THE INDUCTION MOTOR. 391 



In order to increase the power factor of a motor, there 

 are two ways which it is possible to adopt, both of which 

 diminish the no-load currents. These are, either to 

 increase the number of windings of the primary, or to 

 reduce the reluctance of the magnetic path by reducing 

 the air-gap. The first method has the effect of increas- 

 ing the resistance of the winding, and, consequently, 

 diminishing the efficiency of the motor. This disadvan- 

 tage is not possessed by the other method of improving 

 the power factor. 



Relation Between Slip and Torque. It has already been 

 shown on page 361 that with light loads the torque is 

 proportional to the slip. The curve of slip in Fig. 186 is 

 seen to confirm this statement, since it is a straight line 

 rising gradually from the point corresponding to zero 

 torque. 



In the same place, it was further shown that, when 

 heavily loaded, the torque is approximately proportional 



to - The curve representing this relation would be a 



hyperbola. The two theoretical curves, to which the condi- 

 tions approximate at no load and full load, are indicated by 

 the straight line and portion of a hyperbola on the curves 

 plotted in a slightly different manner in Fig. 180 (page 359). 

 The actual shape indicated by a thick line in Fig. 180 passes 

 gradually from one curve to the other. This curve, which 

 is again seen in Fig. 190, shows the characteristic form of 

 the relation always existing between the slip and torque. 



The straight first portion of the curve is the same as 

 would be obtained in a motor without magnetic leakage. 

 It is, therefore, the same line as the one drawn from 

 theoretical considerations in Fig. 177. It is, further, the 

 most practically important portion of the curve, since the 

 working conditions always correspond to points upon it. 

 Thus, although there are two values of the slip corre- 

 sponding to the higher values of the torque, it is only the 

 lower values of the slip which are reached under normal 

 working conditions, i.e., the motor works upon the straight 

 portion of the curve. 



In Fig. 180 is shown a second dotted curve, which 

 applies to the same motor and conditions as the first one, 

 except that a resistance is supposed to have been intro- 



