Alternators are rated according to the voltage they are designed to produce and the maximum current they are capable of providing. The maximum current that can be supplied by an alternator depends upon the maximum heating loss that can be sustained in the armature. This heating loss (which is an I2R power loss) acts to heat the conductors, and if excessive, destroys the insulation. Thus, alternators are rated in terms of this current and in terms of the voltage output — the alternator rating in small units is in volt-amperes; in large units it is kilovolt-amperes.
When an alternator leaves the factory, it is already destined to do a very specific job. The speed at which it is designed to rotate, the voltage it will produce, the current limits, and other operating characteristics are built in. This information is usually stamped on a nameplate on the case so that the user will know the limitations.
Alternator voltage or ac voltage control:
In an alternator, an alternating voltage is induced in the armature windings when magnetic fields of
alternating polarity are passed across these windings. The amount of voltage induced in the windings
depends mainly on three things: the number of conductors in series per winding, the speed (alternator rpm) at which the magnetic field cuts the winding, and the strength of the magnetic field. Any of these three factors could be used to control the amount of voltage induced in the alternator windings.
The number of windings, of course, is fixed when the alternator is manufactured. Also, if the output
frequency is required to be of a constant value, then the speed of the rotating field must be held constant. This prevents the use of the alternator rpm as a means of controlling the voltage output.
Thus, the only practical method for obtaining voltage control is to control the strength of the rotating magnetic field. The strength of this electromagnetic field may be varied by changing the amount of current flowing through the field coil. This is accomplished by varying the amount of voltage applied across the field coil