264 ALTERNATING CURRENTS 



motors of ordinary construction. We cannot, of course, alter at will 

 the number of conductors on the rotor, but we can easily effect what 

 is an equivalent change viz. alter the p.d. impressed on the brushes. 

 This may be done by means of a transformer interposed between the 

 mains and the rotor, and having a variable number of turns in its 

 secondary. A decrease of p.d. in the ratio of r : 1 is clearly 

 equivalent to an increase of C r in the ratio of 1 to r. 



157. Effect of Brush Displacement 



We have so far neglected magnetic leakage, and if this were 

 really negligible, the power factor of our motor would be very high 

 as high as that of a good transformer working on a non-inductive 

 load. Unfortunately, magnetic leakage is by no means negligible, 

 and its effect is to lower the power factor. Now, in the ordinary type 

 of induction motor we have no means of altering the power factor. 

 It is in this connection that one of the most important advantages of 

 the commutator motor comes to light, for by suitably displacing the 

 brushes we can adjust the phase of the rotor and hence also that of the 

 stator current so as to bring the latter into practical coincidence of phase 

 with the p.d., and thereby secure a power factor approaching unity. 



Originally, we assumed the brushes to be placed in such a position 

 that the induced rotor e.m.f. was in exact opposition of phase to the 

 p.d. Now, by displacing the brushes we can introduce a phase differ- 

 ence between the p.d. and e.m.f., thus altering the phase of the result- 

 ant e.m.f. and of the rotor and stator currents. The immediate effect 

 of shifting the brushes, one way or the other, from the position origin- 

 ally assumed, is to increase the resultant e.m.f., and hence increase the 

 current. But although the current is increased, the torque may be 

 either increased or decreased, depending on the direction in which the 

 brushes are displaced. Let us suppose the displacement to have been 

 such as to increase the torque ; acceleration will occur, the induced 

 e.m.f., and with it the current, decreasing until equilibrium is once 

 more established. If the brushes be now further displaced in the 

 same direction, a further increase of torque, accompanied by further 

 acceleration and reduction of the induced e.m.f., will take place. It 

 will thus be seen that by continuing to displace the brushes in the 

 proper direction, the speed may ultimately be brought up to 

 synchronism, and the induced e.m.f. made to disappear; the rotor 

 winding now behaving as a simple non-inductive resistance, since 

 there is no relative motion between it and the rotating field. By 

 displacing the brushes still further, speeds above synchronism may 

 be realized, 



