A Treatise On The Principles And Practice Of Dock Engineering

ELECTRICAL DISTRIBUTION OF ENERGY. 521 The continuons current flows uninterruptedly in one direction, being the reverse of the alternating current which flows alternately in opposite directions. Multiphase currents are a group of the latter type which differ from each other by their relative difference in phase. The continuous current has hitherto proved to be the most satisfactory for dealing with operations so variable in nature as those which prevail in connection with dockwork. Alternating single-phase currents only give good results when utilised at a fairly uniform speed, and without the necessity of overcoming the inertia of heavy bodies at starting. Continuous currents, on the other hand, on account of insulation difficulties with regard to the construction of both armatures and commutators of the generators, and the faet that it is necessary to use rotating transformers for reducing the pressure, are not so weil adapted for the transmission of power to very long distances, though within the ordinary limits of most dock systems, they will be found perfectly effective and sufficient. The dynamos and motors generally utilised may be enumerated as (1) Series wound, (2) Shunt wound, and (3) Compound wound. In the first case, the armature, the field winding, and the external circuit are all in series. In the event of short circuiting, the field current is intensified and the winding may be injured by the heating of the wire. In the second case, the field winding is distinct from the outer circuit, and there is, consequently, a separate current to excite the field magnets. Short circuiting can, therefore, produce no heating effect. The compound machine has two coils on its field magnet. One winding is in series with the external circuit and the armature, the other is in shunt. This machine, from the counter action of its coils, is more regular under the influence of varying currents than either of the other two, but it is only completely regular and automatic at one particular speed. A series-wound motor is suitable for use in positions where great starting power is required, such as in cranes, haulage gear, &c., and also in the case of single motors, driving pumps, and heavy machinery where the load is constant after being once applied. When run off constant pressure circuits, the motors are controlled by a variable resistance placed in series with them and regulated by hand, as required. In the series-wound motor the speed decreases as the current inereases. The torque is greatest at starting when the current is a maximum, being about six times the normal amount, and, as it is proportional to the latter, it varies inversely as the speed. When the load varies the speed is not constant. The shunt-wound motor is not so well adapted for starting against a heavy torque as the series-wound motor. It will, however, run at nearly constant speed under a varying load when supplied with current at constant pressure. With the shunt motor, also, considérable variation in speed can