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8 Common Methods of Transformer Detection

Jun. 28, 2020

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1. Check whether there is any obvious abnormality by observing the appearance of the transformer. Such as whether the coil lead is broken or unsoldered, whether the insulating material has scorch marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed.

2. Insulation test. Use a multimeter R×10k block to measure the resistance between the core and the primary, the primary and the secondary, the core and the secondary, the electrostatic shield and the secondary, and the secondary winding. The pointers of the multimeter should be fixed at the infinite position, otherwise, it means that the transformer insulation performance is poor.

3. Detection of on-off of the coil. Put the multimeter in R×1 gear. During the test, if the resistance value of a certain winding is infinite, it means that this winding has a broken fault.

4. 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 marked with 220V, and the secondary winding is marked with rated voltage values, such as 15V, 24V, 35V, etc., according to these Mark for identification.

5. Detection of no-load current.

Direct measurement method. Open all the secondary windings, put the multimeter in the AC current block 500mA, and string it into the primary winding. 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 greater than 10%-20% of the transformer full load current. The normal no-load current of power transformers of common electronic equipment should be around 100mA. If it exceeds too much, it means the transformer is short circuited.

Indirect measurement method. A 10/5W resistor is connected in series in the primary winding of the transformer, and the secondary is still completely unloaded. Turn the multimeter to the AC voltage block. After power-on, use two test leads to measure the voltage drop U across the resistance R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = U/R.F.

6. Temperature rise detection. Generally, the temperature rise of the low-power power transformer is allowed to be 400C~500C. If the quality of the insulating material used is good, the temperature rise can be increased.

7. Detect and identify the ends of the windings with the same name. 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 connection, the same-named ends of the windings participating in the series must be connected correctly, and no error can be made, otherwise, the transformer cannot work normally.

8. Comprehensive detection and discrimination of short-circuit faults of power transformers. The main symptoms of a short circuit in a power transformer are severe heating and abnormal output voltage of the 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 heat. The simple method to detect whether the power transformer is short-circuited is to measure the no-load current. A transformer with a short-circuit fault will have a no-load current value 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 the iron core will feel hot when touched by hand. It can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.


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