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TRANSFORMER TYPES
Transformers can be constructed so that they are designed to perform a specific
function. A basic understanding of the various types of transformers is necessary
to understand the role transformers play in today’s nuclear facilities.
Types of Transformers
Transformers are constructed so that their characteristics match the application for which they
are intended. The differences in construction may involve the size of the windings or the
relationship between the primary and secondary windings. Transformer types are also designated
by the function the transformer serves in a circuit, such as an isolation transformer.
Distribution Transformer
Distribution transformers are generally used in electrical power distribution and transmission
systems. This class of transformer has the highest power, or volt-ampere ratings, and the highest
continuous voltage rating. The power rating is normally determined by the type of cooling
methods the transformer may use. Some commonly-used methods of cooling are by using oil
or some other heat-conducting material. Ampere rating is increased in a distribution transformer
by increasing the size of the primary and secondary windings; voltage ratings are increased by
increasing the voltage rating of the insulation used in making the transformer.
Power Transformer
Power transformers are used in electronic circuits and come in many different types and
applications. Electronics or power transformers are sometimes considered to be those with
ratings of 300 volt-amperes and below. These transformers normally provide power to the power
supply of an electronic device, such as in power amplifiers in audio receivers.
Control Transformer
Control transformers are generally used in electronic circuits that require constant voltage or
constant current with a low power or volt-amp rating. Various filtering devices, such as
capacitors, are used to minimize the variations in the output. This results in a more constant
voltage or current.
Auto Transformer
The auto transformer is generally used in low power applications where a variable voltage is
required. The auto transformer is a special type of power transformer. It consists of only one
winding. By tapping or connecting at certain points along the winding, different voltages can
be obtained (Figure 8).
Isolation Transformer
Isolation transformers are normally low power transformers used to isolate noise from or to
ground electronic circuits. Since a transformer cannot pass DC voltage from primary to
secondary, any DC voltage (such as noise) cannot be passed, and the transformer acts to isolate
this noise.
Instrument Potential Transformer
The instrument potential transformer (PT) steps down voltage of a circuit to a low value that can
be effectively and safely used for operation of instruments such as ammeters, voltmeters, watt
meters, and relays used for various protective purposes.
Instrument Current Transformer
The instrument current transformer (CT) steps down the current of a circuit to a lower value and
is used in the same types of equipment as a potential transformer. This is done by constructing
the secondary coil consisting of many turns of wire, around the primary coil, which contains only
a few turns of wire. In this manner, measurements of high values of current can be obtained.
A current transformer should always be short-circuited when not connected to an external load.
Because the magnetic circuit of a current transformer is designed for low magnetizing current
when under load, this large increase in magnetizing current will build up a large flux in the
magnetic circuit and cause the transformer to act as a step-up transformer, inducing an
excessively high voltage in the secondary when under no load.
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