Detection of no-load current of the transformer.

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Detection of no-load current of the transformer.

(1) Direct measurement method. Open all the secondary windings and place the multimeter in the AC current block (500mA, stringed into the primary winding. When the plug of the primary winding is inserted into 220V AC mains, the multimeter indicates the no-load current value. This value should not be It is 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 means that the transformer has 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 completely empty. Turn the multimeter to the AC voltage block. After power-on, the voltage drop U across the resistor R is measured with two test leads, and then the no-load current I is calculated by Ohm's law, that is, I null = U/R.

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

7. Generally, the low-power power transformer allows the temperature rise to be 40 °C ~ 50 °C. If the quality of the insulating material used is good, the temperature rise can be increased.

8. Detect and identify 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 of each winding participating in the series must be correctly connected, and no mistake can be made. Otherwise, the transformer will not work properly. I. Comprehensive detection and discrimination of short-circuit faults of power transformers. The main symptoms after a short-circuit fault in the power transformer are severe heat generation and abnormal secondary winding output voltage. Generally, the more short-circuit points between turns in the coil, the greater the short-circuit current, and the more severe the transformer heats up. A simple way to detect if a power transformer has a short-circuit fault is to measure the no-load current (tested earlier in the test method). A transformer with a short-circuit fault will have a no-load current value that is much greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up quickly within a few tens of seconds after the no-load power-on, and the iron core will feel hot when touched by hand. At this time, it is not necessary to measure the no-load current to conclude that the transformer has a short-circuit point.