Discussion on “iron losses in direct-current machines” (Lamme), Schenectady, N. Y., March 7, 1916. (see proceedings for March, 1916)

H. F. T. Erben: I agree with Mr. Lamme´s statements that iron losses may be approximated with extreme accuracy but these approximations must be based upon past experiences. Mr. Lamme calls attention to the extremely distorted wave form often encountered in variable speed motors and instances Fig. 6 of the paper as being a typical case. While the field form as shown in Fig. 6 is a fair representation of the conditions that may be met in a three to one variable speed motor, distortions of greater magnitude will be encountered if speed variations of four to one or five to one are attempted. I have in mind a four to one variable speed motor which at no load and full load and maximum speed had field forms as shown in Fig. 1. It will be noted that at full load the field form was so distorted that there was an actual reversal of flux in the leading pole tip and the flux density at the trailing tip was fifty per cent greater than at no load. When one considers that the teeth in the trailing tip under these conditions are well up to saturation and that the armature teeth are subjected to double frequency, due to the reversal of flux, it is inevitable that there will be extremely high losses. Losses due to distortions of this nature can be obviated by either providing compensating windings or by using very large air gaps.