Page 108 - Louth Transformer Catalogue

LT DATA

WHAT IS A TRANSFORMER?

STEEL CORES
Most transformers have cores made of thin laminations of silicon steel. As the steel core is also
conductive, it has currents induced in it by the changing magnetic flux. Insulating the layers from one
another helps to reduce eddy currents, and the insulation is used to prevent the laminations acting as
a solid piece of steel. The thinner the laminations, the lower the eddy currents and the lower the losses,
however very thin laminations are usually expensive. Most transformers use the traditional ‘E’ and ‘I’
shaped laminations, stacked alternately in order to reduce any air gap; however there are other types
such as ‘C’ cores

‘C’ cores are made by winding silicon steel strips around a rectangular form. Once the required
thickness has been achieved, it is removed and the laminations bonded together. It is then cut in half
to form two ‘C’ cores. The faces are then smoothed to ensure that they fit together with the smallest
possible gap, in order to reduce losses. The coil is placed over one half of the core, which is then
assembled and tightly held together by a steel strap.

TOROIDAL CORES
Toroidal transformers are assembled on a ring shaped core, usually with a rectangular cross section,
although a more expensive circular type is available. The core is constructed from strips of silicon steel
wound in to a coil. The closed ring eliminates any air gaps which are present in ‘E’ ‘I’ laminated cores.

Toroidal transformers are usually more efficient, although more expensive, than laminated ‘E’ ‘I’ types
of a similar power level, other advantages include less mechanical hum, making their usage superior
in audio applications such as amplifiers, as well as size and weight being around half that of ‘E’ ‘I’
laminated transformers.
When fitting a toroidal transformer, it is important not to allow both ends of the mounting bolt to
simultaneously come into contact with metalwork, as this would make a loop of conductive material
flowing through the hole in the toroid, thus allowing a dangerously large current to flow through the
bolt.

SHIELDING

Where minimum electrostatic coupling between the primary and secondary windings is required, an
electrostatic shield can be placed between the primary and secondary, in order to reduce capacitance
between them. The shield consists of a single layer of metal foil connected to earth, and insulated
where it overlaps to prevent it acting as a shorted turn. Transformers may also be placed within an
electrostatic enclosure to prevent interference with nearby electrostatic sensitive devices.

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