Tape Wound Toroidal Cores
Cut-a-way view of an aluminum-cased tape wound core
Examples of TWTC magnetic circuit configurations
TWTC magnetic circuit configurations within finished transformers
The tape wound toroidal core approaches the perfect magnetic circuit configuration as well as permitting the most efficient application of high permeability magnetic alloys. The physical and magnetic characteristics of the toroidal shape reveal many features which contribute to this near-perfect circuit.
For instance, the air gap in the magnetic path is so small that it can be considered non-existent. This minimizes losses, fringing, leakage, distortion, and decreases the magnetizing force necessary to produce a given flux within the material.
In a toroidal core and coil assembly, the entire magnetic path is contained within the electrical winding, further minimizing leakage flux and increasing winding-to-winding coupling. Tape wound cores do generate a small flux in the axial direction, however, this leakage flux can be contained by ring laminations assembled to the top and bottom of the core.
The tape wound core configuration also provides a good degree of self-shielding from external magnetic fields. The single, uniform, magnetic path causes any entering magnetic field to split into two and induce equal but opposite voltages in the two halves of a uniformly distributed winding. Thus, there tends to be no voltage apparently induced in the total winding.
Available Shapes, Sizes and Materials
Magnetic Metals manufactures tape wound toroidal cores and other shapes for power transformers, inductors, drive transformers, saturable reactors, magnetic amplifiers, current transformers, converters and inverters.
We offer a wide range of sizes and materials for tape wound transformer cores. Almost any size can be made for minimum tooling costs. A wide selection of cases or coating materials are also available to meet your requirements.
The magnetic materials used for tape wound cores can be classified in two broad categories: "Square Loop" or "Round Loop". This classification is made to the relative shape of the B-H loop.
Square loop versions tend to have:
- Higher maximum flux capabilities (Bm)
- Wider loops (greater Hc loop width at zero flux)
- Higher squareness ratios (Br/Bm) - ratio of residual flux density to maximum flux density
- Higher core losses
Round loop versions tend to have:
- Lower Bm
- Narrower loops
- Lower squareness ratios
- Higher initial permeability
- Lower core losses
Our technical design engineers are always available to help you select the optimum material for your application, and then incorporate the material’s characteristics in the final design.
Square Loop materials include:
- Amorophous alloys;
- Cobalt iron;
- Microsil™ silicon iron alloy;
- Nanocrystalline alloys;
- Square 50;
- Square 80;
- Super Square 80;
Round Loop materials include:
- SuperPerm™ 49;
- SuperPerm™ 80.
Visit our Advanced Grade Materials page for more information.
Tape wound toroidal cores are fabricated on specially designed machines which wind insulated tape onto a mandrel under controlled tension to provide an extremely uniform cross-section. The wound cores are then annealed in a controlled atmosphere of hydrogen/nitrogen. This develops the specific magnetic characteristics required for the application.
Annealed cores are sensitive to mechanical stresses in varying degrees depending upon the alloy. These stresses cause changes in the magnetic characteristics of the material which may severely alter the performance of the finished core. To prevent these changes from taking place, the annealed tape cores are housed within cases which protect them from the strains of electrical winding and other external disturbances.
These cases are fabricated of various materials depending upon the intended application: plastics; phenolic; nylon; glass-reinforced nylon; and aluminum are typically used.
The non-metallic cases (glass-filled nylon, phenolic, nylon) are the most widely used. The glass filled nylon case has proven superior to the phenolic case because of its greater strength. Aluminum cases provide greater environmental protection and this quality can be further enhanced by the application of an epoxy finish over the case.
A damping medium fills the space between the core and the case to minimize the motion of the core within the case, thus reducing the possibility of change in electrical characteristics under shock and vibration.
Pulse or High Frequency Applications
Magnetic Metals has developed a special process whereby tape wound core pulse or high frequency performance is optimized. To ensure that cores intended for pulse applications are manufactured under this process, the suffix "P" should be added to the standard core number, i.e., 11P4601-P, when ordering.
Gapped Toroidal Cores
The design of a storage choke or transformer which also carries direct current normally requires air gaps which prevent the core from being magnetized by the dc current above 1/2 B max. An air gap in a core has the effect of flattening/ shearing the B-H loop of the material, lowering the residual flux and the permeability, extending the incremental a.c. permeability to higher values of B and H. Precise control of the inductance L, or the remanence Br is therefore possible with gapped toroids.
Constant Current Flux Reset Test
The Constant Current Flux Reset (CCFR) test is widely used to evaluate core performance for magnetic amplifier use. The test is described in IEEE Standard 106. The ac excitation is usually specified at 400 Hertz, but frequencies between 60 and 6000 Hertz may also be specified. Our technical design engineers will ensure maximum flux change and reset as required for your magnetic amplifier specifications.