primary (high-voltage) leads, and those lettered X are the
secondary (low-voltage) leads.
Some transformers have two primary and two secondary
windings (as shown in Figure 14- I 3) so they can
be used for several applications. These are called dualvoltage
transformers. Connections must be made correctly
with dual-voltage transformers. If connected improperly,
it is possible to create a dead short that will
usually ruin the transformer when it is energized.
Consider a dual-voltage transformer rated at 240/
480 V on the primary, and 120/240 V on the secondary.
Each of the two primary windings is, therefore, rated at
240 V. Each secondary winding is rated at 120 V. The
transformer must be connected so that each primary
winding receives the proper voltage. In Figure 14-13, the
transformer is shown with the primary windings connected
in series, with HI and H4 connected to a 480-V
supply. The voltage across HI and H2 is 240 V and the
voltage across H3 and H4 is 240 V. Each winding is
receiving the proper voltage. With each primary winding
receiving the proper 240 V, each secondary winding will
have an output of 120 V. Connecting the secondary
windings in series produces 240 V across XI and X4.
Now consider a case where the primary voltage
available is 480 V, but the desired output is 120 V. In
this case, the primary windings are connected in series,
as in Figure 14-13. The secondary windings are, however,
connected in parallel, Figure 14-14. This is accomplished
by connecting Xl to X3, and X2 to X4. If this is
not done properly, a 240-V dead short will occur. A
voltmeter can be used to make sure the connection is
correct. Connect X 1 to X3, and then connect a voltmeter
between X2 and X4. Energize the primary and read the