D. C. COMMUTATING MACHINES 185 tion produces a magnetic field at the brushes. The e.m.f. gener- ated by the rotation of the armature through this field opposes the reversal of the current in the short-circuited armature coil under the brush, and thus impairs commutation. If therefore the commutation constants of the machines are not abnormally good — high field strength, low armature reaction, low self-in- ductance and frequency of commutation — the machine does not commutate satisfactorily under load, with the brushes midway between the field poles, and the brushes have to be shifted to the edge of the next field poles, as shown in Fig. 95, until the fringe of the magnetic flux of the field poles reverses the armature reac- tion and so generates an e.m.f. in the armature coil, which re- verses the current and thus acts as commutating flux. The commutating e.m.f. and therefore the commutating flux should be proportional to the current which is to be reversed, that is, to the load. The magnetic flux of the field pole of a shunt or compound machine, however, decreases with increasing load at the pole corners toward which the brushes are shifted, by the demagnetizing action of the armature reaction, and the shift of brushes therefore has to be increased with the load, from nothing at no load. At overload, the pole corners towarp! which the brushes are shifted may become so far weakened that even under the pole not sufficient reversing e.m.f. is generated, and satisfactory commutation ceases, that is, the sparking limit is reached. In general, however, varying the brush shift with the load is not permissible, and with rapidly fluctuating load not feasible, and therefore the brushes are set permanently at a mean shift. In this case, however, instead of increasing proportionally with the load, the commutating field is maximum at no load, and gradually decreases with increase of load, and is correct only at one particular load. At constant shift of the brushes, the commutation of the constant potential machine, direct-current generator or motor, is best at a certain load, and usually becomes poorer at lighter or heavier loads, and ultimately becomes bad by inductive sparks due to insufficient commutating flux. In machines in which very good commutating constants cannot be secured, as in large high-speed machines (steam turbine driven direct-current generators) , this may lead to bad sparking or even flashing over at sudden overloads as well as when throwing off full load. 186 ELEMENTS OF ELECTRICAL ENGINEERING 49. This has led to the development of the commutating pole, also called interpole, that is, a narrow magnetic pole located between the main poles at the point of the armature surface, at which commutation occurs, and excited so as to produce a commutating flux proportional to the load, and thus giving the required commutating field at all loads. Such machines then give no inductive sparking, but regarding commutation are limited in overload capacity only by the current density under the brush. Such commutating poles are excited by series coils, that is, coils connected in series with the armature and having a number of effective turns higher than the number of effective series turns per armature pole, so that at the position of the brushes the FIG. 100. — Magnetic force distribution with commutating pole. m.m.f. of the commutating pole overpowers and reverses the m.m.f. of the armature, and produces a commutating m.m.f. equal to the product of the armature current and difference of commutating turns and armature turns, and thereby produces a commutating flux proportional to the load, as long as the mag- netic flux in the commutating poles does not reach too high magnetic saturation. In Fig. 100 is shown the distribution of m.m.f. around the cir- cumference of the armature, and in Fig. 101 the distribution of magnetic flux calculated in the manner as described in para- graphs 46 and 47. M represents the main poles, C the com- mutating poles. A main field excitation FQ is assumed of 10,000 ampere-turns per pole, and an armature reaction Fa of 6000