260 ELEMENTS OF ELECTRICAL ENGINEERING. 



rents supplied to the stator windings have a frequency of / 

 cycles per second, we have : 



in which n is the speed of the stator magnetism in revolutions 

 per second. 



The number of polar regions, or the number of poles as it is 

 usually expressed, is equal to the number of bands of stator con- 

 ductors in each phase of the stator winding. Thus each phase 

 has four bands of conductors in Figs. 227, 228, 229, 230 and 

 231, and the windings represented in these figures are therefore. 

 called four-pole windings (/ = 4). 



125. Relation between speed and torque of the polyphase induc- 

 tion motor. When an induction motor is running at zero load, 

 but little torque is required to drive it, and the rotor revolves at 

 a speed but very little less than the speed of the stator magnetism 

 (the speed of the stator magnetism is called the synchronous 

 speed). The rotor cannot,* however, reach the full synchronous 

 speed, because at synchronous speed the rotor conductors would 

 move along with the stator magnetism ; there would be no elec- 

 tromotive force, and therefore no current induced in the rotor 

 conductors ; and consequently the stator magnetism could exert 

 no driving force on the rotor conductors. 



When an induction motor is running with a load, the driving 

 torque must be large, so that the current in the rotor conductors 

 must be large in order that the stator magnetism may exert the 

 necessary driving force upon the rotor conductors ; therefore the 

 induced electromotive forces in the rotor conductors must be suf- 

 cient to produce the necessary rotor currents, and consequently 

 the rotor must run appreciably below synchronism in order that 



*The effect of magnetic hysteresis in the rotor iron is to produce a certain slight 

 torque the value of which is independent of the relative speed of stator magnetism and 

 rotor, and this hysteresis torque may, and in same cases does, bring the rotor of an 

 induction motor up to full synchronous speed when the motor is without load. 



