It is better to use high-quality cores that are made from premium-grade virgin steel without any air gaps and loss sheets of steel to cause vibration. Ferrite Cores are also selected to make toroidal power transformers. Quiet operation and low losses are the main two reasons for engineers to use toroidal transformers. Related post: Introduction To Toroidal Power Transformers.
Aluminum and copper magnet wires are commonly used for power transformers, but copper wires are preferred in design toroid transformers due to their electrical merits. Magnet wires have to be enameled to prevent the wound wires around the core from shorting out with each other. If the price is a major concern, some would choose to use aluminum wires.
The winding process of the toroidal transformer is important to avoid any leakage inductance, so special care must be given to the winding technique. The winding of the enameled wire must be evenly distributed around the core surface, and the winding strength of the machine must be properly adjusted to avoid perforation of the enameled wire in the winding process.
Bank Winding Technique
Bank winding is the most popular winding technique, in which the wire is wound to form a pie-shaped segment around the toroid. An odd number of segments are wound around the toroid clockwise and follow the even number of segments counterclockwise. This winding process is repeated until it is completed. The bank winding technique greatly reduces the maximum turn-to-turn voltage gradient or stress on the conductor insulation, thus providing protection against voltage spikes which are very common in any application.
Interwinding Insulation Technique
The interwinding insulation technique places a layer of mylar insulation at the halfway point of a single primary winding or between individual windings in a dual primary configuration.
The insulation material has to do with transformers’ safety and performance. Better insulation materials provide for stronger impact resistance and reduce the possibility of short shorts and leakage, which enables long service life.
Toroidal transformers require insulation in different places or stages of construction, such as insulation thickness, number of layers, and creepage required. Insulation is provided between the toroidal core and the first winding (ground insulation), between successive windings (Inter-winding insulation), and on the outside of the last winding (outer wrap). Paper, varnish, polyester, and mineral oil are some popular insulation materials used for protecting windings.
Sometimes, a shielding layer is formed around the toroidal transformer circumference to better reduce the magnetic leakage, by using a thin sheet of grain-oriented silicon steel fixed with an outer wrap. By comparing with other EI transformers, toroids possess low magnetic leakage due to their unique constructure.
Encapsulation or Potting
Toroidal transformers are either open-core or encapsulated with thermoplastic or resin epoxy materials. The purpose of encapsulating a toroidal transformer is to protect it from vibrations, shocks, and water.
Impregnation is the last step in building a toroidal transformer. Transformers are impregnated to boost their performance and efficiency, provide better insulation, and prevent them from short circuits, noise, oxidation, and corrosion. For instance, a toroid transformer is impregnated to avoid any hum in audio applications.