1, color code inductor detection
Put the multimeter in R×1 and connect the red and black pen to either end of the color code inductor. At this time, the pointer should swing to the right. According to the size of the measured resistance value, it can be specifically divided into the following three cases for identification:
A. The resistance value of the color code inductor tested is zero, and there is A short-circuit fault inside.
B. The DC resistance value of the color code inductor to be measured is directly related to the diameter of the enameled wire used for winding the inductor coil and the number of winding turns. As long as the resistance value can be measured, the color code inductor to be measured can be considered normal.
2. Detection of midcycle transformer
A. Switch the multimeter to R×1 gear, and check the on-off condition of each winding one by one according to the arrangement law of the winding pins of the midcycle transformer, and then judge whether it is normal.
B. Test insulation performance. Put the multimeter in R× 10K mode and do the following state tests:
(1) Resistance between the primary winding and the secondary winding;
(2) the resistance value between the primary winding and the housing;
(3) Resistance value between the secondary winding and the housing.
The above test results can be divided into three situations:
(1) Resistance is infinite: normal;
(2) the resistance value is zero: there is a short-circuit fault;
(3) the resistance value is less than infinity, but greater than zero: there is leakage fault.
3, power transformer detection
A. Check whether there is obvious abnormal phenomenon by observing the appearance of the transformer. For example, whether the coil lead is broken or unsoldered, whether the insulation material has burnt marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is corroded, whether the winding coil is exposed, etc.
B. Insulation test. The multimeter R× 10K block is used to measure the resistance value between the core and the primary, the primary and the secondary, the core and the secondary, the electrostatic shielding layer and the VAT secondary, and the secondary windings. The pointer of the multimeter should be fixed at the infinite position. Otherwise, the transformer insulation performance is poor.
C, coil on and off detection. When the multimeter is placed in R×1 gear and tested, if the resistance value of a certain winding is infinite, it indicates that the winding has an open-circuit fault.
D, discrimination of primary and secondary coils. The primary pin and secondary pin of the power transformer are generally led from both sides respectively, and the primary winding is marked with 220V, the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. They are identified by these markers.
E. Detection of no-load current. (a) direct measuring method. Open all the secondary windings, put the multimeter in an ac current stop (500mA), string into the primary winding. When the primary winding plugs into the 220V AC mains, the multimeter indicates the no-load current value. This value should not be greater than 10% ~ 20% of the transformer's full load current. The normal no-load current of the power transformer for common electronic equipment should be about 100mA. If it exceeds too much, it indicates that the transformer has a short-circuit fault. (b) the indirect measuring method. A 10/5W resistor is connected in series in the primary winding of the transformer, and the secondary is still completely empty. Turn the multimeter to ac. After the power is added, the voltage drop U at both ends of the resistor R is measured with two meter pens, and then the no-load current I empty is calculated by Ohm's law, that is, I empty =U/R.
F. Detection of no-load voltage. The primary power transformer is connected to 220V mains, and the no-load voltage value of each winding (U21, U22, U23, U24) is measured with a multimeter alternating voltage in turn. The allowable error range is generally: the high-voltage winding ≤± 10%, the low-voltage winding ≤± 5%, and the voltage difference between the two groups of symmetrical windings with the center tap should be ≤± 2%.
G. The allowable temperature rise of general small power transformer is 40℃ ~ 50℃, and the allowable temperature rise can be increased if the insulation material is of good quality.
H. Detect and discriminate the same end of each winding. When using a power transformer, sometimes two or more secondary windings can be connected in series in order to obtain the required secondary voltage. When the power transformer is used in series, the namesake ends of the windings participating in series must be connected correctly and there can be no mistake. Otherwise, the transformer will not work properly.
I. Comprehensive detection and discrimination of short-circuit faults of power transformers. The main symptoms of short circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Usually, the more short circuit points between turns in the coil, the greater the short circuit current, and the more serious the transformer heating. The simple method to detect and determine whether a power transformer has short-circuit fault is to measure the no-load current (the test method has been described above). The no-load current value of a transformer with a short-circuit fault will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer in the no-load power within a few seconds will quickly heat, touch the iron core will have a hot feeling. At this time without measuring the no-load current can be determined that the transformer has a short circuit point.
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