This chapter has presented an introduction to the subject of alternators. You have studied the characteristics and applications of different types. The following information provides a summary of the chapter for your review.

MAGNETIC INDUCTION is the process of inducing an emf in a coil whenever the coil is placed in a magnetic field and motion exists between the coil and the magnetic lines of flux. This is true if either the coil or the magnetic field moves, as long as the coil is caused to cut across magnetic flux lines.

The ROTATING ARMATURE-ALTERNATOR is essentially a loop rotating through a stationary magnetic fealties cutting action of the loop through the magnetic field generates ac in the loop. This ac is removed from the loop by means of slip rings and applied to an external load.

The ROTATING-FIELD ALTERNATOR has a stationary armature and a rotating field. High voltages can be generated in the armature and applied to the load directly, without the need of slip rings and brushes.

The low dc voltage is applied to the rotor field by means of slip rings, but this does not introduce any insulation problems.

ROTOR CONSTRUCTION in alternators may be either of two types. The salient-pole rotor is used in slower speed alternators. The turbine driven-type is wound in a manner to allow high-speed use without flying apart.

GENERATOR RATINGS are dependent on the amount of current they are capable of providing at full output voltage; this rating is expressed as the product of the voltage times the current. A 10-volt alternator capable of supplying 10 amperes of current would be rated at 100 volt-amperes. Larger alternators are rated in kilovolt-amperes.

EXCITER GENERATORS are small dc generators built into alternators to provide excitation current to field windings. These dc generators are called exciters.

The SINGLE-PHASE ALTERNATOR has an armature that consists of a number of windings placed symmetrically around the stator and connected in series. The voltages generated in each winding add to produce the total voltage across the two output terminals.

A TWO-PHASE ALTERNATOR consists of two phases whose windings are so placed around the stator that the voltages generated in them are 90 out of phase.

TWO-PHASE ALTERNATOR CONNECTIONS may be modified so that the output of a two-phase alternator is in a three-wire manner, which actually provides three outputs, two induced phase voltages, plus a vectorial sum voltage.

In THREE-PHASE ALTERNATORS the windings have voltages generated in them which are 120 out of phase. Three-phase alternators are most often used to generate ac power.

THREE-PHASE ALTERNATOR CONNECTIONS may be delta or wye connections depending on the application. The ac power aboard ship is usually taken from the ship's generators through delta connections, for the convenience of step-down transformers.

ALTERNATOR FREQUENCY depends upon the speed of rotation and the number of pairs of rotor poles.

VOLTAGE REGULATION is the change in output voltage of an alternator under varying load conditions.

VOLTAGE CONTROL in alternators is accomplished by varying the current in the field windings, much as in dc generators.