Armature-conductor impedances and commutation in d.c. machines

Subject to certain approximations about the configuration of the flux associated with armature conductors in slots, the current-density distribution in the conductors is determined by a 1-dimensional diffusion equation, and solutions of this equation when the total current in the conductor may have any prescribed form of variation are obtained. The associated voltages for typical idealised reversal waveforms of armature current are then derived, and it is shown that the eddy-current effects in deep unlaminated conductors may have a significant influence on these voltages. Failure to take sufficient account of these effects may be a major cause of certain discrepancies in the results of some elaborate analyses of commutation in large d.c. machines that have been published, although further work would be necessary for definite confirmation of this.