120

Fig. 25-5 Illustration of Exciter Operation.

Modern generators use solid-state rectifiers and no longer require slip rings and brushes for external excitation. There are two basic types of brushless generators. In the recent past, most generators of the brushless design depended upon residual magnetism in the main field to produce a small voltage upon initial starting. Today, many generators are permanent magnet generators (PMG) containing a separate alternator for the voltage regulator power supply. In this design, the voltage regulator is unaffected under heavy load and during transient load changes. PMGs typically provide better motor starting and short circuit performance.

Precise speed control is required to match the frequency of the utility grid or to maintain the required frequency in isolated operation. The most common frequency used in North America is 60 Hertz (Hz). In Europe, 50 Hz is used. These frequencies have been selected as norms to allow economical mass production of equipment. There are, however, a number of applications for which equipment standards are based on other frequencies.

The frequency produced by any ac generator is a function of two factors:

1. The number of field pole pairs. The number of pairs of poles that construct the field determines the number of electrical cycles produced by each revolution. A two-pole field produces one complete cycle per revolution, a four-pole field produces two cycles per revolution, a six-pole field produces three cycles, etc.

2. Rotational speed (RPM). For a given number of field poles, the frequency produced will be directly

Where:

Frequency is measured in Hz (cycles per second) Speed is measured in revolutions per minute (RPM)

For example, to produce a frequency of 60 Hz, a two-pole alternator would be driven at 3,600 RPM, a four-pole alternator at 1,800 RPM, a six-pole alternator at 1,200 RPM, etc. To produce 50 Hz, a two-pole alternator would be driven at 3,000 RPM.

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