ency circuits such as switch-mode power supplies, powdered iron cores are sometimes used. These materials combine a high magnetic permeability with a high material resistivity. At even higher frequencies (radio frequencies typically) other types of core made of non-conductive magnetic materials, such as various ceramic materials called ferrites are common. Some transformers in radio-frequency circuits have adjustable cores which allow tuning of the coupling circuit.
Air cores Edit
High-frequency transformers may also use air cores. These eliminate the loss due to hysteresis in the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.
Toroidal cores Edit
Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel or permalloy wound into a coil, or from ferrite, depending on frequency. This construction ensures that all the grain boundaries are pointing in the optimum direction, making the transformer more efficient by reducing the core's reluctance, and eliminates the air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to prevent the core's magnetic field from generating electromagnetic interference.
Toroidal cores for use at frequencies up to a few tens of kilohertz may also be made of ferrite material to reduce losses. Such transformers are used in switch-mode power supplies.
Toroidal transformers are more efficient (around 95%) than the cheaper laminated EI types. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantage is higher cost.
When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork.
The winding material depends on the application. Small power and signal transformers are wound with insulated solid copper wire, often enameled. Larger power transformers may be wound with wire, copper or aluminum rectangular conductors, or strip conductors for very heavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire, to minimize the skin effect losses in the conductors.
Windings on both primary and secondary of a power transformer may have taps to allow adjustment of the voltage ratio; taps may be connected to automatic on-load tap changer switchgear for voltage regulation of distribution circuits.
The conductor material must have insulation to ensure the current travels around the core, and not through a turn-to-turn short-circuit.
In power transformers, the voltage difference between parts of the primary and secondary windings can be quite large. Layers of insulation are inserted between layers of windings to prevent arcing, and the transformer is immersed in transformer oil that provides further insulation and acts as a cooling medium.
Although an ideal transformer is purely magnetic in operation, the close proximity of the primary and secondary