Finite element analysis of electromagnets and contact systems in low voltage current limiting circuit breakers

The magnetic forces generated by short-circuit currents in the fast electromagnet and breaking chamber of a low-voltage current-limiting circuit breaker are computed by a finite-element package. A particular type of electromagnet with a rotating armature is optimized by a combination of experiments and numerical analysis. An analytical expression giving the armature torque as a function of its angle and coil exciting current is deduced from the results of finite-element analysis. This expression is used for the study of the transient state of the electromagnet, providing the evolution of its position, speed, and kinetic energy. The simulated values of armature angle are compared with measured ones. The influence of contact arm arrangement and iron plates of the breaking chamber is investigated by assuming a two-dimensional model