The Diseases Of Electrical Machinery

52 THE DIS EAS ES OF case of single-phase generators with rotating armature there is almost invariably one path only. This is a reason why the number of armature turns is less than that of a continuous-current machine under conditions that are otherwise similar. As is well known, poly- phase generators have several windings meeting at one point; a two-phase machine two, or sometimes four, windings ; and a three-phase machine three wind- ings, which are connected together at the so-called neutral-point. Faults occurring in these windings may be either short-circuits to iron, or short-circuits between the copper of one coil or two coils. They show themselves not by sparking, since the continuous sliding surface almost entirely prevents sparking, but by the unusual heating of the winding. Modem Alternators with Fixed Armature.— Since in practice alternators with rotating armatures are daily becoming rarer, we will consider at once the modern single-phase and polyphase generators with fixed armature and rotating field-magnets. This pro- cedure is rational since the same faults occur in both kinds of machine. Calling the fixed armature the stator, we can con- sider the breakdowns in alternators under three heads, viz. (i) the faults which occur in the stator; (2) the faults occurring in the magnets; (3) the faults of the exciter. Faults Occurring in. the Stator.—The stator of a modern single-phase alternator has one immoveable