MULTIPOLAR MACIIINES.—As the E. 31. F. generated in dynamo-electric machines is de pendent upon the rate of change of the cutting of the lines of force, the speed of the machine can evidently be reduced by increasing the number of poles presented to the a-rmature. Modern electrical engineering is tend ing strong ly to the adoption of multipolar machines, IS they allow of more ready direct connection with the driving-engines.
The illustration shows the latest type of _Multipolar Constant Potential Generator, direct, connected to triple-expansion engines, one at each end of the driving-shaft. In this type, it will be noted. the armature rotates within a field-magnet having 10 poles. The frame of the field-magnet consists of but a sin gle casting, and a very ingenious method has been designed by 3Ir. Edison for winding on the magnetizing, coils. The Gramme ring type of winding is employed, hut, instead of using coils of a number of eonvolotions connected to each segment of the commutator. in this type of maehine each convolution is connected di rectly with the commutator-bar. For this pur pose bare copper strips of T:-shapc are slipped over one side of the core. insulated from one another, and each succeeding U-shaped bar of this character is connected direct by a commu tator-bar. The brushes bear upon the commu tator in a vertical plane. This construction with hare conductors on account of the very small difference of potential, If volt existing between each liar, The number of bars employed in this type of machine is 944, and hence an equal number of cominutator-hars is employed, Each bar is therefore equivalent, to a coil of the usual type of Gramme winding. hut, the of contiguous layers entirely avoids and hemp sparking, the machine operating, without, the slightest indication of such it dist urbanee. Eight brushes arc employed, which can be shifted simultaneously by means of the hand-wheel shown, and which can also lie lifted at once from the surface of the coonnutator. The resistance of the armature is •000 ohm, and that of the field 8.45. Each dynamo is designed for 150 volts pressure and a capacity of 666 ampt/res. The design has been so well worked out that a variation of three quarters of the load can be made without requiring the slightest change in the position of the brushes, The Siemens ilaltipolar Dynamo.—Figs. 67 and 68 show the Siemens ring-dynamo. In
this machine the armature completely surrounds the fixed poles; hence, there is practically no waste of lines, and the close proximity of the magnet-cores near the axis insures a much shorter magnetic circuit than is obtained in a dynamo of the usual type. Praetical experi ence has shown that the radial depth of the ring should by very small, that only one layer of wire should be wound on the armature, and that the commutator must contain a very large number of sections. The armature of the machine illustrated is 25 in. in ternal diameter, 8 in. long, and the total weight of the machine is l ton 4 cwt. At 350 revolutions per min. the output was found to be 16 kilo-watts, and at 480 revolutions it rose to 25 kilo-watts. Assnming about 20 kilo-watts as a safe load for continuous working, the weight of this machine is less by one third than that of the usual type made by Messrs. Sie mens & ITalske. In addition to this saving of weight, the machine has the advantage of be ing easily repaired, since the armature is outside, and can be slipped off the shaft in situ with out any danger of the wires chafing against the field-magnets. After several of these ma chines had been made. Messrs. Siemens & Ilalske proposed to utilize the design for direct driven steam-dynamos. It is evident that a machine with stationary field and overhanging external armature is not suitable for small sizes, and that its greatest advantage and power of competing with existing designs will be in such cases where, on account of the motor, the speed is limited. Fig. 69 slioWS the arrangeinent adopted in connection with a pair of ()lien, direet-aeting steam-engines. The field-magnets are attached to the end of the engine bed plate, and the crank-shaft is prolonged so as to carry the overhanging armature. Outside the armature there is no bearing,. but only the attachment for the brushes, which is secured by means of two columns, also fixed to the engine bed-plate. Messrs. Siemens & Ilalske have built several sizes of this dynaino, among which are four large machines for the installation at the railway station in Frankfort-on-the:gain. Each of these machines is intended for an output of 75 kilo-watts. at a speed of 150 revolutions per min. Large dynamos built on this principle are also in operation at the Berlin electric-light station, working up to 1,500 horse power.