THE INDUCTION MOTOR 403 



inductive by the shape given to the slots in which 

 it is wound, since the rotor currents will largely neutralise 

 the leakage lines and so destroy the self-induction. 

 Hence, it is usual to have a large choking coil in the 

 starting circuit external to the motor, and to make the 

 auxiliary winding itself of only a few turns, so that its 

 self-induction forms only a small part of that of the 

 auxiliary circuit. In this case, it is evident that the 

 higher the self-induction of the coil, the less will the 

 current be in the circuit. Consequently the greater 

 the lag obtained, the smaller will be the strength of. the 

 flux. 



In the case of the Hey I and motor, the starting coil 

 is wound in square holes, so as to give a large leakage and 

 great lag, but the winding is connected directly to the 

 mains and a very strong flux is obtained and a large 

 starting torque. The impedance of the starting phase 

 is thus kept smaller than that of the running phase, 

 although the self-induction is greater, and by the special 

 method of winding the leakage flux is counteracted to a 

 comparatively small extent by the rotor currents. The 

 current is greater in the starting phase than in the main 

 phase, but the total current at starting under full load 

 does not exceed twice the normal full-load current. 



The measurements to be made on single-phase motors 

 are practically identical with those already described for 

 polyphase motors, except that special attention must be 

 paid to starting current and torque, as it is chiefly in 

 this respect that different types vary. It will be found 

 that the maximum torque which can be overcome by a 

 single-phase motor is usually lower than a three-phase 

 or two-phase motor of the same rated output. 



INDUCTION MOTOR DIAGRAMS. 



Graphic Representation of the Induction Motor. As in the 

 case of the static transformer, the relation between the 

 various electrical quantities may be shown graphically 

 by vector diagrams. This method of representation is of 

 great importance practically, as well as theoretically. 



Adopting the same general method of procedure as in 

 the transformer diagrams, Figs. 85-89, we begin the 

 construction as follows : 



For simplicity, assume no losses in the stator winding. 



