i-A] SERIES GENERATOR. 3 



other, i. e., an alternating electromotive force. The simplest form 

 of generator is therefore the alternator, the current being taken 

 from the armature to the line without any commutation. If the 

 armature is stationary, the alternating current from the armature 

 is taken directly to the line; if the armature is revolving, the 

 armature windings are connected to collector rings (or slip rings) 

 from which the current is taken to the line by means of brushes. 



In a direct-current generator the armature windings are con- 

 nected* to the several segments or bars of a commutator, rom 

 which the current is taken by brushes to the line. The alter- 

 nating electromotive force generated in each coil is thus com- 

 mutated, or reversed in its connection to the line, at or near the 

 time of zero value of the electromotive force of the coil. 



The electromotive force in each coil increases from zero to 

 a maximum and back to zero, and at any instant the electromotive 

 forces in the various individual coils have different values rang- 

 ing from zero to a maximum, according to the positions of the 

 coils. The sum of these coil-voltages, as impressed upon the 

 line as terminal voltage, is however practically constant. 



for most purposes. But changed conditions have made it a practical and 

 important machine (i) driven at high speed by the steam turbine, or (2) 

 driven at moderate speed to generate large currents at low voltage for elec- 

 trochemical work. Dynamos of this class are not included in this study. 

 For further information, see " Acyclic Homopolar Dynamos," by Noeg- 

 gerath, A. I. E. E., Jan., 1905 ; also, Standard Handbook, or Franklin and 

 Esty's Electrical Engineering. For description of some structural im- 

 provements, see pp. 560 and 574, Electrical World, Sept. 12, 1908. 



*(3b). The details of armature windings will not be here discussed; 

 they are amply treated in many text and handbooks. In almost all 

 machines a closed coil winding is used. (The Brush and T-H arc 

 dynamos and a few special machines use open coil winding.) In a closed 

 winding, the armature coils are connected in series and the ends closed. 

 There are two ways of connecting the coils in series : wave winding and 

 lap winding. In the wave or series winding there are always two brushes 

 and two paths for the current from brush to brush, irrespective of the 

 number of poles. In the lap or parallel winding, generally used in large 

 .generators, there are as many paths (and brushes) as poles. The two 

 schemes are essentially the same in a bipolar machine. 



