2. The characteristics of the toroidal transformer
The core of the toroidal transformer is made of high-quality cold-rolled silicon steel sheet (the thickness of the sheet is generally 0.35mm or less), seamlessly rolled, which makes its core performance is better than that of the traditional stacked-sheet core. Toroidal transformer coils are evenly wound on the core, the direction of the magnetic lines of force generated by the coils and the core of the magnetic circuit is almost completely overlap, compared with the stacked pieces of excitation energy and core loss will be reduced by 25%, which brings the following series of advantages.
1) high efficiency heart without air gap, stacked coefficient can be as high as 95% or more, the core permeability can be taken 1.5 ~ 1.8T (laminated core can only be taken 1.2 ~ 1.4T), the electrical efficiency of 95% or more, the no-load current is only 10% of the laminated type.
2) small dimensions, light weight toroidal transformer than stacked transformer weight can be reduced by half, as long as the core cross-sectional area is equal to keep the toroidal transformer is easy to change the length of the core, width, height ratio, can be designed to meet the requirements of the dimensions.
3) less magnetic interference toroidal transformer core has no air gap, the windings are uniformly wound in the ring on the core, this structure leads to a small leakage, electromagnetic radiation is also small, no need to add another shielding can be used to high sensitivity electronic equipment, such as applications in low-level amplifiers and medical equipment.
4) vibration noise is small core without air gap can reduce the noise of the core induction vibration, winding uniform tightly wrapped around the annular core, effectively reducing the magnetostriction caused by the "hum" sound.
5) low operating temperature due to iron loss can be done 1.1W/kg, iron loss is very small, low temperature rise of the core, the windings in the lower temperature of the core on the heat dissipation is good, so the transformer temperature rise is low.
6) easy to install toroidal transformer only the center of a mounting screw, especially easy in electronic equipment for rapid installation and removal.
3. Classification of toroidal transformers
The toroidal transformer can be divided into standard, economic and isolation type and so on three categories
1) standard power transformer product series capacity 8 ~ 1500VA, has a small voltage adjustment rate, full-load operation temperature rise is only 40 ℃, allowing short-time overload operation, suitable for demanding applications. The primary and secondary windings are insulated with polyester film insulation of class B (130℃), which is required to be wrapped with at least three layers of insulating tapes, and can withstand AC 4000V, 1min voltage withstand test.
2) economic power transformer product series capacity 50 ~ 1500VA, in order to ensure the performance on the basis of trying to reduce the cost, suitable for continuous operation without overloading the use of occasions, the operating temperature rise of 60 ℃, the insulation material grade for the class A (105 ℃), when the full load when the output voltage error is less than 3%.
3) isolation transformer product series capacity 50 ~ 1000VA, can be divided into industrial and medical equipment with two series. Isolation transformer focuses on its insulation performance, primary and secondary with B grade insulation between polyester film wrapped at least 4 layers, breakdown voltage greater than 4000V, all primary leads must be used double insulated wire. The maximum temperature rise of the transformer is less than 45℃. Medical isolation transformer in addition to the above requirements, but also in line with UL544 standard, that is, the primary and secondary windings should have thermal protection, winding and grounded copper shielding spacing distance should be greater than 13mm.
In addition to medical isolation transformer is also required to be equipped with a temperature protection switch in the primary winding, when the core temperature reaches 120 ℃, the temperature protection switch is disconnected when the temperature returns to normal, the switch automatically reset. When the temperature returns to normal, the switch automatically reset and close. Toroidal transformer due to the absence of air gap, coil uniform winding core, theoretically very small magnetic leakage, there is no coil radiation. However, the toroidal transformer due to the existence of no air gap, anti-saturation ability is poor, in the presence of DC components of the utility is prone to saturation, resulting in a very strong magnetic leakage. Many areas of the domestic utility waveform distortion is serious, so many users use the toroidal transformer feeling is not better than the EI-type transformer, or even worse.
The so-called toroidal transformer is absolutely no leakage, is due to media misinformation, or because of the manufacturers of commercial publicity needs and fabricated, toroidal transformer magnetic leakage is very low only in the utility waveforms for the strict sinusoidal waveforms to be established. In addition, toroidal transformers also have strong electromagnetic leakage at the leads, so the leakage of the toroidal transformer is also somewhat directional, and the actual installation of the toroidal transformer is rotated at an angle to obtain the highest signal-to-noise ratio. If conditions permit, consider installing a shield for the transformer, and do a proper grounding, the metal cover can only choose ferrous materials, general metal such as copper, aluminum, etc. only electric shielding without magnetic shielding effect, can not be used as a transformer shield.
The above analysis is based on the transformer materials, production of good basis, the actual majority of commercially available transformer products due to cost pressures and competitive needs, not in strict accordance with industry norms design, and sometimes even cut corners, analyze the unpredictability of more factors. First of all, the quality of the core material, many enterprises with low permeability H50 iron core, corners and even mixed soft iron production transformer, resulting in transformer no-load current is very high, iron loss is too large, no-load heating is serious; this type of transformer to reduce costs, at the same time as to cover the high iron loss voltage regulation rate is too large problem, greatly reduce the primary and secondary coil turns to reduce the copper loss to reduce the rate of adjustment, this practice further increases the voltage regulation rate, the copper loss, the copper loss to reduce the voltage regulation rate, the copper loss to reduce the voltage regulation rate. This practice further increases the no-load current, and no-load current is large will directly lead to increased magnetic leakage.
The toroidal transformer problem is more complex. A proper toroidal transformer core is tightly wound from an equal width silicon steel strip. Or for cost reasons, most of the low-priced toroidal transformer using several or even dozens of silicon steel strip splicing, and even the use of uneven edges of the edge of the corner material winding, winding good with the machine tool car flat, due to the toroidal transformer coils wrapped around the iron core, do not do destructive dissection is difficult to find. Mechanical processing of silicon material lattice arrangement, adjacent silicon steel strip insulation are seriously damaged, such a toroidal transformer regardless of performance or leakage characteristics will be greatly reduced, even after annealing treatment can not make up for the quality of the serious defects.
The stray electromagnetic waves mainly come from the active speaker's power output leads, speakers and power crossover, wireless transmitter equipment and computer mainframe, the reason is not discussed in depth here. Spurious electromagnetic waves in the form of transmission, induction and power transformer similar to the frequency range of the spurious magnetic field is very wide, there are active speakers reflecting the inexplicable reception of local radio broadcasts is a typical spurious electromagnetic wave interference.
Another source of interference that requires attention is the rectifier circuit. Filter capacitors in the boot into the normal state, charging is only concentrated in the peak AC power, charging waveform is a narrow width of the strong pulse, the larger the capacitance, the pulse intensity is also the larger, from the point of view of electromagnetic interference, the filter capacitor is not the larger the better, the rectifier and the filter capacitor should be shortened as much as possible, while trying to stay away from the power amplifier circuit, the PCB space does not allow as much as possible with the land line envelope.
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