On computation of the radial magnetic field in vacuum interrupters

In the Radial Magnetic Field (RMF) type of Vacuum Interrupters (VI), the contacts are so designed that a magnetic field distribution, oriented in the azimuthal direction to the arc, is produced. The interaction of the arc current with this field produces a circumferential force on the arc. This force rotates the arc, thus minimizing the heating of the contact surfaces. The speed of the rotation depends on the value of the magnetic field. This paper deals with the computation of the magnetic field distribution between spiral RMF VI contacts. An analytical formula is derived for the magnetic field distribution by approximating the limbs of the contact as finite current sheets, using Ampere´s law. Taking advantage of the fact that the limbs are perpendicular to each other, they contribute to mutually orthogonal components of the magnetic field in a plane parallel to the contacts, and thus the net magnetic field can be easily calculated by taking their norm. The results obtained using the analytical formula are validated by 3D FEM analysis of the spiral contacts using commercial software.