Field Transients in Magnetic Systems A Study of Field Transients in Iron Cores Partially Laminated and Partially Solid

The complete mathematical solution of applying suddenly a d-c. voltage to an electromagnetic circuit composed of an electric coil with a partly laminated and partly solid iron core containing an air-gap is presented in this paper. Starting from Maxwell's field equations for the eddy currents and the magnetic field in the iron core, the solution of the time conditions is reached with the aid of Heaviside's operational calculus. The result is expressed by means of a time constant of the magnetic field, composed of the time constant of the electric circuit and a fraction of a new-defined time constant of the solid iron part due to the eddy currents in that part. The importance of this result is illustrated by its application to a numerical example and to the effectiveness of quick response excitation in the case of large synchronous generators. Finally some other important applications are discussed.