280 ELECTRICAL EQUIPMENT 



sitate a change in the present standard frequency. In Europe, 

 however, a few single-phase systems are using this frequency. 



Illumination. Where alternating current is used for lighting, 

 the 60-cycle frequency is generally used. No arc lamp has as 

 yet been developed that will operate with entire satisfaction on 

 frequencies of less than 40 cycles, and incandescent lamps cannot 

 be used to advantage on frequencies of less than 30 cycles. 

 Low-voltage incandescent lamps show no flicker; but the effect 

 of fatiguing the eye is noticeable at 25 cycles, especially in high- 

 voltage lamps. 



In systems where lighting predominates a 60-cycle frequency 

 should, therefore, be selected, while, if most of the energy is to be 

 used for power purposes the condition may be such that 25 cycles 

 would prove to be preferable, in which case frequency-changers 

 can be provided for changing the current required for lighting 

 purposes to 60 cycles. 



2. SYNCHRONOUS GENERATORS 



Alternating-current generators may be classified into two 

 general classes according to their general characteristics: Syn- 

 chronous generators and Induction generators. The former type 

 is used almost entirely while the latter is used only occasionally 

 for special cases as explained under the section of Induction 

 Generators. 



The generator forms one of the most important parts of the 

 equipment in a hydro-electric development and a thorough knowl- 

 edge of its characteristics and design is of the utmost importance. 

 The subject will, therefore, be treated somewhat more in detail 

 than would at first seem desirable. 



General Description. Most alternating-current generators 

 are of the revolving field type. The armature, which is then 

 stationary, consists of a laminated iron core supported by a cast- 

 iron frame, the inside periphery of the core being slotted to carry 

 the armature winding. Inside the stator revolves the rotor or 

 revolving field system, and as synchronous generators are not 

 self-exciting, the field excitation is obtained from some external 

 direct-current source. 



Induced E.M.F. The e.m.f's. and currents are alternating, 

 i.e., have one-half wave or alternation, first positive and then 

 negative, for each pole passed by a given armature conductor. 



