8-A] OPERATION AND LOAD TEST. 259 



form south poles, A little later (after % cycle) the current in 

 Phase II. becomes a maximum, so that BB form north poles and 

 B'B' south poles. (A and C are now also north but weaker, the 

 maximum field being under B.) Later (after % cycle) the cur- 

 rent in Phase III. becomes a maximum and north poles are 

 formed under CC. Each pole is accordingly seen to progress 

 and to be successively under A, B, C, A', B' , C, etc. The primary 

 or stator winding thus produces a revolving or rotating field 

 which tends to drag the rotor around with it, 50. With the 

 usual distributed winding, the field is uniform and revolves with 

 uniform speed. 



4. In a 2-pole model (having two poles per phase) the field 

 makes one revolution in one cycle; in a 4-pole model, as Fig. I, 

 one revolution in two cycles; in a 6-pole model, one revolution 

 in three cycles, etc. 



It is seen that, if n is the frequency in cycles per second and p 

 is the number of pairs of poles (per phase), the rotating field 

 makes n-t-p revolutions per second. This is known as the syn- 

 chronous speed of the motor; compare I, Exp. 3-A. 



5. In revolutions per minute, 



Synchronous speed = 6on-^p; 

 Pairs of poles per phase = 60 n -f- synchronous speed. 



6. Speed and Slip. The rotor of an induction motor revolves 

 at a little less than synchronous speed ; for, at synchronous speed 

 it would revolve at the same speed as the magnetic field, in which 

 case there would be no cutting of lines of force, no secondary 

 current and hence no torque. 



The actual speed of an induction motor is, therefore, less than 

 synchronous speed by a few per cent, called the per cent. slip. 

 The slip increases with the load, thus increasing the cutting of 

 lines of force, the current in both secondary and primary and the 

 torque. 



7. Primary Winding. An induction motor is not commonly 



