ELECTRIC GENERATOR, a machine that converts mechanical into electric power, as the result of the continuous motion of a system of electrical conductors across a magnetic field. The term "dynamo," formerly used widely to designate an electric generator or motor, is now obsolescent. In general, a gen erator consists of a field, comprising a series of alternate north and south magnetic poles spaced around a circular periphery, and of a concentric armature built of laminated steel and carrying a system of electrical conductors at its surface, together with a mechanical structure that permits the field to be revolved with respect to the armature by external means, either armature or field being held stationary in particular cases. The armature con ductors are so connected that the voltages induced by their passage across the magnetic flux from the field poles are additive. The field poles are magnetized by electric currents passing through one or more encircling coils, called the field winding.
History.—Faraday, in 1831, rotated a copper disc edgewise between the poles of a horseshoe magnet and collected a con tinuous current from it by closing a circuit through two rubbing contacts, one on the periphery and the other on the shaft of the disc. This first electric generator was a homopolar machine with radial current paths. Since its field was provided by a permanent magnet, the magnetic density was extremely low, and since the current paths were not definitely controlled, it was extremely inefficient. The next year, H. Pixii developed and constructed the first heteropolar machine, and provided it with a wire armature winding. The voltage produced was alternating, so to secure a continuous voltage he constructed the first commutator, reversing the current every half cycle. In 1845 another real advance was made when Wheatstone replaced the permanent magnet fields with electromagnets, which he made self-exciting in 1857. The introduction of the "ring-winding" of Paccinotti (186o) and Gramme (1870) solved the problem of connecting in series any number of the conductors of a multipolar dynamo, thus adding their induced voltages, while yet affording mechanical means of holding them in place on the surface of the revolving armature. It was used extensively in various forms during the next 20 years, but was finally replaced by the "barrel" or "drum" type winding of Alteneck (1871), which was a development from the earlier "shuttle" winding of Werner von Siemens (1856). The greatest defect of the ring winding is that the currents exist ing in the return conductors on the inside of the armature core produce large magnetic fluxes which greatly impair the gen erator characteristics. The barrel winding overcame the difficulty, and greatly reduced the amount of copper required, by joining the ends of conductors under opposite poles by connections across the ends of the armature. When it was found how much the magnetic densities could be increased by decreasing the air gap length between field and armature, the slotted armature was gen erally adopted. This consummated a great advance, as embedding the armature winding in slots (first proposed by Paccinotti in 1860) not only reduced the air gap length, but also reduced eddy current losses in the copper by removing it from the intense mag netic field; and made the mechanical design immensely more rugged. In order to avoid ruinous eddy current losses, due to the cyclic alternation of the magnetic flux, it was early realized that the armature iron must be laminated. Edward Weston and Edison were the first to appreciate all these factors, however, and the latter's bipolar dynamo at once raised the standard of generator efficiency from about So to the then unheard of figure of 90%. This machine had a much greater ratio of iron to copper weight than earlier generators, and had the first mica insulated com mutator. Hopkinson's paper on dynamo-electric machinery (1886) gave the first rational method of calculating generator performance, and so put designing on a solid foundation. Edi son's inventions of his bipolar dynamo in 1878, of the incan descent lamp in 1879, and the "Edison system" of central station power production in 1882, gave the first real commercial im petus to electric generator and power development, and there after it progressed rapidly. In 1881 C. F. Brush made the first "compound wound" generator, by adding an auxiliary field wind ing in series with the armature, and thus solved the problem of automatic voltage regulation of direct current generators. The invention of the carbon brush by Van Depoele in 1888 revolution ized direct current generator design by improving commutation and reducing commutator wear immensely. In the early '9os, parallel operation by means of external equalizer connections was discovered, and in 1896 Lamme invented the internal equalizer connections, which ensure an equal division of the current between parallel armature current paths, and which made really large gen erators practical. Thereafter larger and larger multipolar gen erators, directly connected to reciprocating steam engines, came into use, reaching a peak of development about 1900. Since then, the invention of the "commutating pole" and many refinements of design have greatly improved direct current generator per formance, but fundamental design features have not changed markedly. The use of direct current is now restricted to the con gested sections of large cities, and to special purposes, as for the drive of variable speed machinery, for electrolytic work, and for electric railways. Most of this power is first generated as alter nating current and then transformed to direct current by means of rotary converters, mercury arc rectifiers, or motor generator sets.
The invention of the first alternating current system of power production and distribution by Zipernowski and Deri, Gaulard and Gibbs in Europe, and by William Stanley in the United States (1885), and of the induction motor by Nikola Tesla (i888), initiated a new trend in generator development.
Steam engines and, occasionally, waterwheels were used to drive alternating current generators, or alternators, of various types.
Among the earliest was Stanley's inductor alternator with sta tionary field and armature windings, which derived their voltage from pulsations induced by a toothed rotor in a unidirectional magnetic field.
Elihu Thomson's alternator of 1878 was the forerunner of many large machines constructed similarly to direct current gen erators, except that commutators were replaced by slip-rings. This, and later most successful designs, employed relative rota tion of armature and field, utilizing a full reversal of the mag netic field in voltage production. Ultimately, the advantages of making the armature winding stationary, avoiding high voltage slip-rings, led to universal use of the modern revolving field form of synchronous alternator.
The first large steam turbine driven alternator in the United States was built by the American General Electric company and installed in Chicago in 1903. It was rated 5,000 kw., and was of the vertical shaft type.
Its immediate success led to the almost universal adoption of steam turbine driven polyphase alternators for large central sta tions, a practice which has since continued.
After a few years, turbine alternators were designed, almost entirely, with horizontal shafts and revolving fields. Improved materials and refinements in design have enabled larger and larger machines to be built, ratings up to 5o,000kw. at 13,200 volts, and 3,000 or 3,60o r.p.m. being common. One hundred sixty thousand kw. single shaft generators at 1,80o r.p.m. have been built, and voltages up to 33,00o have been used. The total installed capacity of steam generating stations in the United States was approxi mately 27,000,00o kw. in Accompanying the growth of large central stations in the cities, hydro-electric plants using waterwheel driven alternating current generators have grown proportionately.
The three phase 10o kw. Lauffen generators designed by C. E. L. Brown (1891) and the two phase 5,000 kw. Niagara generators built by the Westinghouse Company (1894), both vertical shaft machines with external revolving fields, are notable examples of early progress.
W. M. Mordey, H. F. Parshall, Elihu Thomson, B. G. Lamme, G. Kapp, Ganz, and many others contributed much to the rapidly growing art. Modern waterwheel generators are almost universally of the salient pole revolving field type, with double layer (drum) armature windings in open slots, and welded steel frame struc tures.
Typical of the very large machines supplied for modern hydro electric projects are the 77,50o kv-a., 88 r.p.m., Dnieprostroy gen erators, the 82,500 kv-a., 18o r.p.m., generators for Boulder Dam, and the io8,000 kv-a., 120 r.p.m., generators for the Grand Coulee development in the United States. Installed hydro-electric gen erating capacity in the United States in 1939 totalled approxi mately 11,000,000 kilowatts.
In the late '90s, when polyphase alternating current (a.c.) was superseding direct current (d.c.) supply, and before the steam turbine was adopted, steam reciprocating-engine-driven alterna tors were built, with ratings up to several thousand kilowatts and speeds well below 10o r.p.m. Since the advent of the internal com bustion engine, many power plants have been built with oil engine driven generators, and the high efficiency and economy of the Diesel engine has brought it under consideration for large projects. Alternating current generators for this service are similar in construction to waterwheel generators, except that the former must be provided with damper or amortisseur windings and often with extra flywheels to limit the electric oscillations set up by the pulsating torque of the engine. Increasing specialization and refinement of design have marked recent generator development. Improved insulating and ventilating methods have so increased reliability that base power generation is being concentrated in relatively few units of very large size. In the early days, alter nator voltage regulation was a difficult problem. A solution was attempted by the use of self-excited alternators, but automatic voltage regulators have since been highly developed, and are now relied upon almost universally. The advent of the first hydrogen cooled synchronous condenser in 1928, and the construction of some 1,500,000 kw. of turbine generators in ratings of 25,000 kw. and up with hydrogen cooling during the years 1938-40 alone, typify the constant improvements in efficiency and more effec tive use of materials being made.
Generators are logically classified in accordance with the way in which these three objectives are met. Arranged in accordance with the type of mechanical drive, and in order of their impor tance, there are: (a) Steam turbine-driven generators, (b) water wheel-driven generators, (c) engine-driven generators, (d) electric motor-driven generators. In accordance with the type of electric power they deliver, and their electrical design, they are divided into: (e) Synchronous generators (a.c.), (f) generators with com mutators (normally d.c.), (g) induction generators (a.c.), (h) inductor generators (a.c.), (i) homopolar generators (d.c.).
In mechanical construction, any generator can be made with a horizontal or a vertical shaft, with radial (disc type) or axial conductor arrangement, and with the field inside or outside of the armature. The most usual construction has a horizontal shaft, with axial conductors, and outside armature. Exceptions are type f machines, which have the armature inside, and type b machines, which are now commonly made with vertical shafts. Of the various types, the engine-driven continuous current gen erator (c, f) was the most important before 1900, in the days of the reciprocating steam engine, and the recent Diesel engine progress is bringing it renewed prominence for marine and railway locomotive power plants, especially tugboats and switching en gines. The development of the electric transformer and the steam turbine brought type a, e into the lead about 1905, and improve ments in power transmission have made type b, e important also. Efficient gas and oil engines, notably the Diesel engine, have brought type c, e into prominence too. The advantages in alter nating current power generation and transmission have made it common to produce direct current power, when required, by con version from alternating current power, so that direct current gen erators are now most frequently of type d, f. Other types of generator are of minor importance in the production of primary electric power, 'though useful in special applications, as d, h for the production of high frequency alternating currents. Any generator can be used as an electric motor (q.v.), but in practice only types e, f and g are so used. An intermediate variety of machines much used is the synchronous condenser, really an electrically driven synchronous generator of reactive power, or a generator of type d, e, in which the driving motor and the gen erator are combined into a single machine. Such a machine draws from a system a very small energy current and returns to it a large magnetizing current. Another important intermediate variety of machine is the "synchronous converter," which is really the combination of a generator of type d, f with a syn chronous driving motor. By building a synchronous motor as shown in fig. 1, with revolving armature and a direct current gen erator with common windings, the incoming and outgoing arma ture currents can be made largely to cancel each other, so that a greatly increased output can be secured from a given amount of material. A large proportion of all the direct current power now generated is produced by such converters, although mercury arc rectifiers are preferred for most new installations.
The general procedure in generator design may then be sum marized thus: (a) The cubic volume of the machine is determined by equation (4), the values of k, B and A being fixed by expe rience. (b) The number of poles, P, is found by equation (2) from the given speed of the drive and the desired electrical frequency.
(c) The ratio of D to L is usually selected so as to make L between one and three times the circumferential pole pitch, depending chiefly on ventilation and mechanical stress limitations.
(d) The armature and field windings and the magnetic circuit details are then laid out in such a way as to secure evenly balanced magnetic and electric densities throughout, while providing ade quate space for insulation and a practical mechanical construc tion. Copper (rarely aluminium) is used for the conductors, var nished cloth, treated paper, or built up mica for insulation, laminated silicon steel for the armature, and commercially pure iron or steel (generally laminated) for the field; these materials give least power losses consistent with economical manufacture.
Shaft seals are employed which hold the loss of hydrogen down to very low values, usually by means of lubricating oil pumped through the small shaft clearance, the entrained air and hydro gen being continuously removed by a vacuum pumping system through which the oil is circulated.
The stationary armature is built of segmental punchings of 2 to 4% silicon steel, o•o2 5 in. or less in thickness, and insulated from each other by thin paper or enamel. The iron losses in the stator, due to the cyclic alternation of the magnetic flux, are a large factor in the total losses and the heating, so every effort is made to reduce them. Due to the great lengths of core, the armature windings are almost always made of half coils, or bars, which are soldered together at both ends after insertion in the slots. To avoid excessive eddy current losses in the copper, the conductors must be made of numerous insulated strands, and these must be transposed so that they occupy the same average positions in the slots, each manufacturer using his own methods of accomplishing these ends. The armature slots are very deep in proportion to their width, to secure large cooling surfaces and to increase the armature leakage reactance. The rotor and stator coil end con structions present some of the most difficult problems in turbine generator design. High leakage reactance is desirable to reduce the excessive currents that flow on short circuits, often of the order of ten times normal, but high leakage fluxes produce stray power losses and local heating of the end structure, and increase the magnetic flux that must be carried by the rotor. The stray losses are minimized by use of double layer fractional pitch arm ature windings, non-magnetic stator flanges, and non-magnetic rotor retaining rings. External reactors are frequently used to limit short circuit currents. Another method found useful in large power stations is to employ a double armature winding, the two electrically similar halves being connected to separate station buses. In this way, the generator behaves like two separate machines, with a very close transformer coupling and a common source of excitation, giving reduced fault currents but maintaining good voltage regulation and system stability. Large generators are almost always wound for 3 phase and Y connection, and often have the neutral point grounded. Especial care is taken to choose rotor and stator slot numbers, winding distributions, etc., that will give sinusoidal voltage wave form, avoiding any high fre quency components that would produce inductive interference in neighbouring communication circuits. The stator frame which sup ports the armature punchings is generally built up of steel plates and ribs welded together in modern practice, and of cast iron sections in former practice. The larger machines are frequently made with split frames, or with separate inner and outer frames, to facilitate shipment. The outside of the frame is covered with steel sheeting, and the interior space is utilized for ventilating air passages. Higher steam pressures and improvements in tur bine design have led to a steady increase in speeds, requiring many mechanical refinements, such as improved rotor balanc ing methods, the frequent use of step-down gears in place of direct connection for generators below 5,000 kw. rating, and greater use of alloy steel rotor forgings. The large dimensions and high operating stresses of the 3,00o and 3,60o r.p.m. gener ator rotors require complex heat treating and inspection processes of the highest degree of refinement.
The armature winding insulation is practically always made of mica flakes cemented to paper tape or sheets, a number of layers being taped or wrapped on the conductors and bonded with var nish. The solvent in the varnish is subsequently removed by vac uum treating and baking processes. Great care is required in the selection of the varnish and the coil treatments, to prevent vola tilization of the solvent and puffing of the insulation when sub jected to the operating temperatures, usually of the order of C. Asphalt base varnishes are generally employed, on ac count of their superior ageing properties. Corona protection, such as a layer of asbestos tape and conducting varnish, is applied to the coil exterior to prevent damage from the corona discharges in ionized air between the coil surface and slot edges.
The theory of operation of such a "round rotor" synchronous generator may be simply explained by aid of the vector diagrams.
Vector E, the upper part of fig. 3, represents the voltage that would be produced in the armature by the synchronously rotating field magnetomotive force (m.m.f) acting alone (armature current, zero). I is the armature current actually flowing, which creates a drop in voltage equal to 1 times the synchronous impedance. Subtracting this drop vectorially from E, there is obtained the terminal voltage, V. The angle 0, between V and 1, is the load power factor angle, which is indicated in the diagram to be lag ging, corresponding to a reactive, or induction motor, load. If the load consisted largely of static condensers, 1 would be leading, and the impedance drop would make V greater than E. Thus, as a reactive load is put on, with a fixed field current, the terminal voltage falls, falling faster the more the load current lags the ter minal voltage. To hold constant voltage, independent of load, it is necessary to adjust the field current, which is usually done by cutting in and out resistance in the exciter field circuit. The syn chronous reactance, X 3 , expressed as a decimal, is equal to the ratio of the field current required to produce normal armature current on short circuit, to the field current required to force nor mal voltage flux across the air gap on open circuit. It consists of two elements, that due to the fundamental air gap flux pro duced by the armature current, or "armature reaction reactance," and that due to the armature leakage flux, or "armature leakage reactance." When saturation of the magnetic circuit occurs, it is necessary to treat these two elements separately, since only the latter produces a real addition to the flux under load. In Figure 3b, represents the field m.m.f., and the armature m.m.f. The vector sum, I 4, represents the net m.m.f. which produces the actual flux 4), corresponding to the "internal" voltage, Evi dently the larger is, the greater the change in voltage due to change in load, and hence the greater the necessity for voltage regulation. With a fixed field current, as the armature current rises and the voltage falls, their product, which is the generator output, reaches a maximum and then decreases, so that beyond a certain load the generator will suddenly "break down" and drop its motor load, unless the field current is raised. These considera tions impose an upper limit on the value of X which in Europe has been put as high as i .8, but is now generally made t • 2. Generators in large central stations near the load centre may have considerably higher synchronous reactance values than per missible for machines connected to their loads through long trans mission lines. For new stations, the desirable generator reac tance values are determined by a system study, in which the short circuit currents, the power demands and stability limits, and the voltage regulation at various points of the power system are cal culated. To facilitate this analysis, usually made with the aid of a miniature electrical network, designers have defined "equivalent" synchronous reactance values for each load and voltage condition, which give the actual generator terminal voltage drop per ampere of load current for the degree of magnetic saturation then exist ing.
High speed circuit breaker and selective relaying methods en able short circuits to be cleared in a small fraction of a second, thus lessening the need for high reactance in the generator. The use of 'automatic voltage regulators and accurate methods of fre quency control permit close adjustment of generator loading and more effective use of power interchanges with hydro-electric plants and other systems. These developments, in turn, have re quired accurate analyses of transient as well as steady state con ditions. Since several seconds must elapse before a change in excitation voltage applied to the field of a large generator can produce its full effect on the armature voltage, and an even longer period is required for adjusting the speed by means of the turbine governor, the problems of co-ordinating the actions of relays and circuit breakers with the voltage regulators and speed governors require a high order of technical skill for their solution.
Waterwheel Driven Alternators.—A cross section of a typ ical large waterwheel driven alternator is shown in fig. 4. Such machines have been built with horizontal shafts, and for very high head plants they are still made, but vertical shafts are employed in almost all recent installations, thus securing better utilization of the full head of water. The weight of the rotor and downward thrust of the water are usually carried by a thrust bearing at the upper end of the shaft, while guide bearings above and below hold the rotor in a central position. Such thrust bearings carry loads of about 40o pounds per square inch, and are cooled by immersion in oil. The field normally revolves, inside the stationary armature. The armature frame was formerly made of cast iron, but since 1925 there has been a marked trend toward built-up frames of welded steel plates. The weight of the thrust bearing is supported by upper bearing brackets resting on the outer rim of the stator frame. The central rotor structure, or "spider," has generally been made from steel castings, but present tendencies are toward the use of built-up structures of steel plate. The rim frequently "floats" on the spider arms, the arms merely taking the weight and the torque, leaving the rim free to expand radially under thermal and centrif ugal stresses. Such a separation of the functions of rim and arms enables each to be designed to withstand definite stresses, whereas when arms and rim are integral, the division of stress is indeter minate, and larger factors of safety are required. The rim is then generally made of segmental overlapping punchings held together by through pins and attached to the spider arms by rectangular keys. The "salient" field poles are held on dovetails fitting into slots in the rim, or, in low speed machines, by radial bolts passing through the rim. As most waterwheels will attain nearly double normal speed if full load is dropped, and the water is not shut off at once, the generator rotor must meet severe overspeed tests, and so its mechanical design is of the greatest importance. The field winding is most economically made with one coil per pole. On large machines, the field coils are made of edgewise wound copper strip, insulated by varnished paper between turns ; but wire wind ings are employed on small machines. Generally the field current is supplied by a direct current "exciter" mounted on the same shaft. As some water may leak through the closed turbine gates, it is necessary to supply brakes to stop and hold the generator rotor at rest, when it is taken out of service. These are mounted below the rotor, and a separate "braking ring" should be provided on the lower end of the rotor spider, which can withstand the extreme heat that may be generated. Ventilation is provided by air drawn in between the poles from the ends and blown out through radial air ducts in the stator.
Hydrogen cooling is less practical for these generators than for the steam turbine type, due to their much greater volume, and since reduction of the windage loss is here unimportant. The armature is made of silicon steel laminations as for turbine gener ators. The number of slots per pole must be small, to preserve a satisfactory ratio of copper to insulation space, and so the flux pulsations due to the teeth are important. In American practice, open slots and machine-wound coils are used, and good voltage wave shape is preserved by fractional pitch, fractional slots per pole windings, while the pole face losses are held down by the use of in. or thinner pole piece laminations. Eddy current losses in the armature copper are avoided by using several turns per coil and employing transpositions in the end windings or coil connec tions. In European practice, partially closed armature slots and single-layer chain windings are most often employed; ,good wave form being obtained by building the pole tips in staggered sections, displaced one armature slot pitch. Solid cast steel poles, but often with laminated tips, are used. Eddy current losses in the arma ture copper are then controlled by transpositions in the slots, just as for turbine generators. In Europe, and to an increasing extent in the United States, "amortisseur windings" are used on water wheel-driven generators. These consist of copper bars passing through slots in the pole faces, and solidly connected together by short circuiting rings at both ends. As such windings enclose the air gap flux, opposing eddy currents are induced in them by any change of the flux, and so they are often called "damper" wind ings. They facilitate synchronizing, damp out speed oscillations, or "hunting," and greatly reduce the peak voltages occurring across the open armature phase during a single phase short circuit. When such a short circuit occurs, the air gap flux is forced into the leakage paths when the pole is opposite the shorted phase, but it returns to its normal path with extreme rapidity as the pole comes opposite the open phase, and so induces a very high peak voltage in the latter. Finally, an amortisseur winding reduces the duty of circuit breakers, as its effect in increasing the initial short circuit current dies away before the breaker can open, while it materially delays the restoration of voltage across the opening breaker contacts. On large turbine generators, the massive rotor iron serves the same purposes effectively. The theory of the elec trical operation of a salient pole generator is more complicated than for a round rotor, due to the variable permeance of the air gap. As the permeance may be represented by the sum of a con stant term and a sinusoidally varying term completing one cycle in each pole pitch (two cycles per electrical cycle), the vector dia grams of fig. 3 must be changed to that of fig. 5 for accurate work. The round rotor theory can be used fairly well for calculating normal operating characteristics, however, except for the torque angle relations. The relation between torque and angle of dis placement, between the field axis and the flux axis is expressed: Ed (X d — X q) T= d 8+ sin 28 q where V and Ed are the terminal and nominal voltage, and 6 is the angle between them, as indicated in fig. 5. Xd and are the synchronous reactances in the direct and quadrature field axes, respectively. If all values are expressed as ratios to full load values, T will be the ratio of actual to full load synchronous torque. The second term of the formula accounts for the ability of a sali ent pole machine to synchronize and carry an appreciable load without field excitation. It becomes zero for a round rotor, when X d = X The entire steady state operation of a salient pole syn chronous machine may be foretold from a knowledge of its two synchronous reactances, Xd and X and its power losses, while its transient operating characteristics can be foretold if the corres ponding transient reactances, X'd and and the zero phase sequence rgactance, X,, are also known. A complete theory of synchronous machines, including transient and steady state phe nomena, is given in five papers by R. E. Doherty and C. A. Nickle.
The diagram (fig. 6) shows curves enabling the approximate dimensions of a waterwheel generator for any given speed and out put to be determined. The diagram shows specifically the relation between capacity and diameter of a vertical type generator. The efficiencies of modern generators of this type are very high, all except very small ones being above 95% and some reaching 98%. The advent of the Kaplan propeller type turbine with ad justable pitch blades has encouraged the development of lower head projects by enabling a somewhat higher generator speed to be used for a given fall of water. These wheels have higher run away speeds, requiring generators to withstand 25o or even 280% of normal speed, instead of 200 to 225% runaway speeds typical of older fixed blade turbines. There has also been a tendency to increased hydraulic thrust values. The Dnieprostroy genera tor thrust bearings have a capacity of 2,000,000 pounds, while the 6o.000 kv-a., 75 r.p.m., Bonneville generator bearings are able to carry 3,000,00o pounds.
The inductor alternator basically consists of a homopolar gen erator with a toothed field structure, so that, while the flux always crosses a single air gap in the same direction, it pulsates in the armature teeth from a low to a high value at the field tooth fre quency. The armature conductors are so connected as to add the alternating voltages due to these pulsations. The flux does not alternate, but only pulsates ; therefore only half the possible work ing range of flux variation is utilized, and so the magnetic struc ture is approximately twice as heavy as for an equivalent alterna tor. This handicap in size prevents its use for ordinary purposes, but the inductor alternator is useful for the generation of high frequency power. The frequency generated is fixed by the number of teeth on the revolving field, and the speed. Since the field can be made of a solid forging, without electrical conductors, it can be run at a very high speed. Alexanderson has built large i oo,000 cycle generators of this type for radio telegraphy, and, in general, the inductor alternator construction affords the best means of generating considerable amounts of electric power at frequencies above 50o cycles per second. The induction generator is simply an induction motor that is driven above its synchronous speed. The armature consists of a laminated cylindrical rotor with a short circuited winding. The field is usually stationary, and is similar to the armature of a normal synchronous generator. It is not self exciting, but receives its magnetizing currents from the power system to which it is connected. Hence, an induction generator must draw a reactive current from the system before it can de liver any power, and it can only supply any given amount of power at a fixed leading power factor. To reduce the magnetizing current required, a small air gap is used, and this in turn requires a larger number of slots and thinner laminations than a synchron ous alternator has. The inability of the induction generator to de liver a lagging current, or to operate without a synchronous ma chine to supply its excitation, limits its usefulness.
The development of greatly improved permanent magnet ma terials, such as Alnico, has encouraged the production of special forms of generators in fractional kilowatt sizes, such as magnetos for aeroplane engine ignition. Also, the widespread use of small low cost gas engines has caused the development of a variety of small a.c. generators, some with saturated magnetic circuits to make them self-regulating.
BIBLIOGRAPHY.-R. R. Lawrence, Principles of Alternating Current Bibliography.-R. R. Lawrence, Principles of Alternating Current Machinery, 614 p. (New York, 192o) ; M. Walker, Specifications and Design of Dynamo-electric Machinery (New York, 1915) ; R. E. Doherty and C. A. Nickle, "Synchronous Machines," A.I.E.E. Trans., vol. 45, P. (1926) , vol. 46, p. 1-18 (1927) ; vol. 47, P. 457 492 (1928) ; vol. 49, P. 700-714 (1930) ; A. R. Stevenson, Jr., "Short Method of Calculating Flywheels," G. E. Review, vol. 28, p. 58o; J. A. Fleming, Fifty Years of Electricity (London, 1921) ; M. A. Savage, "Economic Developments in Turbine Generators in the United States," Second World Power Conference Transactions, vol. 12, paper p. (Berlin 1930) ; M. A. Savage and W. J. Foster, "Design Features that Make Large Turbine Generators Possible," A.I.E.E. Trans., vol. 49, p. (1930) ; Rudolf Richter, Elektrische Maschi nen, vol. 2 (Berlin, 193o) ; Miles Walker, "The Electric Generator," London Electrician (Sept. 25, 1931) ; M. G. Say and E. N. Pink, The Performance and Design of Alternating Current Machines (1936) ; A. E. Clayton, The Performance and Design of Direct Current Ma chines (1938) . (P. L. A.)