Power frequency transformer

Power frequency transformers are also called low frequency transformers to show that they are different from high frequency transformers used in switching power supplies. Power frequency transformers have been used in large quantities in traditional power supplies in the past, and the stability of these power supplies uses linear adjustment, so those traditional The power supply is also called a linear power supply. The power frequency generally refers to the frequency of the mains electricity, which is 50Hz in our country and 60Hz in other countries. The transformer that can change the voltage of the alternating current at this frequency is called a power frequency transformer.

Principle

1. Reducing the amount of copper can be achieved in two ways. One is to reduce the wire diameter, which means that the copper resistance increases, and the copper loss will increase. The second is that reducing the number of turns will increase the no-load current, and the same no-load loss will increase. If the transformer is in the standby state for a long time, the waste of power resources is very large. Every year, the waste of electricity in our country due to the long-term standby power-on state of household appliances is in the billions.

2. When the transformer is designed, the copper loss and the iron loss should be equal, so that the transformer has the lowest loss and the most stable operation. If a transformer is designed, in order to save copper wires, a smaller wire diameter and a method of reducing the number of turns should be adopted. , So that there is a lot of space for the core window, which means that the size of the core is selected too large, resulting in a waste of the core. Due to the large size of the core, the average circumference of the winding is also large, which will also cause the copper wire Increase in dosage. According to the price, the cost of the core is higher than the cost of the copper wire.

Therefore, in the design, under the condition of ensuring that the performance meets the customer's requirements, small iron cores should be selected as far as possible. 41 can be used, and 48 should never be used. Regarding the no-load current, it is better to keep it as low as possible in terms of saving standby losses.

Design and winding of power frequency transformer

In various household appliances, power frequency transformers are involved in some simple calculations, whether they are self-designed winding or repairing a burnt out transformer. Although the calculation formula in the textbook is rigorous, it is complicated and inconvenient in actual use.

1. The choice of iron core

Choosing the right core according to the power you need is the first step in winding a transformer. If the iron core (silicon steel sheet) is selected too large, the transformer will increase in volume and cost, but if the iron core is too small, the loss of the transformer will be increased, and the load capacity will be deteriorated.

In order to determine the size of the iron core, first calculate the actual power consumption of the transformer secondary, which is equal to the sum of the product of the transformer secondary winding voltage and load current. If it is a full-wave rectifier transformer, it should be calculated as 1/2 of the transformer secondary voltage. The power consumption of the secondary winding is added to the power loss of the transformer itself, which is the apparent power of the transformer primary. Generally, the transformer whose secondary winding power is less than 10w, its own loss can reach 30-50% of the actual secondary power consumption, and its efficiency is only 50-70%. The secondary winding power is below 30W, the loss is about 20-30%, the loss below 50W is about 15-20%, the loss below 100W is about 10-15%, and the loss above 100W is about 10% or less. The above loss parameters are about ordinary plug-in transformers. of. If you follow the order of R-type transformer, C-type transformer, and toroidal transformer, the loss parameters will decrease in turn.

The core can be selected based on the total primary power of the transformer calculated above. The core area S=a×b(cm2). As shown in the attached picture. The relationship between the apparent power of the transformer and s is selected by the following empirical formula: s=K√P1

P1 is the total apparent power of the transformer primary, the unit is: VA (Volt-Ampere), s is the cross-sectional area of ​​the core that should be selected, and K is a coefficient. Different values ​​can be selected according to the size of the transformer Pl. At the same time, considering the influence of the insulating paint and the gap between the silicon steel sheets, the relationship between K and P1 is:

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. Then determine the number of turns per volt, so that the winding transformer has a reasonable excitation current. The commonly used empirical formula is: N=(40～55)/S, N is the number of turns per volt.

According to the different quality of silicon steel sheet, the coefficient is 40-55. For relatively high-grade high-silicon steel, observe the scaly crystals on the surface with eyes. It is extremely brittle, and it breaks after only one or two bendings. The fractures are uneven, and the coefficient is 40. If the surface of the silicon steel sheet is smooth, it is not easy to break after bending 4 to 5 times, and the section is a neat straight line, and the coefficient is more than 50.

Calculate the number of turns per volt and multiply it by 220V to get the number of primary turns, and multiply it by the number of secondary required voltages to get the number of turns of each secondary winding. Because the wire has resistance, there will be a voltage drop when the current flows. The calculated number of secondary turns should be increased by 5-10% (according to the load current selection, the larger current can increase by a larger proportion).

3. Selection of wire diameter

According to the load current of each winding, choose enameled wires of different diameters. The following empirical formula can be used to find out:

d=0.8√I，

Unit: l--Ad (wire diameter)--mm.

4. Winding method and matters needing attention

Since the insulation strength of the enameled wire is greatly improved, the flame retardant plastic skeleton winding method is mostly used for low-power transformers below 50W, but high-strength enameled wire must be selected, and the wire should still be arranged one by one during winding. So as not to increase the potential difference between the wires.

For transformers above 50W, since the number of turns per volt is reduced and the voltage difference between the wires is high, it is best to use each layer of insulating paper (0.05mm thick cable paper, kraft paper) method, and the upper wire should be absolutely avoided in the winding Slide into the lower level. The insulation between the windings should be determined by the winding voltage. There should be 4 layers of cable paper of 0.1mm or more between the primary and secondary stages, and adhesive tape should not be used. In the low-power transformer of the above-mentioned winding method, if the secondary has more than two sets of windings, two layers of cable paper are also used for insulation between each set. If the transformer is used in audio or audio-visual equipment. In the multilayer winding method, an electrostatic shielding layer should be inserted between the primary and secondary layers.

After winding up. You also need to pay attention to inserting silicon steel sheets and insert them tightly to avoid electromagnetic noise. Regardless of double E shape or EI shape, the ports should be in close contact. It should be cross-inserted without gaps. The last 4 to 5 pieces can be inserted from the middle to avoid damage to the wire package. Then it is dried and dipped. For transformers below 50W, internal heating method can be used for drying. The method is: short-circuit all the secondary windings of the transformer, and connect the 60～100W/220V bulb in series with the mains to make it automatically heat up. The larger the bulb, the higher the temperature, but it is safer to keep the temperature below 80 degrees in a closed state.

What is the difference between a power frequency transformer and an ordinary transformer

The design and winding of power frequency transformers among various household appliances, whether power frequency transformers are designed and wound by themselves or repairing a burnt out transformer, some simple calculations are involved. Although the calculation formulas in the textbook are rigorous, they are actually used. It seems complicated and inconvenient.

High frequency transformer

The difference between power frequency transformer and high frequency transformer:

1. The power frequency transformer uses silicon steel sheet as the magnetic core material; the high frequency transformer uses the ferrite core material.

2. The working frequency of the power frequency transformer generally refers to the power frequency of 50HZ or 60HZ; the working frequency of the high frequency transformer is generally above 1KHZ, even tens of KHZ or hundreds of KHZ, and the application range is different for different frequencies.