The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY, 65 which, as is unfortunately well known, is under certain conditions particularly liable to faults. Single-phase motors of the sorts now in use possess, in addition to the stator winding for normal running, another or second stator winding, which is only in circuit when starting up. By various means an artificial phase difference is created between the currents into the two windings, so that the motor starts as a two-phase motor. There are two usual methods of obtaining the required phase difference, either by inserting a capacity in the main circuit, or by inserting a re- actance coil in the auxiliary circuit. The best results are obtained by applying both at once. Good results are, however, obtained by inserting a choking coil in the auxiliary winding, and an inductionless resistance in the main circuit. The above applies particularly to motors with short-circuited rotor. Motors with slip-rings are much better for starting, and require, in addition to the starting resistance between the slip- rings only, either a choker in the auxiliary circuit, or a capacity in the main winding of the stator, in order to start with full torque. The cause of the faults to which single-phase motors are liable lies, as is well known, much more rarely in breakdowns in the winding, than in the auxiliary apparatus, the capacity being the weak part. These capacities consist of insulating pots in which a series of iron plates are placed, side by side, insu- ated one from another. The pot is then filied with F