The alternating current must change to a direct current in many cases as in railroad work because the induction motor is not so satisfactory as the direct current series motor and the alternating current series motor is slow in coming into general use.
In all kinds of electrolytic work, transformation must be made, and in many cities where the direct current system was started, it is still continued for local distribution, but the large main stations generating alternating currents and frequently located some distance away from the center of distribution have replaced a number of small central stations.
Transformation may be made by any of the following methods:
- 1. Rotary converters;
- 2. Motor generator sets;
- 3. [A]Mercury vapor rectifiers;
- 4. [9]Electrolytic rectifiers.
[9] NOTE.—Rectifiers are explained in detail in Chapter LIV.
Strictly speaking, a converter is a revolving apparatus for converting alternating current into direct current or vice versa; it is usually called a rotary converter and is to be distinguished from the other methods mentioned above.
Broadly, however, a converter may be considered as any species of apparatus for changing electrical energy from one form into another.
According to the standardization rules of the A. I. E. E. converters may be classified as:
- 1. Direct current converters;
- 2. Synchronous converters;
- 3. Motor converters;
- 4. Frequency converters;
- 5. Rotary phase converters.