I. Principle of Power Frequency Transformer
Power frequency transformers, also known as low-frequency transformers, are distinguished from high-frequency transformers used in switching power supplies. Power frequency transformers were widely used in traditional power supplies, which employed linear regulation for stabilization; hence, these traditional power supplies were also called linear power supplies. Power frequency generally refers to the frequency of AC mains electricity, which is 50Hz in my country and 60Hz in other countries. A transformer that can change the voltage of AC power at this frequency is called a power frequency transformer.
1. Reducing the amount of copper used can be achieved in two ways: First, reducing the wire diameter means increasing copper resistance and thus copper loss. Second, reducing the number of turns increases the no-load current, which in turn increases no-load loss. If the transformer remains in standby mode for extended periods, the waste of electrical resources is enormous. Every year in my country, billions of yuan are wasted due to household appliances remaining in standby mode for extended periods.
2. Transformer design should ensure that copper losses and iron losses are equal. This minimizes transformer losses and ensures stable operation. If, after designing a transformer, a smaller wire diameter and fewer turns are used to save copper wire, leaving significant space in the core window, this indicates an oversized core, resulting in waste. A larger core also leads to a larger average winding circumference, further increasing copper wire usage. Based on price, the cost of the core is higher than the cost of the copper wire.
Therefore, during the design process, while ensuring that the performance meets customer requirements, smaller iron cores should be selected whenever possible; if a 41mm core can be used, a 48mm core should never be used. Regarding no-load current, it is best to keep it as low as possible to save on standby power consumption.
II. Design and winding of power frequency transformers
Among various household appliances, power frequency transformers, whether designed and wound by oneself or repaired from burnt-out transformers, involve some simple calculations. Although the calculation formulas in textbooks are rigorous, they are complicated and inconvenient to use in practice.
1. Selection of iron core
Choosing the appropriate iron core based on the required power is the first step in winding a transformer. If the iron core (silicon steel sheet) is too large, the transformer will be larger and the cost will increase. However, if the iron core is too small, the transformer's losses will increase, and its load-carrying capacity will decrease.
To determine the core size, the actual power consumption of the transformer's secondary winding must first be calculated. This is equal to the sum of the products of the voltages of each secondary winding and the load current. For a full-wave rectifier transformer, it should be calculated as half the secondary voltage. Adding the transformer's own power losses to the secondary winding power consumption gives the transformer's primary apparent power. Generally, for transformers with secondary winding power below 10W, their own losses can reach 30-50% of the actual secondary power consumption, resulting in an efficiency of only 50-70%. For secondary winding power below 30W, losses are approximately 20-30%; below 50W, approximately 15-20%; below 100W, approximately 10-15%; and above 100W, approximately less than 10%. These loss parameters pertain to ordinary laminated transformers. If following the order of R-type transformers, C-type transformers, and toroidal transformers, the loss parameters decrease sequentially.
The core area can be selected based on the calculated total primary power of the transformer. The core area S = a × b (cm²). See the attached diagram. The relationship between the transformer's apparent power and s can be selected using the following empirical formula: s = K√P1
P1 is the total apparent power of the transformer primary winding, in VA (volt-amperes). s is the required cross-sectional area of the core. K is a coefficient that varies depending on the size of the transformer. Considering the influence of the insulating varnish and gaps between the silicon steel sheets, the relationship between K and P1 is as follows:
P1 K value
Below 10VA: 2-2.2
Below 50VA, 2-1.5
1.5～1.4 below 100VA
2. Calculation of turns per volt
After selecting the core 's', determine the number of turns per volt to ensure the transformer has a reasonable excitation current. A commonly used empirical formula is: N = (40~55)/S, where N is the number of turns per volt.
The coefficient for silicon steel sheets varies from 40 to 55 depending on their quality. Higher-grade high-silicon steel exhibits scaly crystals on its surface when visually inspected. It is also extremely brittle, breaking after only 1-2 bends with an uneven fracture surface; a coefficient of 40 is used for this type. If the silicon steel sheet has a smooth surface, it remains relatively unbreakable even after 4-5 bends, producing a clean, straight fracture surface; a coefficient of 50 or higher is used for this type.
After calculating the number of turns per volt, multiply it by 220V to get the number of primary turns. Multiply it by the required secondary voltage to get the number of turns in each secondary winding. Because the wires have resistance, there will be a voltage drop when current flows through them. Therefore, the calculated number of secondary turns should be increased by 5-10% (depending on the load current; a larger percentage can be increased for larger currents).
3. Selection of wire diameter
Select enameled wire of different diameters based on the magnitude of the load current in each winding. This can be determined using the following empirical formula:
d=0.8√I,
Unit: l--Ad (wire diameter)--mm.
4. Winding method and precautions
Because the insulation strength of enameled wire is greatly improved, most small power transformers below 50W adopt the flame-retardant plastic skeleton winding method. However, high-strength enameled wire must be selected, and the wire should still be laid out one turn at a time during winding. Large diagonal crossings are strictly prohibited to avoid increasing the potential difference between conductors.
For transformers with a power rating of 50W or higher, due to the reduced number of turns per volt and the higher voltage difference between conductors, it is best to use insulating paper (0.05mm thick cable paper or kraft paper) between each layer. During winding, it is crucial to prevent upper conductors from slipping into lower ones. The insulation between windings should be determined based on the winding voltage. At least four layers of 0.1mm cable paper should be used between the primary and secondary windings; adhesive tape should be avoided. For small-power transformers using the above-mentioned lap winding method, if the secondary winding has two or more sets of windings, two layers of cable paper insulation should also be used between each set. If the transformer is used in audio or audio-visual equipment, an electrostatic shielding layer should be placed between the primary and secondary windings in multi-layer winding methods.
After winding, the silicon steel sheets must be inserted firmly to avoid electromagnetic noise. Whether double-E or EI type, the ends must be in tight contact. They should be inserted in a staggered pattern without gaps. The last 4-5 sheets can be inserted from the middle to avoid damaging the coil. Then, dry and impregnate. For transformers below 50W, internal heating can be used for drying. The method is as follows: short-circuit all secondary windings of the transformer and connect them in series with a 60-100W/220V light bulb to the mains power supply, allowing it to heat up automatically. The larger the light bulb, the higher the temperature, but it is safer to keep the temperature below 80 degrees Celsius in a sealed environment.
