EXCITERS 353 



The series field excitation of regulating exciters should not 

 exceed 30 per cent of the total excitation, and the resistance 

 of the rheostat should be about three times that of the resistance 

 of the exciter shunt field when hot. 



For regulating exciters, which are not to be operated in parallel, 

 the shunt-wound type is entirely satisfactory, provided it has been 

 designed with this point in view, that is, for low saturation. 



Speed. The speed of an exciter depends on the method of its 

 drive and on its capacity. Extremely slow or high speeds mean 

 excessive cost with the addition of mechanical difficulties for 

 high speed. This is especially important in hydro-electric in- 

 stallations, where the exciters are turbine driven, in which case 

 they must be designed to withstand the increased stresses due to a 

 double-speed. This fact should not be neglected when making a 

 decision on the speed of a water-wheel-driven exciter. 



Method of Drive. While the exciters can be either belt-driven 

 or direct-connected to the machines driving them, the latter prac- 

 tice is almost exclusively used except in the very smallest plants. 

 The direct connection may be either to the main generators, to 

 separate water wheels or to motors, usually of the induction type. 

 Sometimes, although rarely, an exciter may be found that is 

 connected both to a motor and a turbine, the latter running idle 

 when the motor is carrying the load, and vice versa. 



Mechanical Design. The mechanical design of exciters does 

 not differ from other direct-current generators. They may be 

 either of the horizontal or vertical type, the latter construction 

 being used for units direct connected to vertical main generators 

 or directly to vertical water wheels. When intended for direct 

 connection to horizontal water wheels they are almost invariably 

 of the pedestal bearing type, the shaft being provided with the 

 necessary coupling. Care should be taken in designing the bear- 

 ings to see that the water thrust, if any, is provided for. The 

 same construction is also generally used for large motor-driven 

 sets, Fig. 208, the two units being mounted on a common base. 

 Occasionally only two bearings are used and a common shaft. 

 For horizontal units direct-connected to the main units, shaft 

 and bearings are generally omitted, the exciter armature being 

 mounted on an extension to the generator shaft and the frame 

 supported on an extension to the generator subbase, as shown in 

 Fig. 209. 



