Power transformer parameter identification

The marking of the power transformer's nominal power, voltage, current and other parameters will fall off or disappear. Some commercially available transformers do not label any parameters at all. This brings great inconvenience to the use. The following describes the method for judging the parameters of the unmarked power transformer. This method also has reference value for the optional power transformer.

First, identify the power transformer

1. The core of the common power transformer is identified from the shape of the E-shaped and C-shaped. The E-shaped iron core transformer has a shell structure (core wrapped coil), and uses D41 and D42 high-quality silicon steel sheets as iron cores, which are widely used. The C-shaped iron core transformer uses a cold-rolled silicon steel strip as the iron core, and has a small magnetic leakage and a small volume, and has a core structure (coil wrapped iron core).

2. Number of terminals from the winding identification The power transformer has two windings, a primary and a secondary winding, so there are four terminals. In order to prevent hum and other interference, some power transformers often add a shielding layer between the primary and secondary windings, and the shielding layer is the grounding terminal. Therefore, the power transformer terminals are at least four.

3. The silicon steel sheet of the E-shaped power transformer is cross-inserted from the lamination of the silicon steel sheet, and no air gap is left between the E sheet and the I sheet, and the entire core is tightly stitched. There is a certain air gap between the E and I of the audio input and output transformers, which is the most intuitive way to distinguish between power and audio transformers. As for the C-shaped transformer, it is generally a power transformer.

Second, the power estimate

The amount of power transmitted by the power transformer depends on the material and cross-sectional area of ​​the core. The cross-sectional area, whether it is an E-shell structure or an E-core structure (including a C-shaped structure), refers to the cross-sectional (rectangular) area of ​​the segment of the core wrapped by the winding. After measuring the cross-sectional area S of the core, the power P of the transformer can be estimated by P=S2/1.5. The unit of S in the formula is cm2.

For example, the core cross-sectional area of ​​a power transformer is measured as S=7cm2, and the power is estimated. P=S2/1.5=72/1.5=33W, except for various errors, the actual nominal power is 30W.

Third, the measurement of each winding voltage

To make use of an unmarked power transformer, finding the primary winding and distinguishing the output voltage of the secondary winding is the most basic task. An example is given to illustrate the method of judgment.

Example: A power transformer is known with a total of 10 terminals. Try to judge the voltage of each winding.

The first step: distinguish the number of winding groups, draw a circuit diagram.

Use the multimeter R × 1 block measurement, where the connected terminal is a winding. It is measured that there are 3 groups in which two or two are connected, one in which three are connected, and one terminal is not connected to any other terminal. According to the above measurement results, draw a circuit diagram and number it.

It can be seen from the measurement that the transformer has four windings, wherein the numbers 5, 6, and 7 are a tapped winding, and the No. 10 terminal is not in communication with any of the windings, and is a shield lead-out terminal.

Step 2: Determine the primary winding.

For a buck-type power transformer, the primary winding has a thinner wire diameter and more turns than the secondary winding. Therefore, for a step-down transformer like Figure 4, the primary resistance is the largest.

Step 3: Determine the voltage of all secondary windings.

Connect the AC to the primary winding through a voltage regulator and slowly boost up to 220V. The no-load voltage of each winding is measured in turn and marked at each output. If the transformer does not heat up for a long time under no-load conditions, the performance of the transformer is basically intact, and it is further verified that the determined primary winding is correct.

Fourth, the determination of the maximum current of each secondary winding

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