2 8o ALTERNATING CURRENTS 



due to the alternations of the field component OA to produce a certain 

 resultant e.m.f. which gives rise to the actually existing armature 

 current. 



As regards the field winding, which is connected across the mains, 

 the impressed p.d. may be regarded as made up of the following 

 components : (1) the component required to balance the effective 

 resistance drop in the field ; (2) that required to balance the effective 

 reactance drop ; (3) the component required to balance the e.m.f. 

 induced in the field winding by the alternating flux due to that 

 portion of the armature current which results from the rotation of the 

 armature in the field. 



By considering these three components, a circle diagram similar 

 to that deduced above for the series-wound motor may be established. 

 The two diagrams will not, however, be identical in every detail. 

 The torque, for example, in the case of the repulsion motor is not 

 proportional to the square of the stator current. 



The torque will clearly depend on the angle a which the line 

 of brushes makes with the neutral line, and it is not difficult to see 

 that there are two extreme positions of the brushes for which the 

 starting torque vanishes. The condition, in fact, necessary for the 

 development of a starting torque is the simultaneous existence of 

 the two field components OA and OB. If, then, o = 0, i.e. if the 

 brushes be placed along the neutral line, the component A vanishes ; 

 while if a = 90, the component OB vanishes. Thus in either of 

 these two positions there will be no starting torque. For some 

 intermediate position, the torque will reach a maximum value ; this 

 is found to correspond to a = 60 to 70. 



The maximum voltage of the series-wound motor is, as we have 

 seen, limited by considerations of sparking to about 200 volts. 

 Hence, if such a motor be used for traction purposes in connection 

 with a high-voltage overhead trolley line, a transformer must 

 necessarily be employed to reduce the high voltage. In the repulsion 

 motor, however, the armature is entirely disconnected from the field, 

 so that there is nothing to prevent the latter from being wound for 

 a very high voltage. A transformer may thus be dispensed with. 

 The possibility of connecting a repulsion motor directly across a 

 high-voltage line is its chief advantage over the series-wound motor. 



The speed regulation of the repulsion motor may be effected by 

 the simple device of altering the position of the brushes: Since, 

 however, this motor is very sensitive to comparatively small changes 

 in the brush position, this method of speed regulation may prove 

 somewhat troublesome, a very small brush displacement giving rise 

 to a considerable change of speed. The difficulty is partly remedied 

 by using a special arrangement of brushes, due to Deri,* and shown 



* Eleldrotechnische Zeitschrift, vol. xxvi. p. 72 (1905). 



