Semiconductors

Semiconductors have made possible a continuous control of voltage to the traction motors. This is called chopper control, and gives direct-current locomotives the same fine speed control as alternating-current locomotives. Using semiconductors, direct-current electrification is possible without any rotating machines, and all the advantages of both alternating and direct currents can be exploited.
If a commutator is not used on a generator, merely slip rings to connect with its windings without any switching, its output voltage alternates smoothly from positive to negative, which can be made to be a pretty good sine curve by using non-salient-pole windings. It is easier to generate such an alternating current by rotating the field inside a stationary armature, so that the main current does not have to flow through the sliding contacts. The sliding contacts, which are called slip rings, need handle only the lower voltage and current of the field windings. High voltages can be generated because it is much easier to insulate the stator, and because these voltages do not have to be handled by the slip rings. This is such an advantage that practically all electricity is generated as alternating current, and the generators are called alternators. These advantages were even reflected in automobiles, which previously used 6V dynamos, but now all use 12V alternators. Typically, the stators are non-salient-pole, while the rotating fields have salient poles.
Alternating current has the compelling advantage that its voltage can be changed easily and efficiently by a transformer, which is a closed iron core surrounded by two windings (first and fourth principles). The ratio of the voltages in the two windings is the same as the ratio of the number of turns, and the ratio of the currents inversely, so that the power remains the same. Since there are no mechanical parts, the efficiency of transformers is very high, and maintenance very low. Alternating current is transformed to higher voltage and smaller current for transmission, and back to lower voltages for use. Transformers with taps can be used to obtain a series of voltages if desired. In fact, an almost continuous voltage variation without loss is possible.
If direct current is required for motors, alternating current can be converted to direct current in four ways. First, a motor-generator set can be used, running at constant speed. A better solution is the rotary converter, essentially a DC generator rotated by AC supplied to it. These were the only practical ways at first for large power requirements, especially for electric railways. Later, rectifiers based on electrical discharges, notably the mercury-arc rectifier, made conversion possible without rotating machines, and with currents appropriate for locomotives. Both of these methods have now been completely superseded by solid-state (silicon) rectifiers, which are trouble-free and easily controlled. Single-phase 50/60 Hz alternating current can be supplied at high voltage, reduced in voltage by a transformer, rectified by solid state rectifiers, and applied to direct-current traction motors, making a very serviceable and economical locomotive that is today's standard.