INTRODUCTION 1. By the direction of the energy transmitted, electric machines have been divided into generators and motors. By the character of the electric power they have been distinguished as direct- current and as alternating-current apparatus. With the advance of electrical engineering, however, these subdivisions have become unsatisfactory and insufficient. The division into generators and motors is not based on any characteristic feature of the apparatus, and is thus not rational. Practically any electric generator can be used as motor, and conversely, and frequently one and the same machine is used for either purpose. Where a difference is made in the construction, it is either only quantitative, as, for instance, in synchronous motors a higher armature reaction is often used than in synchro- nous generators, or it is in minor features, as direct-current motors usually have only one field winding, either shunt or series, while in generators frequently a compound field is employed. Further- more, apparatus have been introduced which are neither motors nor generators, as the synchronous machine producing wattless lag- ging or leading current, etc., and the different types of converters. The subdivision into direct-current and alternating-current apparatus is unsatisfactory, since it includes in the same class apparatus of entirely different character, as the induction motor and the alternating-current generator, or the constant-potential commutating machine and the rectifying arc light machine. Thus the following classification, based on the characteristic features of the apparatus, as adopted by the A. I. E. E. Standard- izing Committee, is used in the following discussion. It refers only to the apparatus transforming between electric and electric and between electric and mechanical power. 1st. Commutating machines, consisting of a magnetic field and a closed-coil armature, connected with a multi-segmental commutator. 121 122 ELEMENTS OF ELECTRICAL ENGINEERING 2d. Synchronous machines, consisting of a undirectional mag- netic field and an armature revolving relatively to the mag- netic field at a velocity synchronous with the frequency of the alternating-current circuit connected thereto. . 3d. Rectifying apparatus, that is, apparatus reversing the direc- tion of an alternating current synchronously with the frequency. 4th. Induction machines, consisting of an alternating mag- netic circuit or circuits interlinked with two electric circuits or sets of circuits moving with regard to each other. 5th. Stationary induction apparatus, consisting of a magnetic circuit interlinked with one or more electric circuits. 6th. Electrostatic and electrolytic apparatus as condensers and polarization cells. Apparatus changing from one to a different form of electric energy have been defined as: A. Transformers, when using magnetism, and as B. Converters, when using mechanical momentum as inter- mediary form of energy. The transformers as a rule are stationary, the converters rotary apparatus. Motor-generators transforming from elec- trical over mechanical to electric power by two separate machines, and dynamotors, in which these two machines are combined in the same structure, are not included under converters. 2. (1) Direct-current commutating machines as generators are Usually built to produce constant potential for railway, incan- descent lighting, and general distribution. As motors commutat- ing machines give approximately constant speed — shunt motors — or large starting torque — series motors. When inserted in series in a circuit, and controlled so as to give an e.m.f. varying with the conditions of load on the system, these machines are "boosters," and are generators when raising the voltage, and motors when lowering it. Commutating machines may be used as direct-current con- verters by transforming power from one side to the other side of a three- wire system. Alternating-current commutating machines are used as motors of series characteristic for railway and other varying speed service, or with shunt characteristic for constant speed and adjustable speed work, especially where high starting torque efficiency is required. They usually are of single-phase type. (2) While in commutating machines the magnetic field is, INTRODUCTION 123 almost always stationary and the armature rotating, synchronous machines were built with stationary field and revolving armature, or with stationary armature and revolving field, or as inductor machines with stationary armature and stationary field winding but revolving magnetic circuit. Generally now the revolving field type is used. By the number and character of the alternating circuits con- nected to them they are single-phase or polyphase machines. As generators they comprise practically all single-phase and poly- phase alternating-current generators; as motors a very important class of apparatus, the synchronous motors, which are usually preferred for large powers, especially where frequent starting and considerable starting torque are not needed. Synchronous machines may be used as compensators or synchronous condensers, to produce wattless current, leading by over-excitation, lagging by under-excitation, or may be used as phase converters by operat- ing a polyphase synchronous motor by one pair of terminals from a single-phase circuit. The most important class of converters, however, are the synchronous commutating machines, to which, therefore, a special chapter will be devoted in the following. Inserted in series to another synchronous machine or synchro- nous converter, and rigidly connected thereto, synchronous ma- chines are also occasionally used as boosters. Synchronous commutating machines contain a unidirectional magnetic field and a closed circuit armature connected simul- taneously to a segmental direct-current commutator and by collector rings to an alternating circuit, generally a polyphase system. Thus these machines can either receive alternating and yield direct-current power as synchronous converters or simply " converters," or receive direct and yield alternating-current power as inverted converters, or driven by mechanical power yield alternating and direct current as double-current generators. Or they can combine motor and generator action with their converter action. Thus a combination is a synchronous con- verter supplying a certain amount of mechanical power as a synchronous motor. Usually, they convert from three-phase or single-phase alternating to direct-current power. (3) Rectifying machines are apparatus which by a synchro- nously revolving rectifying commutator send the successive half waves of an alternating single-phase or polyphase circuit in the same direction into the receiving circuit. The most impor- 124 ELEMENTS OF ELECTRICAL ENGINEERING tant class of such apparatus were the open-coil arc light ma- chines. They have been practically superseded by the mercury arc rectifier. (4) Induction machines are generally used as motors, poly- phase or single-phase. In this case they run at practically constant speed, slowing down slightly with increasing load. As generators the frequency of the e.m.f. supplied by them differs from and is lower than the frequency of rotation, but their opera- tion depends upon the phase relation of the external circuit. As phase converters, induction machines can be used in the same manner as synchronous machines. Another occasional use be- sides as motors is, however, as frequency converters, by changing from an impressed primary polyphase system to a secondary polyphase system of different frequency. In this case, when low- ering the frequency, mechanical energy is also produced; when raising the frequency, mechanical energy is consumed. (5) The most important stationary induction apparatus is the transformer, consisting of two electric circuits interlinked with the same magnetic circuit. When using the same or part of the same electric circuit for primary and secondary, the transformer is called an auto-transformer or compensator. When inserted in series into a circuit, and arranged to vary the e.m.f., the trans- former is called potential regulator or booster. The variation of secondary e.m.f. may be secured by varying the relative number of primary and secondary turns, or by varying the mutual in- ductance between primary and secondary circuit, either elec- trically or magnetically. The stationary induction apparatus with one electric circuit are used for producing wattless lagging currents, as reactors, reactive or choke coils. (6) Condensers and polarization cells produce wattless leading currents, the latter, however, usually at a low efficiency, while the efficiency of the condenser is extremely high, frequently above 99 per cent. ; that is, the loss of power is less than 1 per cent, of the apparent volt-ampere input. Unipolar, or, more correctly, non-polar or acyclic machines are apparatus in which a conductor cuts a continuous magnetic field at a uniform rate. They have not become of industrial importance. Regarding apparatus transforming between electric energy and forms of energy differing from electric or mechanical energy: The transformation between electrical and chemical energy is INTRODUCTION 125 represented by the primary and secondary battery and the elec- trolytic cell; the transformation between electrical and heat energy by the thermopile and the electric heater or electric fur- nace; the transformation between electrical and light energy by the incandescent and arc lamps. In the following will be given a general discussion of the charac- teristics of the most frequently used and therefore most impor- tant classes of apparatus. A further discussion and calculation of these apparatus is given in "Theory and Calculation of Alternating Current Phenomena," while a discussion of those characteristics and modifications of these apparatus, which, though important, are less frequently met, and a discussion of the numerous less common types of apparatus, which could not be included in the following, is given in "Theory and Calculation of Electrical Apparatus." Some important features, as the nature of the reactance of apparatus, mechanical magnetic forces, wave shape distortions caused by some features of design, in apparatus, etc., are dis- cussed in "Theory and Calculation of Electric Circuits." A. SYNCHRONOUS MACHINES