Why do the filter electrolytic capacitors of frequency converters get damaged?

How to determine whether the filter electrolytic capacitor is damaged? Generally, when an electrolytic capacitor shows the following manifestations, it can be judged as damaged: the appearance is cracked, the aluminum shell is bulging, the plastic outer tube is cracked, electrolyte is flowing out, the safety valve is opened or pressed out, the top part of the small capacitor is cracked, the terminal is severely rusted, the cover plate is deformed or fallen off. All these indicate that the electrolytic capacitor is damaged. When measuring an open circuit or short circuit with a multimeter, the capacity is significantly reduced and the leakage is serious.

1.There are several situations of damage and failure of electrolytic capacitors:

(1) The short circuit or open circuit damage inside the electrolytic capacitor is characterized by burning out the switch tube and other current-limiting components, such as fuses and current-limiting resistors in the switching power supply. When the voltage of a filter capacitor operating in high voltage and large current rises for some reason and exceeds its withstand voltage value, it will break down and short-circuit and get damaged. Or when the rectifier diode is damaged, the polar electrolytic capacitor will be equivalent to operating in an AC circuit and heat up under a large reverse leakage current, thus short-circuiting and getting damaged. Due to the large current flowing through the capacitor during a short circuit, the capacitor usually bursts or causes the sealing rubber stopper to bulge out. After a filter capacitor is short-circuited, faults such as the fuse or current-limiting resistor burning out, the thick film block of the power supply or the switch tube, or the rectifier tube being broken down often occur. The main manifestation is the "three no's" of the entire machine. This kind of fault is common among various types of switching power supplies.

(2) Low efficiency or slight leakage caused by the reduction of electrolytic capacitor capacity. The main reason is that the parameters of the capacitor have changed, but it has not completely failed. To some extent, it still has an effect, but it fails to achieve the expected effect, which leads to the occurrence of the existing fault phenomenon. Moreover, such faults are difficult to diagnose and eliminate.

(3) Failure, complete leakage or leakage caused by the loss of capacity of electrolytic capacitors is one of the most difficult faults to identify and repair when capacitors in power supplies malfunction. This is because when measuring capacitor components and testing with a multimeter, everything is normal, but after installing the capacitor in the circuit, its capacity completely disappears. This is one of the most difficult soft faults to repair in circuits, that is, the component cannot withstand voltage. As soon as there is voltage, it is easy to disappear completely.

2. The reasons for the damage of electrolytic capacitors are as follows:

(1) The quality of the components themselves is poor (large leakage current, high loss, insufficient withstand voltage, containing impurities such as chloride ions, poor structure, short service life).

(2) The rectifier bridge before filtering was damaged, and alternating current directly entered the capacitor.

(3) The voltage divider resistor was damaged, and uneven voltage division caused a certain capacitor to break down first, followed by the breakdown of related other capacitors.

(4) Improper installation of capacitors, such as damage to the outer insulation, the casing being connected to an inappropriate potential, poor electrical connection and welding points, resulting in poor contact, overheating and damage.

(5) Poor heat dissipation environment causes the temperature of the capacitor to rise too high, leading to damage over time.

3. When replacing electrolytic capacitors, the following points should be noted:

(1) When replacing filter electrolytic capacitors, it is best to choose the same model as the original one. When the same model cannot be obtained for the time being, the following points must be noted: the withstand voltage, leakage current, capacity, external dimensions, polarity, and installation method should be the same, and a variety that can withstand a larger ripple current and has a long service life should be selected.

(2) During the process of replacing electrolytic capacitors, pay attention to ensuring that the electrical connections (screw connections and welding) are firm and reliable. The positive and negative terminals must not be connected incorrectly. The clamps used for fixation should be firmly fixed and must not damage the outer insulation sheath of the capacitor. Connect the voltage divider resistor as it was originally and measure the resistance value to ensure uniform voltage division.