Discussion on “determination of power efficiency of rotating electrical machines” (Olin), Boston, June 28, 1912. (see proceedings for July, 1912)

C. M. Green: I have had quite a little experience and difficulty in making efficiency tests on old arc machines particularly the Brush arc generator. The input and output method, to the best of my knowledge, is the only method b y which the efficiency of these machines can be determined, due to the fact of the large influence which the current in the armature has, at ordinary loads, on the eddy currents in the pole shoes and core loss. Furthermore, the field excitation on the Brush, etc., machines with no current in the armature, and rated volts, runs about 25 per cent of that with normal current and rated volts in the armature, so that you may see that the effect of the armature current upon the field windings is very abnormal. There is absolutely no question about the difficulty of making input and output efficiency tests. It is extremely difficult to get results which will check day in and day out. There is a continual variation of at least 2 or 3 per cent in the efficiency.