A new method for observing the phenomena of commutation

The paper describes a new method for measuring the armature coil current in a d.c. machine as it reverses during commutation. A small search coil, fixed relative to the armature, is magnetically coupled with one of the armature coils. The e.m.f. induced in the search coil, which is proportional to the rate of change of armature coil current, is integrated to give a signal proportional to the current itself, and this is displayed on the screen of a cathode-ray tube. The method overcomes several of the difficulties associated with the use of a shunt for measuring armature coil current during commutation. A method is described for triggering the oscillograph time-base which permits examination of the current reversals as they occur in a number of consecutive revolutions of the machine. The paper is illustrated with oscillograms showing thearmature coil current during commutation in a particular aircraft generator, and the cause of poor commutation in this machine is deduced.