A. Check whether the appearance of the transformer is abnormal. 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.
B. Insulation test. Multimeter R × 10K gear shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and slit secondary and secondary windings respectively. The pointer of multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.
C. Coil on-off detection. 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.
D. Distinguish primary and secondary coils. The primary and secondary pins 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.
E. Detection of no-load current.
(1) Direct measurement method. Open all the secondary windings, put the multimeter in the AC current gear (500mA) and connect it in series with 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 should not be greater than 10% ~ 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 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 all secondary windings are still no-load. Turn the multimeter to AC voltage. After power on, use two probes to measure the voltage drop u at both ends of resistance R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = u / R.
F. No load voltage detection. Connect the primary of the power transformer to 220V mains power, and connect the AC voltage of the multimeter to measure the no-load voltage value (U21, u22, U23, u24) of each winding in turn. 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%.
G. 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.
H. Detect and judge the same name end of each winding. When using a power transformer, sometimes two or more secondary windings can be used in series in order to obtain the required secondary voltage. When the power transformer is used in series, the same name end of each winding in series must be correctly connected without mistake. Otherwise, the transformer cannot work normally. 1. 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 heating of the transformer. The simple way to detect and judge whether the power transformer has short-circuit fault is to measure the no-load current (the test method has been introduced earlier). For the transformer with short-circuit fault, 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 heat rapidly within tens of seconds after no-load power on. Touching the iron core by hand will feel hot. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.
