SI \dLK-ni ASK MOTORS 295 



conductors must How in such a direction as to oppose this flux 

 in the same manner as the secondary ampere-turns of any static 

 former oppose the primary ampere-turns. The effect of 

 the rotor conductors is the same as if they were connected as 

 shown in Fig. 273, each conductor being connected with one on 

 the opposite side of the armature to form a closed turn. To 

 oppose the flux <t> M , the current must be flowing inward on the 

 right -hand side of the armature and outward on the left-hand 

 side of the armature, as indicated in the figure. 



Assume that the armature rotates in a clockwise direction. 

 There will he an emf. induced in the rotor conductors due to their 

 cutting the flux </>.,/. This induced emf. is called the speed elec- 

 >'ivt' force because it is induced entirely by the cutting of 

 the flux </> .1, due to rotation. Applying Fleming's right-hand rule, 

 inf. acts inwards on the upper half of the armature and 

 outwards on the lower half, as shown in Fig. 274. This emf. is 

 alternating and is a maximum when <. v is a maximum. As the 

 rotor conductors are short-circuited upon themselves, alternating 

 currents flow in them as a result of this induced emf. The rotor 

 reactance being high as compared with its resistance, these cur- 

 rents lag the induced emf. by very nearly 90. Moreover, these 

 currents produce a flux </> x , at right angles to </> A /, as shown in 

 71 just as the ampere-conductors of a direct-current motor 

 produce a field at right angles to the pole axis when the brushes 

 are in the geometrical neutral. In practice, the stator completely 

 surrounds the rotor, the air-gap being uniform. At synchronous 

 speed, <t> A is substantially equal to </>. v but is 90 from </>. u in space. 



The speed electromotive force E A is obviously, a maximum 

 when <t>v is a maximum. The current /, does not reach its 

 maximum until nearly <><! later in time, because the rotor re- 

 actance i- high as compared with its re-i-tance. In Fig. 273, 

 <t>.v is shown as having reached its maximum and acting verti- 

 cally downwards. After a quarter period, $ A reaches its 

 maximum and is acting 90 in -parr from 1 lie flux <J> M , as shou n 

 in Fin. -71. It will b Mi'/ed that two such fields, acting 



axes 90 from eaofa other in space and differing in liine- 

 l>v an a- 'i(), will produce a rotating magnetic field. 



This field rotate! clockwise in ,.; and 274. As the rotor 



slip increases, <f> A decreases in magnitude because the -peed is 



