TIIK ALTERNATOR 



115 



The semi-closed or overhung type of slot, shown in Fig. 

 119 (6), is often necessary, especially in induction motors. 

 The larger area of tooth face reduces the air-gap reluctance and 

 also redurrs 'he tufting of the flux which tends to produce ripples 

 in the electromotive force wave. It is usually necessary to 

 place the conductors in the slot one at a time, which is ex- 

 pensivr :md uneconomical of slot space. It is also difficult to 

 apply insulation. 



In both types of slot the conductors are usually held in the slot 

 by wooden or fibre wednes, as shown in the figures. The effect 

 of the semi-dosed slot may t _ t i _ i 



be obtained by the use of 

 open slots and magnetic 



iredges. The<e wedn> 



only partly of iron so 

 that the slot is not 

 entirely closed. 



The internal tempera- 

 tures of modern alter- 

 nators are SO high that Fl - 120 Passa^ of v,M,til:,ti.. K airthrough 



UM ClUCtB of a turbo-alterimtor. 



bUllt-Up mica is found to 



le the insulation best able to withstand the high-temperatures 

 and high-volt.- simultaneously. Such mica is pi 



around the active part of the conductor, forming a solid, 



ntntnnrin 



The problem of ventilating a largo unit is not an easy one. 



iKM)-kw. unit, having an efficiency ofWi.."> percent., requires 



TOO kw. to he dissipated. Such a unit might require from 60,000 



to 70. (MM) cu. ft. of air per minute. This air is usually supplied 



by separate Mowers, and to remove the dirt and increase the 



coolii, of the air. it is often passed through an air 



inn of a curtain of \\at. i'e 1 -jo shows the 



passage of the air through the axial ducts Lack of the lamina- 



. 115) and out through a center radial duct, in a 



turbo-alternator. 



57. Rotating-field Structure. From the standpoint of their 



In Id construction. ton may !>< divided into three classes: 



d engine-driven alternator (7. r ) to !.">() r. p.m.) ; 



the liiedilim-.-peed 1 >el t -( 1 H Veil and Wa 1 e|'-\\ | |erl-d 11 \ ell 



