Ans. The teeth protect the inductors, retain them in place against the electrical drag and centrifugal force, and the construction permits a reduction of air gap to a minimum, thus reducing the amount of copper required for the field.
Fig. 1,481.—General Electric revolving field and exciter armature. This is an example of direct connected exciter construction. In this arrangement the armature of the exciter is carried on the alternator shaft at the end farthest from the pulley. In the smaller sizes the magnet frame is bolted to the bearing bracket, but in the larger sizes special construction is used depending upon the conditions to be met. On all alternators of standard design, the field is built for 125 volts excitation and on account of the increased danger from induced voltage, in case the machine is used as a synchronous motor, the builders consider any higher voltage undesirable.
Fig. 1,482.—Section of General Electric Alternator showing method of dovetailing core laminations to frame. The latter is made in two general styles, known as the box type and skeleton type. The box type consists of a single casting for the smaller sizes, but for large capacity alternators the frame castings are usually divided into upper and lower sections. The skeleton type consists of two side castings between which substantial spacing rods are set at regular intervals. The core consists of the usual sheet iron lamination slotted and assembled; they are mounted on the inner periphery of the frame, making lap joints (that is "staggered" as in fig. 1,480), each section being dovetailed to the frame. Heavy clamping rings or end plates are mounted on both sides of the core by means of bolts, and supporting fingers extend along the slot projections. The design is such as to provide for air circulation as shown in figs. 1,483 and 1,484.
Armature Windings.—In general, the schemes for armature windings for alternators are simpler than those for direct current machines, as in the majority of cases the inductors are an even multiple of the number of poles, and the groupings are usually symmetrical with respect to each pole or each pair of poles. Furthermore, as a general rule, all the inductors of any one phase are in series with one another; therefore, there is only one circuit per phase, and this is as it should be, since alternators are usually required to generate high voltages. These general principles establish the rule, that in the circuit in a single phase armature, and in the individual circuits in a polyphase armature, the winding is never re-entrant, but the circuits have definite endings and beginnings. In exceptional cases, as those of polyphase converters, re-entrant circuits are employed, and the armature windings are so constructed that a commutator can be connected to them exactly as in direct current machines. These armatures are usually of the lap wound drum type.
Fig. 1,483.—Section of General Electric alternator frame showing air ducts and supporting fingers extending along the slot projections. The air circulation is provided for by means of ducts formed by suitable spacing blocks inserted at intervals between the laminations, as shown here and in fig. 1,484. The armature coils are form wound and designed so they can be readily replaced in case of injury. They are taped and treated with an impregnating compound, in the usual way, then inserted in the armature slots in an armour of horn fibre and retaining wedges of wood are dovetailed into the slot walls.
Alternator windings are usually described in terms of the number of slots per phase per pole. For instance, if the armature of a 20 pole three phase machine have 300 slots, it has 15 slots per pole or 5 slots per each phase per pole, and will be described as a five slot winding. Therefore, in order to trace the connections of a winding, it is necessary to consider the number of slots per pole for any one phase on one of the following assumptions: 1, that each slot holds one inductor; 2, that there is one side of a coil in each slot; and 3, that one side of a coil is subdivided so as to permit of its distribution in two or more adjacent slots.