Film capacitor safety and work in AC

1. Film capacitor safety

The film capacitor uses a certain proportion of oil wax or sulphur hexafluoride as the impregnating agent, and the impregnating agent is poured into the capacitor shell by vacuum impregnation, which can effectively eliminate the partial discharge phenomenon at the edge of the core, and the crystallite after curing The wax forms a stress zone outside the core. When the component self-healing, due to the existence of certain stress, it can quickly extinguish the arc, preventing the evaporation zone from expanding and self-healing and causing component accidents. Compared with the liquid impregnating agent, the oil wax impregnating agent has stable performance, does not burn, and effectively solves the oil leakage problem.

In the event that the film capacitor fails due to self-healing, the internal metallized film is softened by heat and deflated to cause the capacitor to expand, and the safety device can cut off the power supply in time to protect the entire device. There are many types of safety devices, such as mechanical and electrical types. The fuse of the film capacitor is integrated with electric insurance and has a dual function. It is placed inside the capacitor case, and the fuse is used to start the safety device and cut off the power supply. In case the mechanical insurance is out of control, the electric insurance starts immediately. The power is also turned off to protect the entire device.

When the film capacitor is out of operation, the self-discharge device can reduce the initial peak voltage of the capacitor when it exits operation to less than 50V within 3 minutes to ensure safe operation and maintenance.

2. Film capacitors work in AC

The simple capacitor is composed of the plates at both ends and the insulating dielectric in the middle. Under the premise of not exceeding the breakdown voltage of the capacitor, the plate will be charged after energization, forming a voltage (potential difference), but because the middle is an insulating substance, it is It is not electrically conductive, so the capacitor will have the characteristics of AC and DC blocking. It is the nature of the capacitor. In a DC circuit, the film capacitor is equivalent to an open circuit.

Any substance is relatively insulated. When the voltage at both ends of the substance is increased to a certain extent, the substance can be electrically conductive. We call this voltage the breakdown voltage, and so is the capacitance. After the capacitor is broken down, it is not an insulator. But in general, capacitors work below the breakdown voltage. Such voltages are not seen in the circuit and can be viewed as insulators.

In the AC circuit, because the direction of the current changes with time, the process of charging and discharging the film capacitor is time-consuming. At this time, a changing electric field is formed between the plates. In fact, the current is passed through the field. The form passes between the film capacitors, and this electric field is also a function of time.