Core Characteristics

The composition of a transformer core depends on such factors as voltage, current, frequency, size limitations and construction costs. 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 transformers are used when the voltage source has a high frequency (above 20 kHz). An iron-core transformers are usually used when the source frequency is low (below 20 kHz). A soft-iron-core transformer is very useful where the transformer must be physically small, yet efficient. The iron-core transformer provides better power transfer than does the air-core transformer. A transformer whose core is constructed of laminated sheets of steel dissipates heat readily, thus providing efficient transfer of power. The majority of transformers you will encounter in aircraft equipment contain laminated steel cores. These steel laminations are insulated with a non-conducting 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 transformer core is one that offers the best path for the most lines of flux with the least loss in magnetic and electrical energy.

Hollow-Core Transformers

There are two main shapes of cores used in laminated-steel-core transformers. One is the hollow-core, so named because the core is shaped with a hollow square through the centre. The following figure illustrates this shape of core. Notice that the core is made up of many laminations of steel.

FIGURE

The following figure illustrates how the transformer windings are wrapped around both sides of the core.

FIGURE

Shell-Core Transformers

The most popular and efficient transformer core is the shell core, as illustrated in the figure below. As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. The laminations are insulated from each other and then pressed together to form the core.

FIGURE