SUMMARY
As a study aid and for future reference, the important points of this chapter have been
summarized below.

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____BASIC TRANSFORMER__ - The basic transformer is an electrical device that transfers
alternating-current energy from one circuit to another circuit by magnetic coupling of the
primary and secondary windings of the transformer. This is accomplished through mutual
inductance (M). The coefficient of coupling (K) of a transformer is dependent upon the
size and shape of the coils, their relative positions, and the characteristic of the core
between the two coils. An ideal transformer is one where all the magnetic lines of flux
produced by the primary cut the entire secondary. The higher the K of the transformer, the
higher is the transfer of the energy.

The voltage applied to the primary winding causes current to flow in the primary.

This current generates a magnetic field, generating a counter emf (cemf) which has the
opposite phase to that of the applied voltage. The magnetic field generated by the current
in the primary also cuts the secondary winding and induces a voltage in this winding.

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____TRANSFORMER CONSTRUCTION__ - A TRANSFORMER consists of two coils of insulated wire
wound on a core. The primary winding is usually wound onto a form, then wrapped with an
insulating material such as paper or cloth. The secondary winding is then wound on top of
the primary and both windings are wrapped with insulating material. The windings are then
fitted onto the core of the transformer. Cores come in various shapes and materials. The
most common materials are air, soft iron, and laminated steel.

The most common types of transformers are the shell-core and the hollow-core types. The
type and shape of the core is dependent on the intended use of the transformer and the
voltage applied to the current in the primary winding.

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____EXCITING CURRENT__ - When voltage is applied to the primary of a transformer,
exciting current flows in the primary.

The current causes a magnetic field to be set up around both the primary and the
secondary windings. The moving flux causes a voltage to be induced into the secondary
winding, countering the effects of the counter emf in the primary.

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____PHASE__ - When the secondary winding is connected to a load, causing current to flow
in the secondary, the magnetic field decreases momentarily. The primary then draws more
current, restoring the magnetic field to almost its original magnitude. The phase of the
current flowing in the secondary circuit is dependent upon the phase of the voltage
impressed across the primary and the direction of the winding of the secondary.

If the secondary were wound in the same direction as the primary, the phase would be
the same. If wound opposite to the primary, the phase would be reversed.

This is shown on a schematic drawing by the use of phasing dots. The dots mean that the
leads of the primary and secondary have the same phase. The lack of phasing dots on a
schematic means a phase reversal.

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____TURNS RATIO__ - The TURNS RATIO of a transformer is the ratio of the number of turns
of wire in the primary winding to the number of turns in the secondary winding. When the
turns ratio is stated, the number representing turns on the primary is always stated
first. For example, a 1:2 turns ratio means the secondary has twice the number of turns as
the primary. In this example, the voltage across the secondary is two times the voltage
applied to the primary.

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____POWER AND CURRENT RATIOS__ - The power and current ratios of a transformer are
dependent on the fact that power delivered to the secondary is always equal to the power
delivered to the primary minus the losses of the transformer. This will always be true,
regardless of the number of secondary windings. Using the law of power and current, it can
be stated that current through the transformer is the inverse of the voltage or turns
ratio, with power remaining the same or less, regardless of the number of
secondaries.

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____TRANSFORMER LOSSES__ - Transformer losses have two sources-copper loss and magnetic
loss. Copper losses are caused by the resistance of the wire (I^{2}R). Magnetic
losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant
after the coil has been wound and therefore a measureable loss. Hysteresis loss is
constant for a particular voltage and current. Eddy-current loss, however, is different
for each frequency passed through the transformer.

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____TRANSFORMER EFFICIENCY__ - The amplitude of the voltage induced in the secondary is
dependent upon the efficiency of the transformer and the turns ratio. The efficiency of a
transformer is related to the power losses in the windings and core of the transformer.
Efficiency (in percent) equals P_{out}/P_{in} X 100. A perfect transformer
would have an efficiency of 1.0 or 100%.

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____POWER TRANSFORMER__ - A transformer with two or more windings wound on a laminated
iron core. The transformer is used to supply stepped up and stepped down values of voltage
to the various circuits in electrical equipment.

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____AUTOTRANSFORMER__ - A transformer with a single winding in which the entire winding
can be used as the primary and part of the winding as the secondary, or part of the
winding can be used as the primary and the entire winding can be used as the secondary.

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____AUDIO-FREQUENCY TRANSFORMER__ - A transformer used in audio-frequency circuits to
transfer af signals from one circuit to another.

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____RADIO-FREQUENCY TRANSFORMER__ - A transformer used in a radio-frequency circuit to
transfer rf signals from one circuit to another.

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____IMPEDANCE-MATCHING TRANSFORMER__ - A transformer used to match the impedance of the
source and the impedance of the load. The mathematical relationship of the turns and
impedance of the transformer is expressed by the equation: