If the primary voltage is less than the secondary voltage, the transformer is called a
step-up transformer, and if the primary voltage is greater than the secondary it is called a
step-down transformer. Some transformers are provided with a tapped primary [See Figure
9 (b) ], for the purpose of adjusting the transformer to
the supply voltage. By using the proper tap, the ratio of the number of secondary
turns to the number of primary turns is adjusted to give the proper secondary voltage when the supply voltage is too high or too low. When it is desirable to have the
secondary voltage variable over a considerable range the secondary is sometimes
tapped [as in Figure 10 (a) ]. A transformer may have several secondary windings
all insulated from each other [Figure 9 (b) ], and the voltage generated in each
secondary winding will be independent of the voltage generated in the other
secondary windings; in other words each secondary winding behaves as though the
other secondary windings were not present. Another type of transformer called
the auto-transformer is sometimes used. In this transformer both the primary and
secondary windings use some turns in common. The same laws regarding the
voltage ratio of the windings hold for the auto-transformer as for transformers
where the windings are entirely separate. As shown in Figure 10 (b), the primary
uses only a fraction of the total number of turns while the secondary uses all the
turns of the winding. This condition could just as well be reversed. The connections used depend upon the voltage change desired.
The figures used in conjunction with this discussion of transformers show the
primary and secondary windings on different legs of the iron core. In practice the
windings may be as shown, but they are usually wound one on top of the other.
One method is about as good as the other so far as operation is concerned, and
all the statements made regarding transformer action are true for transformers
in which the primary and secondary are on the same leg of the core, as well as
when they are on separate legs.
Transformers used to transform the voltage of power circuits are known as
"power transformers." Transformers used to change the voltage of speech frequencies are known as "audio transformers." The electrical action in both types is
the same. The difference arises from the difference of the frequencies involved and
the use to which the transformers are put. A power transformer should not be used
on circuits of lower frequency than that for which it was designed unless the primary voltage is reduced a corresponding amount, that is, a 60 cycle transformer with
a 110 volt primary should not be used on a 110 volt, 25 cycle circuit, because it
would become over-heated and burn out.
20. Impedance.- If a 110 volt DC circuit was connected to a 110 volt primary of
a power transformer, the transformer would probably burn out immediately, and
the current would be over twenty times the current for which the primary was
wound. Therefore, it is evident that direct current and alternating current behave
differently in certain types of circuits. It was implied in the discussion of Ohm's
law that resistance alone limited the current in DC circuits, but it is evident from
the behavior of DC in transformers that there is some other property that limits the
alternating current in a transformer circuit. This property of the transformer circuit which limits the alternating current is known as impedance.