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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.