Advanced theory of the delay of the commutating pole flux of a d.c. machine, and the improved machine that applies it

When the response of the commutating flux is delayed, the appropriate electromotive force for commutation is not induced in the armature winding. The time delay of the commutating pole flux is due to eddy currents induced in the yoke or liners, and results in brush spark. The theory of time delay of the commutating pole flux has been derived by Rudenberg. However, it has been pointed out that the observed value of this time delay is much smaller than the calculation given by Rudenberg´s theory. Therefore, the authors have modified Rudenberg´s theory, adopting new assumptions concerning the flux distribution in the airgap and considering the effect of fringing flux. The derived theory is compared with experiment and shows a good agreement. Furthermore, to test the influence of a nonmagnetic liner, the cutoff test of the conventional 1700 kW mill motor and the improved one in which a `high resistance nonmagnetic liner¿ is installed has been conducted. As a result, it was concluded that the nonmagnetic liner plays a more important role in the flux delay than the magnetic liner. It was concluded that this phenomenon cannot be explained using Rudenberg´s theory, but, with the authors´ theory, it can.