In its most basic form consists of:

A primary coil or winding.

A secondary coil or winding.

A core that supports the coils or windings.

Refer to the circuit in the illustration below as you read the following explanation: The primary winding is connected to a 60 hertz ac voltage source. The magnetic field (flux) builds up (expands) and collapses (contracts) about the primary winding. The expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding.

This voltage causes alternating current to flow through the load. The voltage may be stepped up or down depending on the design of the primary and secondary windings.

Basic transformer action.


Two coils of wire (called windings) are wound on some type of core material. In some cases the coils of wire are wound on a cylindrical or rectangular cardboard form. In effect, the core material is air and this type is called an AIR-CORE.

Transformers used at low frequencies, such as 60 hertz and 400 hertz, require a core of low-reluctance magnetic material, usually iron. This type is called an IRON-CORE. Most power types are of the iron-core type. The principle parts and their functions are:

The CORE, which provides a path for the magnetic lines of flux. The PRIMARY WINDING, which receives energy from the ac source. The SECONDARY WINDING, which receives energy from the primary winding and delivers it to the load. The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical damage.


The composition of the core depends on such factors as voltage, current, and frequency. Size limitations and construction costs are also factors to be considered.

Commonly used core materials are air, soft iron, and steel. Each of these materials is suitable for particular applications and unsuitable for others. Generally, air-core types are used when the voltage source has a high frequency (above 20 kHz).

Iron-core types are usually used when the source frequency is low (below 20 kHz). A soft-iron-core type is very useful where the unit must be physically small, yet efficient. The iron-core type provides better power transfer than does the air-core type.

A unit whose core is constructed of laminated sheets of steel dissipates heat readily; thus it provides for the efficient transfer of power. The majority of these units you will encounter in Navy equipment contain laminated-steel cores. These steel laminations (see the next illustration below)are insulated with a nonconducting material, such as varnish, and then formed into a core. It takes about 50 such laminations to make a core an inch thick. The purpose of the laminations is to reduce certain losses which will be discussed later in this chapter.

An important point to remember is that the most efficient core is one that offers the best path for the most lines of flux with the least loss in magnetic and electrical energy.

Below is a short video on basic operation. You should note which direction current flows when the unit conducts.

Hollow core construction in a transformer.

Hollow Core



Counter emf

Coefficient of coupling

Effect of a load

Power Relationship


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