D. C. COMMUTATING MACHINES 189 netic flux at the armature circumference therefore always has the same shape, and its intensity is proportional to the current, except as far as saturation limits it. As the result thereof, shifting the brushes to the edge of the field poles, as in Fig. 95, brings them in a field which is proportional to the armature cur- rent and thus has the proper intensity as a commutating field. Therefore with series-wound machines commutating poles are not necessary for good commutation, but the shifting of the brushes gives the same result. However, in cases where the direc- tion of rotation frequently reverses, as in railway motors, the direction of the shift of brushes has to be reversed with the re- versal of rotation. In railway motors this cannot be done with- out objectionable complication, therefore the brushes have to be set midway, and the use of the magnetic flux at the edge of the next pole, as commutating flux, is not feasible. In this case a commutating pole is used, to give, without mechanical shifting of the brushes, the same effect which a brush shift would give. Therefore in railway motors, especially when wound for high voltage, as 1200 to 2400 volts, a commutating pole is sometimes used. This commutating pole, having a series winding just like the main pole, changes proportionally with the main pole. When reversing the direction of rotation, however, the armature and the commutating poles are reversed, while the main poles remain unchanged, or the main poles are reversed, while the arma- ture and the commutating poles remain unchanged; that is, the separate commutating pole becomes necessary because during the reversal of rotation it has to be treated differently from the main pole. 52. The commutating pole counteracts the armature reaction only at the place of commutation, but not elsewhere, and the field distribution resulting from the armature reaction thus is not eliminated by the commutating pole, except locally. Thus in machines having very low field excitation, and relatively high armature reaction, as alternating-current commutating machines, adjustable speed motors of wide speed range at the high-speed position, boosters near zero voltage, etc., the load losses resulting from excessive field distortion, the tendency to instability of speed, and the liability of flashing at the commutator at sudden changes of load are not eliminated by the commutating pole, but a more complete neutralization of the armature reaction is necessary. 190 ELEMENTS OF ELECTRICAL ENGINEERING Such is given by a compensating winding. This is a dis- tributed winding, located in the field pole faces closely adjacent to the armature, as shown in Fig. 102. It is connected in series but opposition to the armature winding, and of the same number of effective turns as the armature. By such a compensating winding, the armature reaction is completely eliminated, and with it magnetic distortion, load losses, etc. By giving the compensating winding some more ampere-turns than the armature, over-compensation is produced, giving a mag- netic cross flux under load, opposite to that of armature reaction, that is, a commutating flux. Very commonly in such com- pensated machines merely the ampere-turns of the compensat- ing winding in the slots at the commutating zone are increased, so that the compensating wind- ing all around the armature ex- actly neutralizes the armature reaction, except at the commu- tating zone, where it over-com- pensates and thus gives a local commutating flux. Such ma- chines, when properly designed, are characterized by absence of load losses, stability at all speeds, instant recovery at sudden load changes, and absence of sparking at commutator even at mo- mentary overloads of several hundred per cent. FIG. 102. — Compensated com- mutating machine with fractional pitch armature winding.