General. The adaptability of the dc generator for
specific uses has led to the development of many special generators. These
machines over the years made a significant contribution to industrial progress.
However, most of these special applications have disappeared
or are now being met with other devices such as silicon controlled rectifiers
or programmed control of field currents to the main dc generator.
Synchronous Converters. Of all the special
generators, this was one of the earlier and most widely used. It was the
principal dc power source for streetcars and interurban lines.
It was a most ingenious device, combining in a single
armature and winding an ac motor taking its current from the lines through slip
rings at the rear and a dc generator providing dc power from a commutator on
the front end.
Because the flow of the currents was in opposition, the
resulting rotor winding could be small in cross section. A single stator
provided flux for both functions. With the decline of street railway systems,
the synchronous converter disappeared.
Rotating Regulators. These dc machines had trade
names like Rototrol, Regulex, and Amplidyne. They, too, have been replaced by
solid-state devices. In addition to having fields for feedback intelligence,
response was enhanced using self-excited shunt fields tuned to the air-gap line
or by means of cross-magnetization from armature reaction.
Three-Wire Devices. Because three-wire dc circuits
are no longer in use, balancer sets and threewire generators are relics in
school labs or museums.
Homopolar or Acyclic DC Generators. The single-pole
machine principle still fascinates electrical engineers and several research
and development labs continue to study new arrangements of its basic parts.
Fundamentally, it consists of a single conductor moving
through a uniform single direction flux with a collector at each end of the
conductor. The output is a steady ripple-free pure dc current and no
commutation.
Currents reaching 270,000 A at 8 V were provided by one
commercial unit. Recent efforts have been mainly to use liquid metals to take
the large currents from the rotating collectors and to obtain higher voltages
by connecting units in series.
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