THE INDUCTION MOTOR. 401 



perpendicular to A A 1 , and in phase with the flux generated 

 by the stator current. 



When the rotor rotates there are consequently two 

 separate effects, each tending to produce currents in the 

 rotor winding and consequently each tending to produce 

 an alternating rotor flux. The relation between these 

 fields will be as follows, from what has just been' said : 

 One field, having the same direction as the stator flux 

 and 90 out of phase with it, and a second field perpen- 

 dicular to the axis of the stator flux, but in phase with 

 it. The second field is due to the rotation of the rotor, 

 and will depend for its direction upon the direction in 

 which the rotation occurs. 



As already explained in the discussion of the produc- 

 tion of a rotating field, the necessary conditions for the 

 existence of such a field are fulfilled by the formation of 

 two alternating magnetic fluxes in two directions per- 

 pendicular to each other, the phase of the fluxes differing 

 by J period. ^These conditions therefore will exist in 

 the rotor of J the induction motor supplied with single- 

 phase current as soon as it is rotated with sufficient 

 speed to produce the current which forms the second 

 field. At slow speeds the currents produced by the 

 motion of the conductors across the stator field will be 

 weak and the field produced by them will also be weak. 

 The rotating field will consequently be unequal in 

 strength and will have its maximum strength as it rotates 

 into the direction of the axis of the stator field, and will 

 be weakest when it has moved into the position at right 

 angles to this. We shall consequently obtain at first a 

 field rotating with the full speed corresponding to the 

 periodicity of the supply, but varying in strength during 

 each revolution. As the speed rises, the field will become 

 more uniform until ultimately, when the speed attains 

 its steady value, we shall have a rotating field differing 

 only slightly from the uniform field to be found in a 

 two-phase motor. 



The increase in current in the single phase winding is 

 due to the fact that this now has to supply the whole 

 of the magnetising current of the motor, as well as the 

 current necessary to balance the demagnetising ampere- 

 turns of the rotor. 



