How to evaluate the quality of film capacitors? What happens if film capacitors are used incorrectly

Film capacitor quality inspection

Method one:

1. Connect the capacitor to the power supply. If the pointer of the multimeter does not swing at the moment it is switched on, it means that the capacitor has failed or is disconnected. If the needle keeps indicating the power supply voltage but does not swing, it indicates that the capacitor has been short-circuited. If the hands of the watch swing normally but do not return to the zero position, it means that the capacitor is leaking. Because the higher the indicated voltage value, the greater the leakage power.

2. The auxiliary DC voltage used in the measurement of a capacitor with a small capacity cannot exceed the withstand voltage of the capacitor under test, so as to avoid the capacitor breakdown and damage due to the measurement.  To accurately measure the capacitance of a capacitor, a capacitance bridge or Q meter is required. The simple detection method mentioned above can only roughly judge the quality of the pressure gauge capacitor.

3. For a large-capacity fixed capacitor, use the resistance file (R×1000) of a multimeter to measure the two electrodes of the capacitor. Look at the swing of the needle. The larger the swing, the greater the capacitance of the capacitor.  If the test rod keeps touching the capacitor lead, the needle should be near ∞. Otherwise, it indicates that the capacitor has leakage. The smaller the resistance value, the greater the leakage and the worse the quality of the capacitor.

4. When checking the quality of the capacitor, for electrolytic capacitors with lower withstand voltage, the resistance file should be placed in the R×100 or R×1K file. Connect the red test lead to the negative end of the capacitor and the black test lead to the positive end. The pointer of the multimeter will swing and then return to or near the zero position, which shows that the quality of the electrolytic capacitor is qualified. The larger the capacity of the electrolytic capacitor, the longer the charging time, and the slower the pointer swings.

Method Two:

In the absence of special instruments, the quality and quality of the capacitor can be tested and judged with the resistance file of a multimeter. For a fixed capacitor with a large capacity (above 1μF), the resistance profile (R×1000) of a multimeter can be used to measure the two electrodes of the capacitor. The needle should swing in the direction of small resistance, and then slowly swing back to around ∞. Then exchange the test rod and try again. Look at the swing of the hands. The larger the swing, the greater the capacitance of the capacitor.  If the test rod keeps touching the capacitor lead, the needle should be near ∞. Otherwise, it indicates that the capacitor has leakage. The smaller the resistance value, the greater the leakage and the poor quality of the capacitor; if the needle does not move at all during the measurement, Which indicates that the capacitor has failed or is open; if the needle swings but cannot return to the starting point, it indicates that the capacitor has a large leakage and its quality is not good.

Method three:

The capacitance of the test circuit must be cut off first, and then the high-voltage capacitor must be released first, and then the rectified filter capacitor (or other circuits with large resistance at both ends) and other circuit designs are measured in the circuit.  If the capacitance cannot be determined in the circuit measurement, the measurement can only be removed.

Method four:

The capacitance of an electrolytic capacitor is usually measured with the R x 10, R x 100 and R x 1K of a multimeter.  The red and black pens are connected to the negative pole of the capacitor (the capacitor should be discharged before each test), and the quality of the capacitor can be judged by the pendulum.  If the needle swings to the right and then slowly returns to the left, the capacitor is usually good.  If the needle does not swing after swinging, it means that the capacitor has been disconnected.  If the needle is placed back in a certain position after the needle swings, it indicates that the capacitor has leaked. If the needle does not swing, it means that the capacitor electrolyte has dried up and lost its capacity.

Errors in the use of film capacitors and solutions

At low temperatures, the viscosity of the impregnant inside the capacitor increases, the internal voltage decreases, and the capacitor's ability to withstand electricity decreases. The heat generated during the operation of the capacitor can be dissipated in time, and the temperature of the medium inside the capacitor can be reduced to achieve the purpose of extending the actual service life of the capacitor.

When breaking the capacitor bank, if the circuit breaker has a heavy breakdown, the terminal of the capacitor will have a high operating overvoltage of 3 times, 5 times, 7 times..., and strong partial discharge and dielectric damage will occur inside the capacitor. Cause capacitor breakdown. The quality of the circuit breaker used to switch the capacitor is closely related to the actual service life of the capacitor. The national standard stipulates: Use a circuit breaker suitable for switching capacitors. When the circuit breaker is in the breaking operation, no heavy breakdown that may cause excessive overvoltage occurs."

Therefore, in order to prevent excessive inrush current and overvoltage, after the capacitor is withdrawn from the network, the capacitor is discharged in time.  Before putting the capacitor into grid operation again, the remaining voltage on the capacitor should not exceed 10% of its rated voltage.