Since Mitchell Faraday invented the transformer, the transformer production process and the materials used have undergone major changes. At first, laminated transformers were widely used because of the easy winding on the coil skeleton. However, due to the air gap in the magnetic circuit of the laminated transformer, the effectiveness of the magnetic circuit is not high. Although the C-core transformer reduces the air gap and retains the advantage of easy winding on the coil skeleton, there is still an air gap in the magnetic circuit.
The invention of the high-speed ring-type transformer winding machine made it possible to quickly and smoothly wind the wire around the closed ring-type core, thus taking advantage of all the advantages of an airless core. Compared with the laminated transformer of the same capacity, the air-gap core transformer has the advantages of small volume, light weight and small magnetic leakage. In recent years, due to the sharp rise in labor costs and the continuous improvement of winding technology, the price difference between ring transformers and laminated transformers is very small. At present, compared with laminated transformers and C-type transformers, the price of 15-50VA ring transformers is more expensive, the price of 200-500VA ring transformers is slightly lower, and the price of 500VA ring transformers has a great advantage. However, in many applications, the technical characteristics of ring transformers are more important.
Ring core
The ring core is made of a silicon steel strip, much like a wound clock spring, which is then wrapped and impregnated and encased in a plastic case, and the core can be coated.
In 1990, the most significant invention of iron core materials was: in the low carbon content of 0.04% hot rolled low carbon steel, a small amount of silicon was infiltrated to form an alloy, which improved the performance of hot rolled silicon steel strip. Although the new material has overcome the problem of magnetic degradation, its magnetic permeability and ductility are reduced, and it is difficult to press and form.
In 1930, the magnetic characteristics of the cold-rolled silicon steel strip in the rolling direction were greatly improved, the grain orientation, the working magnetic flux density and the magnetic permeability of the silicon steel strip were greatly improved, and the loss was reduced. Unfortunately, these advantages can only be obtained if the magnetic flux is consistent with the direction of the grain. The study of laminated core magnetic circuit shows that about 40% of the core is 90° to the optimal direction; The other 40% acts as a flux loop, and the remaining small portion of the core can reach its full potential state. However, in a ring core, the magnetic flux direction is always the same as the grain direction, but when the grain orientation is 90° from the magnetic flux direction, the effectiveness of GOSS is worse than that of hot-rolled silicon steel strip.
The average working magnetic density of the GOSS core is about 1.6T, and the corresponding hot-rolled silicon steel strip is about 1.3T, and the corresponding loss of the two materials is 0.45W / 1B and 1.25W / 1B, which is much smaller than the loss of copper.
The main reason for the low iron loss is that the ring core has a continuous magnetic circuit, while the laminated core transformer has an air gap between the EI sheets. Therefore, the existence of large air gap reluctance is the main disadvantage of laminated transformers.
In magnetic components, the resistance of the air gap is much greater than that of the iron core. The magnetoresistance of the air gap not only reduces the efficiency of the laminated core transformer, but also increases the magnetic density of the edge of the air gap. A portion of the flux leaks into the space around the transformer, creating parasitic electromotive forces on wires, printed boards, and passive components. In order to reduce the impact of the air gap, C-core can be used. The steel strip is wrapped around the mandrel, dipped and cut in half, polished off the cut surface, polished and etched. In this way, when the two parts of the core are in contact within the coil skeleton, the gap is small. The ring core has no air gap, and has reverse winding winding, which can further reduce the parasitic magnetic field, and the electrical noise can be reduced to 1/7 ~ 1/10 of the laminated transformer.
The ring core not only has excellent magnetic properties, but also has many mechanical advantages: the inter-chip pressure can be strictly controlled and can reach the same level of 95%. Using impregnation technology, the core can become a solid whole, in the winding and processing process without deformation; Because the ring core is very strong, the influence of magnetostrictive force is reduced. This reduces vibration and audio noise.
There are many fixing methods for ring transformers, the most common method is to use the center hole of the ring transformer. It should be noted that screws and other fasteners should not make the top clamp plate of the transformer and the bottom plate grounded at the same time, so as not to form a short circuit ring. This situation deserves special attention.
The diameter/height/width ratio of the ring core can be changed as required. Generally, the weight of the transformer decreases with the increase in height, but the increase in core height, average turn length of the wire and copper weight offset the reduction in iron weight.
The most commonly used ratio of iron core diameter, height and width with excellent performance is: 4×2×1 or 5×3× 1.5 and 6×2×0.5; In the application, there is no limit to width or height.
Insulation requires a large center hole, surface area requirements are not strict but height limits, can be used. The core design is also limited by the winding machine, and the core must have a center hole large enough for the line shuttle to pass smoothly.
The winding of a ring transformer
When winding the wire, the wire passes through the core center hole, is wound to the core, the outer surface, and is wound back to the center hole. The winding is completed by using an open ring type wire shuttle which can be inserted into the iron core. Wrap the appropriate length of wire into the groove of the bobbin, with one end of the wire fixed to the core: Wrap the wire from the bobbin around the core. When the required number of turns is finished, the excess wire is removed from the bobbin, and then the opening of the bobbin is opened and the iron core is removed from the bobbin. The size of the winding and core is limited by the winding machine, the requirements for the wire shuttle are related to the wire and insulation, and the maximum height of the core is also limited by the diameter of the wire shuttle. Another function of the winding is to strengthen the iron core to reduce electromagnetic vibration, reduce audio noise, and also act as an electrostatic shield between the windings. Using special winding techniques, it is possible to reduce the magnetic field effect of the winding. The maximum winding speed of the standard ring winding machine is 2000 turns/min. The winding made of wires with good extensibility and insulation has a very high yield.
Apply
An example of a ring transformer that is clearly superior to a lamination core transformer is a power transformer for a low-voltage high-brightness Marine navigation light that can be installed on the top of the mast of an omnidirectional radio beacon machine. Other transformers are difficult to install in this position, but the ring transformer has a central hole, as long as the barrel is single over the antenna rod, connected to the power supply.
The low radiation field of ring transformer is its inherent advantage. When the laminated transformer is used, if the radiation noise is too large, shielding technology can be used, and when this method still cannot meet the requirements, ring transformers should be used.
Because the ring transformer has low noise retention, it is used in the United States for high-fidelity audio equipment and video display terminals; Japan uses it for power amplifiers, electronic test instruments and general equipment. When the inherent low noise characteristics of the bad transformer cannot meet the requirements, low carbon steel or micrometal tank shielding can be added.
It should be noted that annular transformers cannot be used in half-wave rectifier circuits. Because it produces a hysteresis loop that is asymmetrical to the S/H axis. In this way, the core becomes unidirectional polarized and saturated. This shortcoming can be overcome by using bridge rectifier. The remanence of ring transformers with large capacity made of cold rolled strip is higher than that of laminated transformers made of hot rolled strip. In this way, when the magnetic density in the core is higher; After the transformer is disconnected from the power supply, the core will retain 80% of the magnetic flux value, if the transformer is energized again in the direction of remanence, the core magnetic flux will soon reach the saturation value from the remanence value, so that the instant starting magnetization current is only limited by the low group of resistance of the primary winding, this starting current will blow the fuse, therefore, in the primary circuit, should be added to the surge prevention
Safety or soft starting resistance.
At present, limited by the winding machine, the maximum capacity of the ring transformer can be made is 10kVA. For general electronic devices. This capacity is sufficient.
Major advantage
The main advantages of ring transformers are lower radiation field and higher efficiency. In the case of half the size and weight, the given capacity can be reached, and if a transformer with a slightly larger capacity is used, the temperature of the transformer can be reduced. The center hole fixation allows the ring type transformer data to be easily installed on the printed circuit board.
The ring type transformer has a high degree of flexibility and can be designed according to the requirements of the chassis and the overall assembly. Because the production of ring transformers does not require a die, nor does the coil skeleton injection mold, the production cycle is short, suitable for small and medium-sized batch production, and can meet the needs of contemporary electronic equipment continuous modification.
