Current and torque of D-C machines on short circuit

Abstract

This paper presents formulas, in terms of easily evaluated coefficients, for the armature current, field current, and torque of d-c machines on short circuit. The method of the paper makes it unnecessary to calculate the mutual inductance between the armature and field windings. Heretofore, this mutual inductance (between the two windings which normally are in quadrature with each other) has made it exceedingly difficult to calculate current and torque on short circuit. Under short-circuit conditions, the armature and field windings are not, in effect, in quadrature with one another and are mutually inductive partly because of saturation of the flux path at one side of the poles and partly because of commutation phenomena. These circumstances create difficulties uniquely overcome in the paper. A basic feature of the paper is the expression for the relation between flux, armature amperes, and field amperes. Calculations cannot be made without establishing this relationship (plotted for a specific case in Figure 9) and a simple expression for it has not been presented before to the author's knowledge. Another important feature, also believed to be new, is the development of an expression for the resistance to current flow created by the reactance voltage of the alternating current in the armature winding. A third feature is a formula for estimating the time constant of the armature circuit using an average coefficient of inductance. The results are applied to several machines and the calculated performance compared with test data. In most cases, particularly with large machines having bar windings, approximate calculations can be made without need of manufacturer's data.