Single-Phase Transformers (part 1)

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Understanding how transformers change values of voltage and current is important to the information presented in later units. Many AC motors operate on these principles. We will…

  • discusses transformers and how they are divided into three major types.
  • determines values of voltage and current and illustrates different methods that can be employed to determine these values.
  • describes how windings determine the primary and secondary voltage and how the nameplate rating provides key information when providing protection.
  • discusses installation and testing.



  • Autotransformers
  • Constant-current transformer
  • Control transformer
  • Distribution transformer
  • Excitation current
  • Flux leakage
  • Inrush current
  • Isolation transformers
  • Laminated
  • Primary winding
  • Secondary winding
  • Step-down transformer
  • Step-up transformer
  • Tape-wound core
  • Toroid core
  • Transformer
  • Turns ratio
  • Volts-per-turn ratio

Learning Goals:

  • discuss the different types of transformers.
  • calculate values of voltage, current, and turns for single-phase transformers using formulas.
  • calculate values of voltage, current, and turns for single-phase transformers using the turns ratio.
  • connect a transformer and test the voltage output of different windings.
  • discuss polarity markings on a schematic diagram.
  • test a transformer to determine the proper polarity marks.


Transformers are among the most common devices found in the electrical field. They range in size from less than one cubic inch to the size of rail cars. Their ratings can range from milli-volt-amperes (mVA) to giga-volt amperes (GVA). It’s imperative that anyone working in the electrical field have an understanding of transformer types and connections. This unit presents transformers intended for use in single-phase installations. The two main types of voltage transformers, isolation transformers and autotransformers, are discussed.

+++++1 All values of a transformer are proportional to its turns ratio. Primary 1000 turns; Secondary 250 turns

+++++2 An isolation transformer has its primary and secondary windings electrically separated from each other. Load Secondary Primary

Single-Phase Transformers

A transformer is a magnetically operated machine that can change values of voltage, current, and impedance without a change of frequency. Transformers are the most efficient machines known. Their efficiencies commonly range from 90% to 99% at full load. Transformers can be divided into three classifications:

1. Isolation transformer

2. Autotransformer

3. Current transformer

All values of a transformer are proportional to its turns ratio. This does not mean that the exact number of turns of wire on each winding must be known to determine different values of voltage and current for a transformer.

What must be known is the ratio of turns. For example, assume a transformer has two windings. One winding, the primary, has 1000 turns of wire; and the other, the secondary, has 250 turns of wire (). The turns ratio of this transformer is 4 to 1, or 4:1 (1000 turns/250 turns=4). This indicates there are four turns of wire on the primary for every one turn of wire on the secondary.

Transformer Formulas:

Different formulas can be used to find the values of voltage and current for a transformer. The following is a list of standard formulas, where ...

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 primary

IS = current in the secondary

The primary winding of a transformer is the power input winding. It’s the winding that is connected to the incoming power supply. The secondary winding is the load winding, or output winding. It’s the side of the trans former that is connected to the driven load).

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