330 ELECTRICAL EQUIPMENT 



it should, properly speaking, be put into the runner of the wheel 

 or an external flywheel provided. It is, however, invariably 

 found that such an arrangement is objectionable and nearly always 

 more costly than to design the generator rotor for the desired fly- 

 wheel effect, which means additional material in the rotor rim or 

 by increasing the diameter of the same. A good value of the WR 2 

 for water-wheel-driven generators has been given as 10,000,000 

 per Kw. maximum rating, divided by the square of the speed ex- 

 pressed in revolutions per minute; thus 



wr>2 Y / \ 10,000,000 

 WR 2 perKw. (max.) = ( R3 ^ )2 - 



Field Winding. Two methods are used for winding the field 

 coils; viz., the wire wound and the strip wound. For small 

 machines, where even for moderate exciting voltages it is neces- 

 sary to have many turns of small section, the cotton-covered wire- 

 wound coil is usually selected. The necessary insulation may be 

 placed on the assembled pole piece and the winding wound directly 

 thereon. Heavy metal and fiber collars are provided at the ends 

 and serve to clamp the conductors together and prevent move- 

 ment due to mechanical stresses. 



The wire-wound field coil, however, has its limitations both 

 mechanically and electrically. As the centrifugal force of the 

 field coil increases, the vertical component of the force will reach 

 a critical value where the crushing stress on the cotton insulation 

 around the individual wires becomes excessive, while at the same 

 time the horizontal component tends to tear the wires from the 

 pole. From the electrical standpoint the limitation is that of 

 heating. It is evident that the heat generated in the inner layers 

 of the winding can reach the outside surface of the winding only 

 by passing through the insulation of each succeeding layer. This, 

 of course, results in a very considerable difference" in temperature 

 between the inner and outer layers and in order to operate the 

 former at safe temperatures it is necessary to adapt compara- 

 tively low-current densities in the copper; this, in turn, resulting 

 in a heavy winding and consequently high centrifugal forces. 



In order to obviate these difficulties, inherent to the wire- 

 wound field, it is customary to construct the winding of copper 

 strip wound on edge, as shown in Fig. 190. The method of insu- 

 lating this type of winding is similar to that described for wire- 



