The Tesla Multiphase Current Motors

The prominence which has been given to power transmission by multiphase currents in the experiments between Frankfort and Lauffen, seems to have led to the impression, even among those otherwise well informed, that this was the first example of this method of transmission. It may therefore be well to put on record the fact that in this country multiphase motors manufactured by the Westinghouse Co. have been in successful commercial operation for several years, and a three phase power plant has been in daily service in the shops of the Westinghouse Co. at Pittsburgh for some time past, driving the shafting and machinery of the winding department.

As is well known, Mr. Nikola Tesla is the inventor of the multiphase motor, and his patents in this country are controlled by the Westinghouse Electric & Manufacturing Company, who have been quietly standardizing the different sizes and have, within the last year or two, placed a large number of them in service.

The generators for these motors are of the Westinghouse multipolar type with toothed, drum armatures wound to give currents of sixty or ninety degrees difference of phase. These machines, one type of which is shown in Fig. 1, are built either entirely self-exciting and self-regulating or separately-exciting and self-regulating. The machine shown is designed for 60 kilowatt capacity.

The motors are constructed with internal poles and grooved armatures. The larger motors have armatures with three collecting rings, and the fields are excited with direct current. The smaller ones have armatures wound with coils closed upon themselves, doing away with all collecting rings and commutators. The engraving Fig. 2, shows a 10 h. p. Tesla motor and Fig. 3 exhibits the armature removed from the enclosing field magnets. These multiphase motors are built in sizes of from 1 to 1,000 h. p.

A 1,000 h. p. three-phase generator or motor, for the same machine can be made either as a motor or generator, is shown in the engraving Fig. 4 and when run as a generator will give 150 amperes at 5,000 volts. In mechanical design this machine is similar to the multipolar direct current railway generators built by the Westinghouse Company. The armature is of the drum type with slots for the wire and is without bands.

With the Tesla multiphase system the power can be subdivided to any extent for motor or lighting purposes, and high pressures may be employed which can be reduced to whatever voltage is desired. The multiphase motors start under load, and run at constant speed. The smaller ones are made entirely alternating, and the larger ones are self-exciting after reaching synchronism.

As the starting torque with the Tesla multiphase motors is comparable with shunt wound direct current machines, they may be applied in every case where direct current shunt wound motors can be used. At starting, the direct current motors require dead resistance in series, which means considerable loss, but the alternating current motor has inductive resistance, and therefore the loss at starting is much less. Multiphase motors can be used for operating mines, factories, and street railways, and the generating station may be located wherever most convenient for obtaining the best results, and if the pressure is not over 5,000 volts, the motors may be supplied direct from the mains. The Tesla motor has a great advantage over other types of motors in that it requires no commutator, and consequently is sparkless. The motor may be boxed up, and kept free from dust and grit, and high pressures can be employed successfully. No starting resistance is required and the motor is small and compact, and requires no attention. Tests made at the Westinghouse Co.'s works demonstrate that the Tesla motors are fully as efficient as the best direct current motors.