In the alternating magnetic field generated by alternating current, there are 100 times of current passing through the zero point (take 50Hz AC as an example), at this time, the attraction of the electromagnet is zero and it is in the released state; after passing the zero point, the current rises again, the attraction Force recovers, and the electromagnet is closed; so repeatedly, there will be noise when the electromagnet is pulled in and pulled out. 

To avoid this phenomenon, it is necessary to inserted a ring in the core side surface of the static iron core to limit vibration of the core in AC constructions. Also known as shading ring, the magnetic flux generated by the induced current in the ring and the magnetic flux outside the ring do not cross zero at the same time, so that the electromagnetic suction force is always greater than the spring reaction force, thus eliminating the jitter of the AC core