ALTERNATING MOTORS.- The Testa Alter nating Motor.—Mr, Nikola Testa was the first to build a practical motor employing currents of different or what are now termed " polvphasal ' currents. One of the types of the Testa motor, as built by the Westinghouse Co.. is shown in perspective in Fig. 44, and with its parts exposed in 45. It consists of a series of magnets built up of laminated sheet-iron and wound with two sets of coils, the ends of which are connected to the two bind ing posts shown. These binding posts form the only connection with the regnlar lighting circuits, with the addition of a single return wire. By the aid of this return wire, two alternating currents are sent through the field of the motor at the saute time, the pulsations of the two currents being equal in strength, but the one lagging a quarter phase behind the other in the two sets of field coils, respect ively. The effect of this is that a rapidly rotating polarity is given to the field, cor responding in period to that of the currents producing it.
The armature core of the mo tor is of the Siemens drum type, mid it is wound with a compar atively few turns of heavy wire, the ends of which are soldered together, forming a closed cir suit having no connection with the energizing current. The alternating currents in the field induce secondary currents in the armature, and by the attraction between these and the revolving polarity of the field, armature rotation is produced, the rate of rotation corresponding very near ly with that of the field. When no work is being done by the motor, the synchronism is exact, or nearly so, and very little cur rent passes either through the armature or field ; but as load is put on and t-he work increases, the armature tends to lag slight ly, causing the passage of more current in proportion to the work clone. The reaction between the armature and field is, therefore, similar to th a t between the primary and secondary of a converter when changes of lamp loads take place in the second ary circuit. The addi tion of the return wire for the motor circuit can be made easily, so as to adapt existing lighting circuits to motor work.
The speed of the motor, as well as its direction of rotation, may be reg ulated by an ingenious adaptation of the con verter principle, an ad justable " ehoki ng coil " arrangement being employed. which avoids the use of resistances and switches. The sim plicity of the winding and general construction of the motor makes it unlikely to get out of repair. Thus the insulation of the armature is of no importance, since the current induced in it, though comparatively large in volume, has a potential of but a few volts, and often less than a volt, regardless of the voltage of the supplying circuits.
The Alternating Three-phase Motor, constructed by C. E. L. Brown, of Switzerland, is shown in part section in Fig. 46, and the armature winding in Fig. 47. Three armature cir cuits are connected, as in a Thomson-Houston armature, and the winding is so arranged that four rotating poles are produced. With 40 cycles the motor makes about 1,200 revolutions per minute. The motor takes 50 volts normally ; a reduction to 30, or an increase to over 100, does not make any practical difference in the speed. Of course, in the first case, the beat ing of the armature wire is greater, and in the second the heating of the iron is increased. The magnetic field rotates, and is produced by the arma ture reaction, thus avoiding all slid ing contacts. The field magnet is composed of a laminated ring with holes, in which are placed insulated copper bars. The free ends on both sides are connected by copper rings. It is not easy to imagine a more sim ple construction. The armature has 90 conductors of about 40 sq. mm. sect ion. The weight of copper is 20 kg., the iron about 100 kg. The breadth of the armature is 20 mm., the outer diameter about 500. The rotating magnet carries 54 copper bars, with a section of 100 sq. mm. The weight of the copper is 15 kg.; that of the iron is 70. Recent trials in Oerlikon with this motor showed that it can easily supply 20 horse-power. The total weight of the motor is 420 kg.