3-7
reduce this force to a safe level so that the windings will not be thrown out of the machine, the salient
pole is used only in low-speed designs.
ALTERNATOR CHARACTERISTICS AND LIMITATIONS
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.
Q6. How are alternators usually rated?
Q7. What type of prime mover requires a specially designed high-speed alternator?
Q8. Salient-pole rotors may be used in alternators driven by what types of prime movers?
SINGLE-PHASE ALTERNATORS
A generator that produces a single, continuously alternating voltage is known as a SINGLE-PHASE
alternator. All of the alternators that have been discussed so far fit this definition. The stator (armature)
windings are connected in series. The individual voltages, therefore, add to produce a single-phase ac
voltage. Figure 3-5 shows a basic alternator with its single-phase output voltage.
Figure 3-5.—Single-phase alternator.
The definition of phase as you learned it in studying ac circuits may not help too much right here.
Remember, "out of phase" meant "out of time."
Now, it may be easier to think of the word phase as meaning voltage as in single voltage. The need
for a modified definition of phase in this usage will be easier to see as we go along.
