The Ferranti Alternating Ma chine is shown in perspective in Fig. 93, and Fig. 94 shows the construc tion of the field-magnet frame. The machine illustrated is designed for an electrical output of 150 horse power, and is therefore capable of feeding 3,000 10-candle-power 35 watt lamps. As will be seen, there are 20 magnets on each side of the armature, the distance between the pole-faces being I in. Between the magnets the armature, which is in. wide and has a diameter of 4 ft., makes 400 revolutions per min., and has a peripheral speed of 4,500 ft. per min. The diameter of the armature-shaft is 4A in, It will be noted that the pulley is placed between two bearings and that the armature is overhung.
Machines of this type are now in course of construction (August, 1891) capable of furnish ing current for 200.000 lamps. 'They will be installed at the Deptford station in London.
The following are a few of the details of the Ferranti-Deptford dynamos: '1'lle small ma chines are 1.2 ft. 6 in. high, 15 ft. over all the large machines will be 45 ft. high over all, :Lad will weigh am tons each. The number of alternations of current will be 4,000 complete cycles per min. (67 per second) in all machines. in the small nuichme there will be 48 poles, and the speed 168 revolutions per Min. The large machines are to be coupled direct, and the speed is 60 revolutions per min. only, the periphe ral speed being obtained by the large ness of their diameter. The coils of the dynamos are built up in the same manner as an ordinary dynamo's, each coil generating 125 volts. These are very strongly mounted mechanically. and most carefully insulated, the prin ciple hieing to bury the conductors in the insulation. The insulation used is sulphur, specially treated. and is so hard that in one ca-se where some met al was found to be mixed with the sulphur it took two days to chip out one coil. The sulphur eats partially Into the cast iron and bronze, arid makes a thorough joint. Besides this, the surface insulation is carefully ar ranged to be of porcelain throughout. The electrical efficiency of the arma ture is very great, two volts being ob tained for every foot of copper.
The _Horde!, Allernatiny Dynamo.
—This excellent machine was designed by 31r. William 31. Mordey for the Brush Electrical Engineering Co. of London. and possesses a number of valuable character istics, Pig. 95 shows the machine complete, and Fig. 96 the armature, which is stationary, and consists of a number of coils of narrow copper ribbon, wound on cores of non-conduct ing material. Each coil is bolted between two brackets, the ends of the conductors being
brought out through porcelain insulators. The brackets are then bolted to a gun-metal sup porting ring, being placed out side of the magnetic field so as to avoid loss from eddy - cur rents. which are still further re duced by the employment of German silver for the brackets and Lolls. The gun-metal sup porting ring, which is bolted to the if the maohine, is in two portions, being divided in at vertical diametrical lino. The,, two parts, after having received the coils, are bolted to and to the bed-plate. the tiehl-inagnet being first placed in position. This design pro vides ample filed it lee for repairs, as it allows not only of single coils of the armature being quickly removed and replaced, hilt renders it easy to take out one half or the whole of the armature.
The field-magnet, shown in 97, eonsists of at single °lee tro-magnet, built up ii•4 follows; A short cylinder of iron, through the axis of which the shaft forms the core of the magnet, and round this core is wound the exeit lug coil. Against each cud of this eylinder is [dared a piece. of a form which will be best understood from Fig. 97. Eaell casting has it number of horns or arms which radiate from the shaft and central part of the casting, and then bend over, forming nine pole-pieces on each side of the armature. These horns on one side, as will be seen, approach within a very short distance of those on the other side of the armature, and in this very narrow polar gap or slit the arma ture is held, the entire field-magnet revolving with the shaft on which it is mounted. The ends of the exciting coil are connected to n collector" rings on the shaft, which are shown to the right of the illustration. It will be observed that this form of field-magnet is very simple. A single ex citing coil suffices for a machine of any size, speed, or number of alternations. Besides its pecul iarity of form it differs from the usual arrange ments in that it has poles of one sign only on each side of the armature ; thus the magnetic leakage between adjacent poles on each side is absolutely ail. By revolving the field-magnet, instead of the more delicate armature, safety and steadiness of running are secured, the heavy magnet acting as an excellent fly-wheel, and effectually neutral izing any pulsation due to irregularity in the stroke of the engine.