Toroidal transformer
A toroidal transformer ( English toroidal transformer ) is a particular design of a transformer , the core in the form of a toroidal core made of a material such as soft iron or of special ceramic materials, the so-called ferrites exists. The windings are electrically isolated over the toroidal core . The individual turns are usually evenly distributed over the entire circumference of the ring. Toroidal transformers are used, among other things, in the area of smaller power transformers in power supplies with apparent powers in the order of magnitude of a few 10 VA to a few 10 kVA. Another exemplary application is pulse transformers in the field of data transmission, where toroidal transformers are used on the physical level in twisted pair cables , for example in the context of Ethernet .
General
Since the core has the shape of a ring and the windings that cover the entire core are tightly packed on top of one another with good magnetic coupling on the ring, the magnetic leakage flux is low compared to other core designs due to the design. The air gap in the core, which should be as small as possible to minimize magnetic resistance , is practically non-existent due to the shape of the core, which is reflected in the low no-load current and an almost vertical and narrow hysteresis curve . In comparison to angular iron cores (E or U cores), these advantages are bought at the cost of a more complex toroidal core winding technology . The electrical conductor, usually in the form of enamelled copper wire , is unwound on the toroidal core using special winding machines. Smaller toroidal cores, such as those used in pulse transformers for data transmission, are also wound by hand. Also current transformer -Transformers are built of toroids.
The no-load current of toroidal transformers is around 40 times lower than that of transformers with square cores of the same power. The current curve is shown in the adjacent picture, whereby the no-load current with a 1 kVA transformer is only 0.026 A eff . is small. (The current is shown on a greatly enlarged scale so that you can clearly see the curve in relation to the voltage.)
Separate electrical winding chambers on a support body, as are necessary, for example, in isolating transformers with increased safety requirements for electrical insulation, are in principle feasible with toroidal core transformers, but not sensible because the advantages of the very direct coupling of the secondary to the primary coil would be lost. The isolation of the individual windings from one another is just as well ensured by inlaid plastic films , previously also oil paper , so that toroidal core isolating transformers with increased safety requirements can also be produced with them. Their weight advantage compared to isolating transformers with square cores is up to 40%, with comparable technical data.
The material for the toroidal cores is made for low-frequency applications in the range of the mains frequency from sheet metal strips of soft iron which are electrically insulated on the surface and which are formed into a spiral-shaped layered toroidal core. As with other core designs, the stratification is necessary to minimize the eddy currents in the core. For higher frequencies, electrically non-conductive ceramic materials such as ferrites or cores pressed from ferromagnetic powders by sintering metallurgy are used.
Larger toroidal transformers are fastened with mounting screws, which fix the transformer centrally in its position and allow mounting on a base plate, or they are screwed onto a base plate by means of a nut in the middle hole. Large toroidal transformers, currently manufactured up to 150 kVA power, are also fully encapsulated and then have several fastening nuts cast in.
Toroidal transformers of all sizes cause high current peaks of up to 80 times their rated current ( inrush current ) when switched on , because their core can get into magnetic saturation more easily than with other transformers due to the high permeability when switched on. The permeability is so high because the hysteresis curve of the core is not sheared (tilted) due to the absence of air gaps. The max. Remanence always occurs when the system is switched off at the end of a half-wave. Household mains fuses can be triggered when toroidal transformers are switched on from a power of approx. 300 VA. These current peaks can be avoided entirely with soft starters or transformer switching relays or reduced with inrush current limiters (NTC) to such an extent that fuses do not trip. However, the latter require a cooling break of 1–2 minutes, which is often not given in technical use.
literature
- Irving M. Gottlieb: Practical Transformer Handbook for Electronics, Radio and Communications Engineers . Newnes, 1998, ISBN 978-0-7506-3992-7 .
