Like many electronic devices, transformers are comprised of a multitude of parts, each working in conjunction with the others to ensure the safe and effective transmission of energy. In order to get a better handle on different types of transformers and why they may be suitable for certain applications, it is helpful to examine the various components at work.The core makes up the bulk of a transformer, so it’s no surprise that selecting the proper material plays an integral role in the transformer’s overall function. A range of cores exist, such as steel laminated, solid, toroidal, and air cores, as well as variations of each within their respective categories.
Steel Laminated Cores
Steel laminated cores are known for their high level of permeability, making them a good choice for transmitting voltage at the audio frequency level, as the permeable core reduces magnetizing current. However, unlaminated steel cores have a high level of eddy current loss, which occur when a conductive material encounters a changing magnetic field, and can result in core heating. Because of the presence of several steel laminations, protected by a non-conducting insulator material between layers, these eddy currents are contained and magnetizing effects are reduced. Although thin laminations are harder to manufacture and are more expensive, they are effective in high frequency transformers.
Several designs are available for steel laminated transformers, each offering its own advantages. An E shaped core is affordable to manufacture, but tends to exhibit more energy loss. A C type core, on the other hand, offers reduced resistance because the metal grains run parallel to the energy flux.
Solid cores, particularly the powdered iron cores used in circuits, have high magnetic permeability as well as electrical resistance. When used in circuits, they tend to work best for transmission levels above main frequencies. For frequencies that tend to range even higher, such as those beyond the VHF (very high frequency) band, powdered iron is replaced by ferrites which are non-conductive, magnetic ceramic materials.
A range of materials are available for use in toroidal cores, including steel, coiled permalloys, powdered iron, or ferrites. These cores can be circular in structure, with the rest of the transformer built around the core ring—the lack of an opening in the core ring means no air gaps—or they can be a long strip of material. The advantage of using a strip is reduced resistance as a result of properly aligned grain boundaries. With a circular core, the windings are generally wound around the core, covering the surface in its entirety.
Toroidal cores are more efficient at handling the same kind of energy load than steel laminated E shape cores, and can be made smaller, lighter, and with a lower magnetic field. However, windings tend to be more expensive for toroidal cores.
In some applications it’s possible to get by without a core at all by simply placing the windings within appropriate range. The air that fills the space where the core would have been becomes the primary magnetic circuit, which doesn’t suffer from loss. However, leakage is high, making air cores a poor choice for transmitting or distributing power. They are often found in radio frequency applications.