SINGLE-PHASE MOTORS 



293 



existing at that instant. The same thing is represented in Fig. 

 271 (6), which shows the flux distribution curves of two fields t 

 and 02, each of which is equal to one-half the maximum field. 

 These two fields glide around the air-gap in opposite directions 

 and with equal velocities. Their algebraic sum < at any instant 

 is the value of the resultant 

 field at that instant and this 

 resultant field is stationary 



01 = 



'max 



J 



=T-*, (6) 



FIG. 271. Representation of a single- 

 phase alternating field by two oppositely- 

 rotating fields. 



It! 



The single-phase field may 

 be considered therefore as 

 made up of two equal rotat- 

 ing fields, revolving in 

 opposite directions. (Ex- 

 periment shows that two 

 such fields actually exist.) Each field acts independently upon 

 the rotor and in the same manner as the rotating field of the poly- 

 phase induction motor. One field tends to cause rotation in a 

 clockwise direction and the other field tends to cause rotation 

 in a counter-clockwise direction. Figure 272 shows the slip- 

 torque curve due to each of the two fields. The torques act 



in opposite directions as 

 shown. At standstill (slip 

 = 1) the two torques are 

 opposite and equal, and 

 the rotor has no tendency 

 to start. If the rotor in 

 some manner be caused to 

 rotate in the direction in 

 which the torque TI is 

 > acting. 7'i will immediately 

 exceed the counter-ton pie 

 !T 2 and the armature will 

 begin to accelerate in the direction of TI. As the armature 

 speeds up, TI predominates more and more over T- and the 

 armature approaches synchronous speed without difficulty. The 

 counter torque due to TI always exists, however, although it has 

 little effect Mtt the synchronous speed of the field which 

 produces TI. 



<r,)0 l 



FlO. 272. Two opposing toniurs in . -i sin^l 



