Multiphase motors have also been constructed by v. Dolivo-Dobrowolski, but they do not differ in principle from those of Tesla, described above. The motor employed in the trans commission of power experiments between Lauffen and Frank fort-on-the-Main, in September, 1891, was designed by Dobro wolski.
The Reehniemal Alternating-current Motor.—This motor, Fig. 48, designed by M. W. C. Rechniewski to work with al ternating currents, does not differ from the ordinary continu ous-current type, except that the field is of laminated iron. The armature is of the drum type, with Pacinotti teeth. For the sake of economy in the manufacture, both the armature and field-magnet cores are stamped out of one sheet. The following figures relating to a machine of 15 horse-power have been furnished : Volts at terminals, 115 ; current, 100 amperes ; revolu tions per minute, 1,400 ; diameter of armature, 8 in. : peri pheral velocity in feet per minute, 2,800 ; weight of iron in field, 440 lbs. ; weight of iron in armature, 108 lbs. ; section of iron in field, 42.5 sq. in. ; section of iron in armature, sq. in. : induction in armature, 3,700,000 lines.
This motor is not, of course, self-regulating.
The Thomson Alternating-current Motor, invented by Prof. Elihu Thomson, is shown in Fig. 49. C U' are the field coils or inducing coils, which alone are put into the alter nating-current circuit. I T is a mass of laminated iron, in the interior of which the arma ture revolves, with its three coils, B, wound on a core of sheet-iron disks. The eom mutator short-circuits the armature coils in succession in the proper positions to utilize the repulsive effect set up by the currents which are induced in them by the alternations in the field coils. The motor has no dead point. and will start from a state of rest and give out considerable power. A curious property of the machine is that at a certain speed, depending upon the rapidity of the alternations in the coil, C, a continuous current passes from one commutator brush to the other, and it will energize electromagnets and perform other actions of direct currents.
Rankin. Kennedy's Motor is shown in Fig. 50. It consists of
two ordinary dynamos, with ring or drum armatures and laminated field magnets ; both armatures are on the same shaft, their coils being connected together. One of the machines acts as the motor, the other taking the place of the commutator ; there arc no brushes and no commu tator, and, therefore, an entire absence of sparking. The motor requires two currents, one at a quarter of a complete alternation in advance of the other ; but it does not require any synchronizing, and it can start with load on from rest. The two currents at different phases are obtained from a transformer, or two lbw wires with a third for a common return, or from a coil wound on the field of one of the com bined machines. Larger machines are made multi polar.
resin Motor with the polypha sal motors above described the difference in phase is obtained by a specially constructed generator. But if the fiehl or energizing circuits of a motor, in which the action is dependent upon the inductive influence upon a rotating armature of independent field magnets exerted successively and not simul taneously, he both derived from the same source of alternating currents, and a condenser of proper capacity he included in one of the same. that approximately the desired difference of phase may be obtained between the currents following directly from the source and those flowing through the condenser. The great size and expense of condensers for this purpose that would meet the requirements of the ordinary systems of comparatively low potential, however, are practically prohibitory to their employment in practice. This difficulty has been overcome by Mr. Nikola Tesla, in the apparatus shown in Fig. 51. Here A B repre sent the poles of an alternating current motor, of which C is the armature, wound with coils. B, closed upon themselves, as is now the general practice in motors of this kind. The poles, A, which alternate with poles. 11, are wound with coils of coarse wire, E, in such direction as to make them of alternate north and south polarity, as indicated in the diagram by X S.