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A transformer is an electric appliance that transmits electric energy or transmits signals from one circuit to another through the principle of electromagnetic induction. It is an important component for transmitting electric energy or as a signal transmission. When the lower coil Zhongtong AC current, the core or core) will be generated in the alternating magnetic flux, the secondary coil induces a voltage (or current). A transformer consists of an iron core (or magnetic core) and a coil. The coil has two or more windings, wherein the winding of the power supply is called a low level coil, and the rest of the winding is called a secondary winding. The winding is composed of two windings.

By changing the number of turns of different voltage ratio of the two coils will be in the second coil L, the transformer is device based on this principle into a transformed AC voltage, current and impedance of the ring number of junior and secondary coils with proper proportion, can increase or reduce the voltage in circuit. You can express by means of a formula;

Lower voltage (U1) / secondary voltage (U2) = lower loop number (N1) / secondary loop number (N2)

It should be noted that any transformer can only transfer power from low to secondary. It can increase or decrease voltage, but it can not increase power. The ratio of the primary to secondary voltage of the transformer is the ratio of the secondary to the lower. Without considering the loss of the transformer, it can be said that the power of the lower input is the power of the secondary output. As shown in Figure 6-25, that is the working principle of the transformer diagram.

The mark of the nominal power, voltage and current of the power transformer will fall or disappear in a long time. Some commercially available transformers do not mark any parameters at all. This caused great inconvenience to the use. The method of distinguishing the parameters of unmarked power transformer is introduced below. This method also has reference value for the purchase of power transformers.

I. identification of power transformers

1. from the shape identification, commonly used power transformer iron core has E shape and C shape two kinds. E shape iron core transformer is shell type structure (iron core wrapped coil), and uses D41 and D42 high quality silicon steel sheet as iron core, which is widely used. The C shape core transformer is made of cold rolled silicon steel strip as core. The magnetic leakage is small and the volume is small. It is core structure (coil wrapped iron core).

2., the number of terminals from the winding identification, power transformers common two winding, that is, a low and a secondary winding, so there are four leading end. In order to prevent AC noise and other disturbances, some power transformers often add a shielding layer between the primary and secondary windings, and the shielding layer is grounded. Therefore, the power transformer terminals are at least 4.

3. from the silicon steel sheet laminated mode shape recognition E power transformer silicon steel sheet is inserted into the air gap *, don't leave E film and I, the core of seamless. There is a certain air gap between the E chip and the I chip of the audio input and output transformer, which is the most direct way to distinguish the power supply from the audio transformer. As for C transformers, they are generally power transformers.

Two, power estimation

The magnitude of the transmission power of the power transformer depends on the material and cross-sectional area of the core. The so-called cross sectional area, whether it is the E shaped shell structure, or E shaped core structure (including C structure), which refers to the cross section that wrapped the core winding (rectangular) area. After measuring the cross sectional area of the core, the power S of the transformer can be estimated at P = S2 / 1.5, P. The unit of S in the formula is cm2.

For example: the core area of a measured power transformer S = 7cm2, to estimate the power, P = S2 / 1.5 = 72 / 1.5 = 33W dead from a variety of errors, the actual nominal power is 30W.

Three 、 measurement of winding voltage

To make use of an unmarked power transformer, it is essential to find out the lower winding and to distinguish the output voltage of the secondary winding. An example is given to illustrate the method of decision making.

Example: a power transformer with 10 terminals is known. Test the voltage of each winding.

The first step is to distinguish the number of windings and draw a circuit diagram.

Multimeter R * 1 block measurement, where the interlinked terminals that is a winding. Now measured: 22, there are 3 groups of interlinked, three interlinked there are 1 groups, there is a terminal and any other terminal is blocked. According to the above measurement results, draw a circuit diagram, and number.

From the measurement shows that the transformer has 4 windings, wherein the label, and, is winding along the tap, and the signal terminals and any windings are not connected, shielding terminal.

The second step: determine the lower level winding.

For step-down power transformers, the lower winding has smaller diameter and more turns than the secondary winding. Thus, a buck transformer such as Fig. 4, whose resistance is the largest, is the lower winding.

The third step: to determine the voltage of all secondary windings.

In the lower winding, through the regulator into the AC, slowly boost until 220V. The no-load voltage of each winding is measured in sequence and marked at each output. If the transformer is in no load for a long period of time without fever, the transformer performance is basically intact, but also further verify the judgment of the lower winding is accurate.

Four. Determine the maximum current of each secondary winding

The output current of the secondary winding of the transformer depends on the diameter of the enameled wire of the winding D. The diameter of the enameled wire can be measured directly from the lead terminal. After measuring the diameter, the maximum output current of the winding can be calculated according to formula I = 2D2. The unit of D in the formula is mm.

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