How to determine the power transformer parameters

Power transformer nominal power, voltage, current and other parameters of the mark, will fall off or disappear. Some commercial transformers do not have any parameters at all. This brings great inconvenience to use. The following describes the identification method of the unmarked power transformer parameters. This method also has reference value for the purchase of power transformers.

First, identify the power transformer 1. The cores of commonly used power transformers are identified from the appearance of E-shaped and C-shaped. The E-shaped core transformer has a shell-like structure (coil wrapped coils) and uses D41, D42 high-quality silicon steel sheet as core, which is widely used. C-shaped core transformers use cold-rolled silicon steel strips as cores, which have a small magnetic leakage and a small volume, and are core-type structures (coil-wrapped cores).

2. Identifying the Number of Terminals from the Winding Identifies the power transformer The common two windings, a primary and a secondary winding, have four terminals. Some power transformers have a shielding layer between the primary and secondary windings to prevent hum and other interferences. The shielding layer is grounded. Therefore, there are at least 4 power transformer terminals.

3. The silicon steel sheet that identifies the E-shaped power transformer from the lamination method of the silicon steel sheet is inserted, and no air gap is left between the E sheet and the I sheet. The entire iron core is tightly fitted. There is a certain air gap between the audio input and output transformer's E-chip and I-chip, which is the most intuitive method of distinguishing between the power supply and the audio transformer. As for the C-shaped transformers, they are generally power transformers.

Second, the power estimation Power transformer transmission power, depending on the core material and cross-sectional area. The so-called cross-sectional area, whether it is an E-shaped shell structure or an E-shaped core structure (including a C-shaped structure), refers to the cross-sectional (rectangular) area of ​​the core column wrapped by the windings. 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: Measured a power transformer core cross-sectional area S = 7cm2, estimate its power, get P = S2/1.5 = 72/1.5 = 33W? Excluding various errors, the actual nominal power is 30W.

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

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

Step 1: Distinguish the number of windings and draw a circuit diagram.

Measured with a multimeter R×1 block, all connected terminals are a single winding. Measured now: There are three groups of two communicating, one group of three communicating, and one terminal is not connected to any other terminal. According to the above measurement results, draw the 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 windings with taps, and the No. 10 terminal is not connected to any of the windings and is a shield terminal.

The second step: determine the primary winding.

For a buck power transformer, the primary winding has a smaller wire diameter and more turns than the secondary winding. Therefore, a step-down transformer such as the one shown in Figure 4 has the largest primary resistance.

Step 3: Determine the voltage of all secondary windings.

Connect the AC power to the primary winding through a voltage regulator and slowly increase the voltage to 220V. Measure the no-load voltage of each winding in turn and mark it at each output. If the transformer does not generate heat for a long time under no-load condition, it indicates that the transformer performance is basically intact, and it further verifies that the determined primary winding is correct.

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