41 Large transformers are filled with transformer oil that both cools and insulates the windings. The windings of signal transformers minimize leakage inductance and stray capacitance to improve high-frequency response. High-frequency transformers supplier operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses.
The electrical conductor used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. 37 Air cores are also used for resonant transformers such as Tesla coils, where they can achieve reasonably low loss despite the low magnetizing inductance. Air-core transformers are unsuitable for use in power distribution, 12 but are frequently employed in radio-frequency applications.
Other advantages compared to E-I 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 greater choice of shapes. Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite 35 A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer’s efficiency by reducing the core’s reluctance The closed ring shape eliminates air gaps inherent in the construction of an E-I core.
Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or amorphous (non-crystalline) metal alloy The higher initial cost of the core material is offset over the life of the transformer by its lower losses at light load. 30 The transformer universal EMF equation can be used to calculate the core cross-sectional area for a preferred level of magnetic flux. 28 Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires.
At higher voltage and power ratings, shell form transformers tend to be more prevalent. When windings surround the core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form. Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction , the frictional energy of which produces an audible noise known as mains hum or “transformer hum”.
Eddy currents are induced in the conductive metal transformer core by the changing magnetic field, and this current flowing through the resistance of the iron dissipates energy as heat in the core. Designing energy efficient transformers for lower loss requires a larger core, good-quality silicon steel , or even amorphous steel for the core and thicker wire, increasing initial cost.