A method for separating no-load losses in electrical machinery

The method proposed makes use of idle operation of the machine as a motor, the voltage being varied, and speed kept constant. After deducting the armature I
²
R losses from the watts input, the remaining watts are plotted against the voltage. A formula is derived based upon the assumption that the watts are equal to constant windage and friction loss plus core loss which latter varies as a constant power of the voltage. In applying the method, tangents to the curve are drawn at two points, and from the slopes of these tangents, the voltages and watts at these points, the exponent of the core loss curve, the core loss, and the windage and friction may be calculated with the use of the equations derived. An example is given of the close agreement with the test curve in the case of an induction motor; and other examples are cited of close agreement of the core loss and windage and friction losses with the losses measured by means of the usual belted method. The fact is pointed out that in some machines more accurate results are obtainable by means of the proposed method than with the usual belted method.