266 



it becomes equivalent to a rotating magnet excited by means of 

 continuous currents. Hence by substituting for an ordinary field- 

 magnet of the revolving type a commutator rotor, and supplying this 

 latter with three-phase currents from a transformer whose primary is 

 connected across the terminals of the armature, we obtain a self- 

 exciting polyphase generator. This is the arrangement used by Latour. 

 Such a generator, it may be noted, need not run at absolutely 

 synchronous speed. 



Since at synchronous speed there is no relative motion of the field 

 and the rotor core, it is evident that there is no need to laminate the 

 core : it may be constructed of solid steel. 



1 60. Hey land Alternator 



In Heyland's self-excited and compounded synchronous alternator, 

 the field-magnet is of the ordinary type of construction employed in 

 separately excited generators, consisting of a flywheel whose rim 

 carries a number of polar projections wound with exciting coils. 

 These field coils are connected in a certain manner explained below 

 to a commutator, and by means of three brushes spaced a distance 

 apart corresponding to | period, exciting currents are fed into 

 the field winding from the secondaries of two transformers, one of 

 which supplies a nearly constant current, sufficient to produce the 

 required generator p.d. on open circuit, while the other supplies 

 a current which increases in simple proportion to the wattless 

 component of the load current, and so maintains the generator 

 p.d. approximately constant. 



In the latest form of this type of generator, the commutator 

 contains eighteen segments per magnetic circuit, i.e. per pole-pair. 

 The first six segments of each group of eighteen are active segments 

 (i.e. segments connected to the field winding) ; the next three are 

 dummy segments, which are not in connection with anything ; then 

 comes another set of six active segments, followed by a set of three 

 dummy segments. Corresponding active segments of each group are 

 interconnected.* The width of each brush is slightly in excess of the 

 width of three segments. 



* Thus, if we suppose the segments to be numbered consecutively, we should have 

 the following twelve groups of interconnected segments : 



Group I: 119 37 55, etc. Group VII: 10 28 46 64, etc. 



II : 2 20 38 56, etc. VIII: 11 29 47 65, etc. 



Ill : 3213957, etc. IX : 12304866, etc. 



IV : 4 22 40 58, etc. X: 13 31 49 67, etc. 



V : 5234159, etc. XI : 14325068, etc. 



VI : 6244260, etc. XII : 15335169, etc. 



There is, of course, no occasion to connect the idle or dummy segments 7, 8, 9; 16, 

 17, 18, etc. 



