough the battery thermal switch, and the rela xe d cont ac ts of both the battery bus relay and th e external power select relay, Above 16 amps the charge r acts as an unregulated transformer-rectifier unit , and when th e battery has sufficient charge that the current tend s to go below 16 amps, the charging c urrent is abruptly reduc ed to zero. The current remains at zero until the battery voltage drop s below the charge voltage, at which time the charger provides the battery with a pulsed charge and the process is repeat ed. The pulsing continues until th e control circuits within the charger change th e opera tion to the low mode, approximately two 30 11