ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 3 
As the voltage of the excitation current should not exceed 
about 50 volts, a pressure transformer 7, TJ, is often 
applied, as indicated in Fig. 2, which gives the con- 
nections of a self excited generator. 
5. The Classification of Alternators.—The arrangement 
of the armature and field system leads to a further classi- 
fication of alternators, namely: (1) Revolving Armature 
type; (2) Inductor type; and (3) Revolving Field type. 
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Fie. 2.—CoNNECTION DIAGRAM OF SELF ExcIrED ALTERNATOR. 
6. Revolving Armature Alternator.—The revolving 
armature alternator has been developed from. the con- 
tinuous current dynamo; slip rings which are two, three 
or four in number, according to the number of phases, 
replace the commutator. 
To obtain a single phase current, the closed armature winding 
is tapped at two points, which are 180 degrees apart on 
its circumference; for a two phase current, at four points, 
