Considering first of all continuous-current motors, it may be said from a theoretical standpoint that the possibilities of continuous-current motors are almost unlimited. The speed of such motors may be economically regulated by varying either the applied pressure or the exciting current. In the case of a constant-voltage supply, the usual method of varying the supply voltage consists in the use of series-parallel connexion. This involves the use of at least two motors and finds its commonest application in traction.
Occasionally, however, some form of the Ward-Leonard system of control is adopted. This, however, entails the use of a variable-voltage generator, which in turn needs an electric motor or a prime mover to drive it. Since each conversion of energy is associated with loss, such systems are not only costly but eventually become more or less wasteful. This is particularly the case when the Ward-Leonard control is used on an alternating-current system of comparatively small power (e.g. that of a private installation) in order to drive, say, a rolling-mill or a winding-engine where the peak load is much in excess of the mean load. Here it often becomes needful to supply a fly-wheel converter set consisting of an induction motor with slip regulator (see below), a variable-voltage generator and a fly-wheel, in addition to the driving motor, the armature of which has often to be divided into two or three parts in order to reduce inertia when rapid reversals are necessary. The function of the slip regulator is to allow the speed of the induction motor to fall when a heavy load comes on, and so to permit the excess load to be taken up by the stored energy in the fly-wheel. Such sets often have to deal with peaks of 20,000 H.P. and may give an overall efficiency of 50-70%. Where the supply systems are sufficiently large, as on the Rand, the fly-wheel can be dispensed with, but the induction motor must then be able to cope with the peaks. The electric winder affords a good example of the problems that have to be met in many cases in order to replace a steam-engine drive.
A much simpler method of controlling the speed of a continuous-current motor is to vary the exciting current. This can be done automatically or manually, and it may be made dependent on or independent of the load; but in every case a single machine only is necessary. The usual continuous-current motor for different speeds is the shunt motor; in this, with a given excitation, the speed is practically independent of the load; but by increasing or decreasing the exciting current the speed is lowered or raised respectively. By this method of shunt control it is possible to obtain speed ranges as high as 1.5 or 1.6. Such wide ranges, however, make the design difficult. At the lowest speed the ventilation is usually very poor, while the exciting current is highest, but fans built on the shaft of the armature can usually overcome any difficulty arising therefrom. It is at the higher speeds that the design becomes a serious problem. In addition to the high peripheral speeds of armature and commutator the very weak field may render the motor unstable, while the commutating poles—which are essential to prevent sparking—may produce hunting. It becomes necessary therefore to provide such motors with compensating windings in order to neutralize armature reaction. Thus despite the economy of this method, the motors become costly when wide speed ranges are demanded.
Series motors in which the exciting winding is in series with the armature winding, and in which in consequence the speed becomes a function of the load, are widely used for drives where there is no danger of the load being removed—e.g. for traction or for fans, cranes, etc., but the only common application of voltage and field control of series motors is for traction work.
The compound-wound motor combines the shunt and series characteristics in varying degree, according to requirements. If a series characteristic is required with merely a limiting top speed, it is only necessary to provide the motor with a small shunt winding in addition to the series winding in order to prevent racing. When, however, an increased torque at starting or a fall in speed in the case of overloads is demanded, a small series winding is added to the shunt winding. In the former case the series turns may be short-circuited if desired after a definite speed has been reached.
Except in cases where a variable voltage is applied to the motor, starting resistances are necessary with continuous-current motors, so that continual starting becomes wasteful. For general speed control the continuous-current motor is doubtless unrivalled, and where circumstances justify the outlay conversion from alternating to continuous current is the best solution. A typical case would be a factory in which several variable-speed motors are installed.
Coming to the alternating-current side, mention must first be made of the question of power-factor rectification. The alternating-current, three-phase system having established itself as the standard method of transmission, vigorous attempts are being made in every country to keep the power-factor of such systems as high as possible, in order to secure the minimum outlay in transmission and generation. Obviously, with three-phase supply it becomes highly important to employ wherever practicable three-phase motors, but in any such application the power-factor must not be overlooked. Broadly speaking, the user does not stand to gain by ignoring this question, for whether the rectification is achieved by him or by the power company, or is not done at all, the consumer has to pay.
Though with alternating current there are more types of motors available than with continuous current, speed control presents a more difficult problem. From the point of view of power-factor correction, the synchronous motor can be regarded as ideal, but here speed control is not available, while there are the additional difficulties of providing facilities for starting and for separate (continuous-current) excitation. Where the conditions at starting do not call for a large amount of torque, it is often possible to bring the motor up to speed as an induction motor by means of eddy currents induced in the pole shoes or by using the damping winding as a squirrel-cage winding. The next stage consists in the provision of a starting motor in the form of an induction motor with two poles less than the synchronous motor. For severe starting conditions, such a starting motor would become too costly, and the present solution is being sought by building the synchronous motor itself as an induction motor. The machine then runs up to speed as an induction motor, is excited by continuous current and pulls into synchronism, whence it continues running as a synchronous motor. In addition to meeting severe starting conditions, this arrangement is also replacing the induction motor where power-factor correction is important. By its simplicity the induction motor is doubtless the alternating-current motor that finds most favour. Where repeated starting or where speed control is necessary the motor is uneconomical, because the input to an induction motor depends on the torque, and is independent of the speed. Nevertheless it is often preferable to incur this waste rather than to install converting sets. It is possible, however, to obtain economical speed control with an induction motor by changing the number of poles or by connecting two induction motors in cascade—in each case, however, with a certain sacrifice in power-factor as well as through the extra cost incurred. There are numerous ways of effecting a change in the number of poles—e.g. by regrouping the coils, by varying the number of phases, by using two or more windings, etc.—and generally it becomes needful to employ a squirrel-cage rotor. Such a rotor, however, does not necessarily mean a low starting torque, for some of the locomotives used on the Simplon tunnel railway have such windings. Generally speaking, it is not usual to obtain more than six speeds with induction motors, while two and four are more usual.
The commutator motor offers theoretically the best solution for obtaining speed control with alternating current, and the possibilities here are the same as with continuous current. Actually, however, the limitations are more severe, because not only do commutation conditions limit the pressure as in the continuous-current motor, but the transformer pressure induced by the alternating flux in the coils undergoing short-circuit imposes further limitations which result in a comparatively small output per pole. The reduced commutator pressure usually entails a transformer between supply and motor, but where speed control is required advantage can be taken of this to vary the applied pressure by using a variable-ratio transformer. The real trouble occurs when the E.M.F. in the short-circuited coils depends upon synchronism, as in three-phase commutator motors and single-phase commutator motors of the repulsion and shunt types. The practical result is that the speed of such motors never varies greatly from synchronous speed, and that their limiting output is a few hundred horse-power. On the other hand, types like the single-phase series commutator motor, free from this restriction, have been successfully built for outputs of over 1,000 H.P. and speed ranges up to four or five times that of synchronism. Despite limitations, alternating-current commutator motors are becoming more widely used, particularly for small outputs; while as cascade or auxiliary motors they have been successfully applied for utilizing the slip energy of large induction motors. Variable-speed sets of this kind will probably be more widely developed in the future, particularly when the properties of alternating-current commutator motors come to be better understood.
Authorities.—As additional authorities may be consulted: Miles Walker, The Specification and Design of Dynamo-Electric Machinery (1915); Hawkins, Smith and Neville, Papers on the Design of Alternating Current Machinery (1919); Alexander Gray, Electrical Machine Design (1913); A. T. Dover, Electric Traction (1917), and G. Klingenberg, Bau grosser Elektrizitätswerke (1920).
ELECTRICITY SUPPLY (see 9.198).—United Kingdom.—In its commercial aspects the history of electricity supply in the United Kingdom from 1910 to 1914 was comparatively uneventful. No fresh legislation was passed; no new supply schemes of the first magnitude were brought forward. The supply undertakings were in the main content with steady development among both industrial and domestic consumers. Advances were more rapid in the lighting field on account of the appearance of the drawn-wire tungsten lamp, first in the vacuum type and later in the gas-filled type. Improvements in cooking and heating apparatus also stimulated the domestic day load. The war, however, arrested the growth of the domestic demand and brought an urgent and practically unlimited call for electric power in factories and workshops extended for war purposes and in new factories erected for the production of munitions of war. During the first months of war the need was met by running all the plant (including reserves) available in public generat-
