The functions of a power transformer are power transmission, voltage conversion, and insulation. As a primary soft magnetic electromagnetic component, it is widely used in power supply technology and power electronics technology. Based on the magnitude of the transmitted power, power transformers can be divided into several categories: above 10kVA is high power, 10kVA to 0.5kVA is medium power, 0.5kVA to 25VA is low power, and below 25VA is micro power. The design of power transformers differs depending on the transmitted power, and this distinction should be clear.
A. Inspect the surface of the transformer to check for any obvious abnormalities, such as whether the coil leads are cracked or detached, whether the insulation material has burn marks, whether the core fastening screws are loose, whether the silicon steel sheets are corroded, and whether the winding coils are exposed.
B. Insulation test: Use a multimeter on the R×10k range to measure the resistance between the core and the primary winding, between the primary winding and each secondary winding, between the core and each secondary winding, between the electrostatic shielding layer and the secondary winding, and between each secondary winding. The multimeter pointer should remain at infinity. Otherwise, it indicates that the transformer's insulation function is poor.
C. Detection of coil continuity: Set the multimeter to the R×1 range. During the test, if the resistance of a certain winding is infinite, it indicates that the winding has an open circuit defect.
D. Identify the primary and secondary coils: The primary and secondary leads of a power transformer are usually led out from both ends separately. The primary winding is often marked with "220V", while the secondary winding is marked with its rated voltage value, such as 15V, 24V, 35V, etc. Identify the coils based on these symbols.
E. Detection of no-load current:
(a) Direct Measurement Method: Open all secondary windings and set the multimeter to AC current range (500mA), connecting it in series with the primary winding. When the primary winding is plugged into 220V AC mains power, the multimeter reading is the no-load current value. This value should not exceed 10% to 20% of the transformer's full-load current. The normal no-load current of power transformers in common electronic equipment should be around 100mA. If it exceeds this value significantly, it indicates a short-circuit defect in the transformer.
(b) Direct measurement method: Connect a 10Ω/5W resistor in series in the primary winding of the transformer, while keeping the secondary winding completely unloaded. Set the multimeter to AC voltage range. After powering on, use the two probes to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current I_no_load, i.e., I_no_load = U/R.
F. No-load voltage test: Connect the primary winding of the power transformer to 220V AC mains power. Use a multimeter to measure the no-load voltage values of each winding sequentially (U21, U22, U23, U24). The values should meet the requirements. The allowable error is generally: high voltage winding ≤ ±10%, low voltage winding ≤ ±5%, and the voltage difference between two symmetrical windings with center taps should be ≤ ±2%.
G. Generally, the allowable temperature rise of a small power transformer is 40℃～50℃. If the insulation material used is of good quality, the allowable temperature rise can be further increased.
H. Detecting and identifying the corresponding terminals of each winding: When using a power transformer, sometimes two or more secondary windings can be connected in series to obtain the required secondary voltage. When using a power transformer in series, the corresponding terminals of each winding connected in series must be correctly connected and cannot be reversed. Otherwise, the transformer will not operate normally.
