3-A] CHARACTERISTICS. 63 



2. Types of Alternators. Synchronous alternators are of 

 the following three types (compare Part I., Exp. i-A) ; 



1. Alternators having a revolving armature and stationary 

 field, used only for small machines. 



2. Alternators having a revolving field and stationary arma- 

 ture, the most common type. 



3. Inductor alternators, having a stationary armature and sta- 

 tionary field, the revolving part or inductor consisting only of 

 iron. 



The first type corresponds to the nearly universal type of 

 direct current generator; there is, however, no commutator and 

 alternating current is delivered from the armature winding to 

 the line by means of collector (or slip) rings and brushes. In the 

 second and third types, the armature is stationary and current is 

 delivered directly to the line without collector rings. The con- 

 tinuity in insulation, thus made possible, is an important ad- 

 vantage in high potential machines. In revolving field alterna- 

 tors, the field current is introduced through slip rings. 



Each type is made in several forms which may be studied by 

 reference to standard works,* or better by examination of actual 

 machines. The form most desirable depends upon conditions 

 of operation, character and speed of prime-mover, etc. In some 

 cases, it is desirable to make the moving mass as small as possi- 

 ble ; in other cases as in direct-connected engine-driven genera- 

 tors a certain fly-wheel effect is advantageous. Alternators of 

 the second type usually have an internal revolving field, a con- 

 spicuous exception being the umbrella form of external revolving 

 field in the vertical-shaft alternators at Niagara. In the old 

 Mordey and Brush form of machine, the stationary armature 

 coils were in a vertical plane between the two parts of the revolv- 



tioned in Electric Journal, p. 550, October, 1908. The steam turbine has thus 

 modified both alternating and direct current generators (33, Exp. i-A). 

 * See also " The Mechanical Construction of Revolving-field Alterna- 

 tors," by D. B. Rushmore, Transactions A. I. E. E., Vol. XXIII., p. 253. 



