Inspection of power transformers

A. Check whether there is any obvious abnormal phenomenon by observing the appearance of the transformer. Such as whether the coil lead is broken, desoldered, whether the insulating material has scorch marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

B. Insulation test. Use the multimeter R×10k to measure the resistance values ​​between the iron core and the primary, the primary and each secondary, the iron core and each secondary, the electrostatic shielding layer and the secondary, and the secondary windings. The pointer of the multimeter should point to infinity. The position does not move. Otherwise, the transformer insulation performance is poor.

C. Detection of coil on and off. Put the multimeter in the R×1 block. During the test, if the resistance value of a winding is infinite, it means that the winding has an open circuit fault.

D. Identify the primary and secondary coils. The primary and secondary pins of the power transformer are generally drawn from both sides, and the primary winding is mostly marked with 220V, and the secondary winding is marked with a rated voltage value, such as 15V, 24V, 35V, etc. Recognize based on these marks.

E. Detection of no-load current.

(1) Direct measurement method. Open all secondary windings, put the multimeter in the AC current block (500mA, and connect the primary winding in series. When the plug of the primary winding is inserted into the 220V AC mains, the multimeter indicates the no-load current value. This value should not be It is greater than 10% to 20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment should be about 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.

(2) Indirect measurement method. A 10/5W resistor is connected in series with the primary winding of the transformer, and the secondary is still fully unloaded. Set the multimeter to AC voltage. After power-on, use two test pens to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = U/R.

F. Detection of no-load voltage. Connect the primary of the power transformer to 220V commercial power, and use a multimeter to connect the AC voltage to measure the no-load voltage value of each winding (U21, U22, U23, U24) in turn, which should meet the required value, and the allowable error range is generally: high-voltage windings≤±10 %, the low-voltage winding is ≤±5%, and the voltage difference between the two sets of symmetrical windings with center taps should be ≤±2%.

G. Generally, the allowable temperature rise of low-power power transformers is 40°C to 50°C. If the quality of the insulating materials used is good, the allowable temperature rise can also be increased.

H. Detect and identify the same-named end of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When the power transformer is used in series, the ends of the same name of each winding participating in the series must be connected correctly, and no mistakes can be made. Otherwise, the transformer will not work properly. I. Comprehensive detection and judgment of short-circuit faults of power transformers. The main symptoms after the short-circuit fault of the power transformer are severe heating and abnormal output voltage of the secondary winding. Generally, the more short-circuit points between turns inside the coil, the greater the short-circuit current, and the more serious the transformer heats up. A simple way to detect and judge whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced earlier). For transformers with short-circuit faults, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer will quickly heat up within tens of seconds after no-load power-on, and it will feel hot when you touch the iron core with your hand. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.