Introduction to the Basic Concept and Measurement Method of Power Transformer

Power transformer is a kind of soft magnetic electromagnetic component. Its function is power transmission, voltage conversion and insulation isolation. It is widely used in power supply technology and power electronics technology. The most basic type of power transformer consists of two groups of coils wound with wires, which are inductively combined with each other. When an AC current (with a known frequency) flows through one group of coils, an AC voltage with the same frequency will be induced in the other group of coils, and the induced voltage depends on the coupling of the two coils and the degree of magnetic cross link.

It is used in almost all electronic products. Its principle is simple, but the winding process of transformer will have different requirements according to different applications (different purposes). The main functions of the transformer are: voltage conversion; Impedance transformation; Isolation; Voltage stabilizing (magnetic saturation transformer), etc. the common iron core shapes of transformers generally include E-type and C-type iron cores.

Nine methods of power transformer measurement

1 Check whether there are obvious abnormalities by observing the appearance of the transformer.

For example, whether the coil lead is broken, desoldered, whether the insulating material is burnt, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

Multimeter R 10K block shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and secondary and secondary windings respectively. The pointer of multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.

Place the multimeter in R Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.

4 Distinguish primary and secondary coils.

The primary pin and secondary pin of the power transformer are generally led out from both sides, and the primary winding is mostly marked with the word 220V, while the secondary winding is marked with the rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.

5 Detection of no-load current.

(1) Direct measurement method.

Open all secondary windings and set the multimeter to AC current (500mA, in series into the primary winding. When the plug of the primary winding is plugged into 220V AC mains power, the multimeter indicates the no-load current value. This value shall not be greater than 10% 20% of the full load current of the transformer. The normal no-load current of the power transformer of common electronic equipment shall be about 100mA. If it exceeds too much, it indicates that the transformer has short-circuit fault.

(2) Indirect measurement.

A 10? / 5W resistor is connected in series in the primary winding of the transformer, and the secondary winding is still no-load. Turn the multimeter to the AC voltage gear. After power on, use two probes to measure the voltage drop u at both ends of the resistor R, and then use Ohm's law to calculate the no-load current I null, i.e. I null = u / R.

Connect the primary of the power transformer to 220V mains power, and connect the AC voltage of the multimeter. The no-load voltage values (U21, u22, U23 and u24) of each winding shall meet the requirements. The allowable error range is generally: high voltage winding 10%, low voltage winding 5%, and the voltage difference between two groups of symmetrical windings with center tap shall be 2%.

7 Generally, the allowable temperature rise of low-power power transformer is 40 50 . If the insulating material used is of good quality, the allowable temperature rise can be improved.

8 Detect and judge the same name end of each winding.

When using the power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the power transformer in series method, the same name ends of each winding in series must be correctly connected without mistake. Otherwise, the transformer cannot work normally.

9 Comprehensive detection and discrimination of short-circuit fault of power transformer.

The main symptoms of short-circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Generally, the more inter turn short-circuit points in the coil, the greater the short-circuit current, and the more serious the transformer heating. A simple method to detect and judge whether the power transformer has short-circuit fault is to measure the no-load current. For the transformer with short-circuit fault, the no-load current value It will be much greater than 10% of the full load current. When the short circuit is serious, the transformer will heat rapidly within tens of seconds after no-load power on, and it will feel hot to touch the iron core by hand. At this time, it can be concluded that there is a short circuit point in the transformer without measuring the no-load current.

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