Fig. 2,135.—Typical American overhead 6,600 volt single phase interurban trolley line, Baltimore and Annapolis short line, Annapolis, Md.
An objectionable feature is that the voltage of a constant current alternator will rise very high if the circuit be opened, because it is then relieved of inductance drop and armature reaction.
To guard against a dangerous rise of voltage, a film cut out or equivalent device is connected to the terminal of each machine so that it will short circuit the latter if the voltage rise too high.
Ques. What advantage have constant current alternators over constant current dynamos?
Ans. The high pressure current is delivered to the external circuit without a commutator, hence there is no sparking difficulty.
The above relates to the revolving field type of alternator. There are, however, alternators in which the armature revolves, the current being delivered to the external circuit through collector rings and brushes. This type of alternator, it should be noted, is for moderate pressures, and moreover there is no interruption to the flow of the current such as would be occasioned by a tangential brush on a dynamo in passing from one commutator segment to the next.
In the revolving field machine, though the armature current be of very high pressure, the field current which passes through the brushes and slip rings is of low pressure and accordingly presents no transmission difficulties.
