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depends mainly on three things: (1) the number of conductors in series per winding, (2) the speed
(alternator rpm) at which the magnetic field cuts the winding, and (3) 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 cod.
Q20. How is output voltage controlled in practical alternators?
PARALLEL OPERATION OF ALTERNATORS
Alternators are connected in parallel to (1) increase the output capacity of a system beyond that of a
single unit, (2) serve as additional reserve power for expected demands, or (3) permit shutting down one
machine and cutting in a standby machine without interrupting power distribution. When alternators are
of sufficient size, and are operating at different frequencies and terminal voltages, severe damage may
result if they are suddenly connected to each other through a common bus. To avoid this, the machines
must be synchronized as closely as possible before connecting them together. This may be accomplished
by connecting one generator to the bus (referred to as bus generator), and then synchronizing the other
(incoming generator) to it before closing the incoming generator’s main power contactor. The generators
are synchronized when the following conditions are set:
1.
Equal terminal voltages. This is obtained by adjustment of the incoming generator’s field
strength.
2.
Equal frequency. This is obtained by adjustment of the incoming generator’s prime-mover
speed.
3.
Phase voltages in proper phase relation. The procedure for synchronizing generators is not
discussed in this chapter. At this point, it is enough for you to know that the above must be
accomplished to prevent damage to the machines.
Q21. What generator characteristics must be considered when alternators are synchronized for parallel
operation?
SUMMARY
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.
