Transformers installed for applications on a farm electrical
system are almost always dry-type transformers.
Only dry-type transformers operating at less than 600 V
are discussed in this unit.
Common two-winding transformers are often called
insulating transformers. The primary winding and the
secondary winding are separate and not connected. An
auto transformer has its winding interconnected so that
the primary and the secondary share the same winding.
Auto transformers, therefore, have an electrically connected
primary and secondary. These two basic types of
transformers are shown in Figure 14-10. A major advantage
of auto transformers over the insulating types is their
lighter weight and compact size. Auto transformers are
used for electric-discharge lighting ballasts.
A special type of auto transformer, called a grounding
auto transformer or zigzag transformer, is occasionally
used to create a neutral wire or a ground for an
underground 480-V, 3-phase system. These transformers
are found occasionally in industrial wiring. Standard in-saluting transformers can be used to make a zigzag transformer.
The wiring of these transformers is covered in
NEC Section 450-5.
Control transformers are special insulating transformers
commonly used to supply power for control of
appliances, equipment, and motor starters, Figure 14-11.
A control transformer is required when the control circuit
voltage is different from the line voltage supplied. Common
control circuit voltages are 24 V and 120 V. A
120-V control circuit to a start -stop push-button station
may be desirable for a 125-hp, 480-V, 3-phase motor
powering an irrigation pump. The control transformer
would step down the 480 V to 120 V for the control
circuit.