Power-frequency transformer

　　Power-frequency transformers, also known as low-frequency transformers, are distinguished from high-frequency transformers used in switch-mode power supplies. Historically, power-frequency transformers were widely employed in conventional power supplies, which relied on linear regulation to maintain stability—hence the term "linear power supplies." The term "power frequency" typically refers to the frequency of mains electricity, which in China is 50 Hz, though other countries use 60 Hz. A transformer capable of adjusting the voltage of alternating current while maintaining this specific frequency is precisely what we call a power-frequency transformer.

　　Principle

　　1. Reducing the amount of copper used can be achieved in two ways: First, decreasing the wire diameter—which would increase copper resistance and, consequently, copper losses. Second, reducing the number of turns would lead to higher no-load current, thereby increasing no-load losses as well. If a transformer remains energized in standby mode for extended periods, the waste of electrical resources becomes significant. In China alone, the annual electricity wasted due to household appliances staying in standby mode adds up to billions of yuan.

　　2. When designing a transformer, the copper losses should be equal to the iron losses to minimize overall transformer inefficiency and ensure stable operation. However, if, after the transformer is designed, a smaller wire gauge and fewer winding turns are used solely to save on copper costs—leaving ample unused space in the core window—it indicates that the core size was chosen too generously, resulting in unnecessary waste of core material. Moreover, because the larger core dimensions lead to a greater average winding circumference, this also inadvertently increases the amount of copper wire required. Given their respective prices, the cost of the iron core typically exceeds that of the copper windings.

　　Therefore, when designing, while ensuring that performance meets the customer's requirements, you should always opt for the smallest possible core size—whenever 41 is sufficient, avoid using 48 altogether. As for the no-load current, minimizing it is still the best approach from the perspective of reducing standby power losses.

　　Design and Winding of Power Frequency Transformers

　　Among various household appliances, power-frequency transformers—whether designed and wound from scratch or repaired after a burnout—require some basic calculations. While the calculation formulas found in textbooks are precise, they often appear complicated and inconvenient when applied in real-world situations.

　　1. Selection of the iron core

　　Choosing the right core based on your required power level is the first step in winding a transformer. If the core (silicon steel laminations) is too large, it will increase the transformer’s size and drive up costs—but if the core is too small, it will boost transformer losses and reduce its ability to handle load effectively.

　　To determine the core size, first calculate the actual power consumption of the transformer's secondary winding, which equals the sum of the products of each secondary winding’s voltage and its corresponding load current. For a full-wave rectifier transformer, the calculation should be based on half of the secondary winding’s voltage. The secondary winding’s power consumption, when added to the transformer’s own internal losses, gives you the transformer’s primary apparent power. Typically, for transformers with secondary winding power ratings below 10W, the internal losses can account for 30% to 50% of the actual secondary power consumed, resulting in an efficiency of only 50% to 70%. For transformers with secondary power ratings between 10W and 30W, losses drop to around 20% to 30%; for those rated between 30W and 50W, losses decrease further to approximately 15% to 20%; and for transformers with power ratings below 100W, losses typically range from 10% to 15%. Above 100W, losses generally fall below 10%. These loss figures are specific to conventional laminated transformers. However, if you’re using R-type, C-type, or toroidal transformers instead, the loss levels will progressively decrease in that order.

　　Based on the calculated total primary power of the transformer, the core can be selected. The core area is given by S = a × b (cm²), as shown in the accompanying figure. The relationship between the transformer's apparent power and the core area s is determined using the following empirical formula: s = K√P₁

　　P1 represents the total apparent power of the transformer's primary winding, measured in VA (volt-amperes). 's' is the core cross-sectional area to be selected, and 'K' is a coefficient that varies depending on the transformer's rated power, Pl. Additionally, accounting for the insulating varnish between silicon steel laminations and the presence of air gaps, the relationship between K and P1 is as follows:

　　P1 K-value

　　Below 10VA, 2 to 2.2

　　Below 50VA, 2 to 1.5

　　Less than 100VA, 1.5–1.4

　　2. Calculation of Turns per Volt

　　After selecting the iron core 's', determine the number of turns per volt to ensure the transformer winding has a reasonable magnetizing current. A commonly used empirical formula is: N = (40–55) / S, where N represents the number of turns per volt.

　　Select a coefficient of 40 to 55 depending on the quality of the silicon steel sheets. For higher-grade, high-silicon steel, you’ll notice flaky crystalline formations on the surface when viewed with the naked eye. Such material is also extremely brittle—bending it just 1 or 2 times will cause it to break abruptly, with jagged fracture edges. In this case, the coefficient should be set at 40. On the other hand, if the silicon steel sheet has a smooth, polished surface and can withstand 4 to 5 bends without easily breaking, resulting in a clean, straight fracture line, the coefficient should be chosen at 50 or higher.

　　After calculating the number of turns per volt, multiply it by 220V to obtain the primary winding’s turn count. Then, multiply this result by the required secondary voltage for each secondary winding to determine the number of turns in each secondary coil. Since the wire has resistance, a voltage drop will occur when current flows through it. Therefore, the calculated number of secondary turns should be increased by 5–10% (choose the exact percentage based on the load current—higher currents may require a larger adjustment).

　　3. Selection of Wire Diameter

　　Based on the magnitude of the load current in each winding, select enameled wire of different diameters. The following empirical formula can be used to determine this:

　　d = 0.8√I,

　　Unit: l--A.d (wire diameter)--mm.

　　4. Winding Method and Precautions

　　Due to the significant improvement in the insulation strength of enameled wire, most low-power transformers below 50W now use a stacked winding method with flame-retardant plastic bobbins. However, it is essential to select high-strength enameled wire and, during winding, carefully arrange each turn neatly—strictly avoiding large-angle diagonal crossings—to prevent increasing the potential difference between adjacent wires.

　　For transformers rated at 50W and above, the reduced number of turns per volt results in a higher voltage difference between wires. To mitigate this, it’s best to insert insulating paper (such as 0.05mm-thick cable paper or kraft paper) between each layer during winding. Importantly, ensure that the upper wire layers never slip down onto the lower ones. Insulation between individual windings should be chosen based on the voltage rating of each winding. For the primary and secondary windings, at least four layers of 0.1mm-thick cable paper should be used as insulation, and adhesive tape should be strictly avoided. In small-power transformers wound using the stacking method—and especially if the secondary side has two or more winding groups—each group must also be insulated from the others with two additional layers of cable paper. If the transformer is intended for use in audio or audiovisual equipment, an electrostatic shielding layer should be incorporated between the primary and secondary windings when employing the multi-layer winding technique.

　　After winding is complete, pay attention when inserting the silicon steel laminations—they must be firmly pressed in place to prevent electromagnetic noise. Whether using a double-E or EI core design, ensure that the core ports are in tight, continuous contact. It’s best to insert the laminations in a crisscross pattern, leaving no gaps whatsoever. For the final 4 to 5 laminations, you can slide them in from the center to avoid damaging the coil assembly. Next, proceed with drying and varnishing. For transformers rated below 50W, an internal heating method is recommended for drying. Here’s how: Short-circuit all of the transformer’s secondary windings, then connect a 60–100W/220V light bulb in series with the mains power supply. This setup will cause the transformer to heat up gradually as the bulb glows. Note that the larger the bulb, the higher the temperature—but in a sealed environment, keeping the temperature below 80°C is the safest approach.

　　What is the difference between a power-frequency transformer and a regular transformer?

　　When designing or rewinding power-frequency transformers for various household appliances—whether you're creating one from scratch or repairing a damaged transformer—the process inevitably involves some basic calculations. While the calculation formulas found in textbooks are precise and rigorous, they can seem overly complex and inconvenient to apply in real-world situations.

　　High-frequency transformer

　　The difference between power-frequency transformers and high-frequency transformers:

　　1. Power-frequency transformers use silicon steel laminations as the core material, while high-frequency transformers employ ferrite core materials.

　　2. The operating frequency of a power-frequency transformer typically refers to a power supply frequency of 50 Hz or 60 Hz; in contrast, high-frequency transformers usually operate at frequencies above 1 kHz, often reaching tens or even hundreds of kHz—though the specific frequency range varies depending on the application.