Types of Alternating-Current Generators

Author: E.E. Kimberly

Alternating-current generators are of two general types, namely, induction and synchronous. The induction generator was discussed in Chapter 18 as much as is justified by its relative importance. The synchronous generator is so named because, in contrast to the induction generator, it is capable of establishing a frequency proportional to its speed and independent of its load.

Fig. 20-1. Engine-Type Synchronous Generator

Synchronous generators may be classified, according to their rotor construction, as smooth-core generators and salient-pole generators.

Smooth-core rotors are of cylindrical form with exciting windings embedded in longitudinal slots. They are used in high-speed generators of large size (750 kv-a and greater at 1800 rpm or more). Generators with salient-pole rotors are not essentially different in construction from synchronous motors described in Chapter 19.

Fig. 20-2. Waterwheel-Driven Vertical Alternating-Current Generator Under Construction

A salient-pole generator is shown in Fig. 20-1. The terminal voltage at no load is simply that produced by the sweeping of the rotor flux across the stator conductors. Further than mitigating excessively high voltage transients caused by line-to-line system faults, amortisseur windings serve no useful purpose if the generator is driven by a turbine. If the generator is driven by a reciprocating engine, however, the amortisseur serves to smooth out the pulsations in angular velocity characteristic of such engines. Fig. 20-2 shows the stator of a large waterwheel-driven alternator under construction.