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Unit 28
Single-Phase Transformers
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Unit 28 Single-Phase Transformers
Objectives:
Discuss the different types oftransformers.
List transformer symbols and formulas.
Discuss polarity markings.
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Unit 28 Single-Phase Transformers
A transformer is a magnetically operated machine.
All values of a transformer are proportional to its
turns ratio.
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Unit 28 Single-Phase Transformers
The primary windingis connected to the
incoming power supply. The secondary windingis connected to the
driven load.
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Unit 28 Single-Phase Transformers
This is an isolation transformer. The secondarywinding is physically and electrically isolated from
the primary winding.
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Unit 28 Single-Phase Transformers
The two windings of an isolation transformer
are linked together by the magnetic field.
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Unit 28 Single-Phase Transformers
The isolation transformer greatly reduces
voltage spikes.
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Unit 28 Single-Phase Transformers
Basic construction of an isolation transformer.
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Unit 28 Single-Phase Transformers
Each set of windings (primary and secondary) is
formed from loops of wire wrapped around the
core. Each loop of wire is called a turn.
The ratio of the primary and secondary voltages is
determined by the ratio of the number of turns in
the primary and secondary windings. The volts-per-turn ratio is the same on both the
primary and secondary windings.
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Unit 28 Single-Phase Transformers
Transformer Symbols
NP
= number of turns in the primary
NS= number of turns in the secondary
EP= voltage of the primary
ES= voltage of the secondary
IP = current in the primaryIS = current in the secondary
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Unit 28 Single-Phase Transformers
Transformer Formulas
EP/ ES= NP/ NS
EPx NS= ESx NP
EPx IP= ESx IS
NPx IP= NSx IS
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Unit 28 Single-Phase Transformers
The distribution
transformer is a common
type of isolationtransformer. This
transformer changes the
high voltage from thepower company to the
common 240/120 V.
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The control transformer is another common
type of isolation transformer. Thistransformer reduces high voltage to the
value needed by control circuits.
Unit 28 Single-Phase Transformers
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Unit 28 Single-Phase Transformers
Polarity dots are placed on transformerschematics to indicate points that have the
same polarity at the same time.
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Unit 28 Single-Phase Transformers
Review:
1. All values of voltage, current, and
impedance in a transformer areproportional to the turns ratio.
2. The primary winding of a transformer isconnected to the source voltage.
3. The secondary winding is connected to theload.
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Unit 28 Single-Phase Transformers
Review:
4. An isolation transformer has its primary
and secondary voltage electrically andmechanically separated.
5. Isolation transformers help filter voltage
and current spikes.
6. Polarity dots are often added to schematic
diagrams to indicate transformer polarity.