406 THE INDUCTION MOTOR. 



always pass through F, no matter how the load varies. 

 Further, the angles at D and B are right angles, and 

 consequently the points D and B will always lie upon the 

 semi-circles A B F and F D O, drawn upon A F and F 0. 

 Their positions will depend upon the magnitude of the 

 primary and secondary currents, that is, upon the load 

 on the motor. Thus we may re-draw the diagram of 

 Fig. 194 as shown in Fig. 195, which is the simplest form 

 of Hey land's diagram. 



There will be two extreme conditions, viz., when the 

 stator current has its maximum and minimum values. 

 The maximum current will flow when the motor is so 

 heavily loaded that the rotor is pulled up, and the rotor 

 currents have their maximum value owing to the large 

 slip. In this case the motor becomes practically a static 

 transformer with short-circuited secondary winding. The 

 primary flux which cuts the rotor conductors will produce 

 rotor currents of such a magnitude that the rotor flux 

 will exactly counteract the primary flux (if we neglect the 

 losses in the rotor winding), and the resultant flux through 

 the rotor becomes nil. Under these conditions the 

 only portion of the primary flux which remains un- 

 neutralised is the leakage flux, and consequently we may 

 write stator flux = leakage flux or Z a = z. At the 

 same time rotor flux = Z r = O. 



This condition would be represented in the diagram 

 if B were made to move to the left until it coincided 

 with A, when B O, the leakage flux, would become equal 

 to 4 0. 



The other extreme case possible is when the motor is 

 unloaded and runs synchronously (again the losses being 

 neglected), in which case the rotor current becomes zero, 

 and the primary current is only the magnetising current, as 

 in the case of an unloaded transformer. The rotor and 

 stator fluxes are then equal, the leakage flux being prac- 

 tically zero. This case would be represented in the dia- 

 gram if B were considered to have moved round to the 

 right until it coincided with the point F. In this con- 

 dition A F = A B and F B = O. 



In both limiting cases the stator flux Z s is the same, 

 since it depends only on the voltage applied to the motor, 

 and in both cases the primary current is entirely spent 

 in producing this flux, since the motor is not doing useful 



