small size, large power, thin thickness or high temperature resistancesuitable for transformers of charging posts, optical storage, automotive electronics, special medical instruments and other products High working frequency, good skin effect and proximity effect, increased insulation strength, good radiation resistance and corrosion resistance Inductance is the ratio of the magnetic flux of the conductor to the current producing the alternating magnetic flux when the alternating current passes through the conductor inside and around the conductor. When DC current flows through the inductor, there are only fixed magnetic force lines around the inductor, which do not change with time; However, when the alternating current passes through the coil, the magnetic field lines around it will change with time. According to Faraday's law of electromagnetic induction - magnetism generates electricity, the changed magnetic lines of force will produce induced potential at both ends of the coil, This induced potential is equivalent to a "new power source ". When a closed circuit is formed, the induced potential will generate induced current. The Lenz's law tells us that the total amount of magnetic lines of force generated by the induced current should try to prevent the change of the original magnetic lines of force. Since the original magnetic line of force changes from the change of the external alternating power supply, from the objective effect, the inductance coil has the characteristics of preventing the change of current in the AC circuit. The inductance coil has the characteristics similar to the inertia in mechanics, and electrically It is called "self induction". Generally, sparks will occur when the knife switch is opened or turned on, which is caused by high induction potential generated by self induction. In general, when the inductance coil is connected to the AC power supply, the magnetic line of force inside the coil will change at all times with the alternating current, causing the coil to continuously generate electromagnetic induction. This kind of electromotive force generated by the change of the current of the coil itself is called "self induction electromotive force". Inductance is only a parameter related to the number, size, shape and medium of the coil, and it is a measure of the inertia of the inductance coil, independent of the applied current