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 They, too, have been replaced by solid-state devices.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.

Some success has been possible, but restricting the sodium potassium to the collector area has proved difficult.

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