TIIK IMJCCTION MOTOR 



101. Synchronous Speed ; Slip. It has just been shown that 



the angular speed of an alternating-current rotating field depends 



upon two factors, the frequency of the current and the number 



of poles for which the machine is wound. The relation between 



1. frequency and poles is given by the following equation: 



(63) 



where N is the speed of the field in revolutions per minute, / 

 the frequency in cycles per second and P the number of poles 

 (see equation (2), page 7). This speed, N, of the rotating 

 field, is called the synchronous speed of the motor. The common 

 synchronous speeds for commercial motors at 25 and at 60 

 cycles per second are as follows : 



Poles 



2 

 4 

 6 

 8 

 12 



Slip. If an armature whose conductors form closed circuits 

 be placed in a rotating field, it will develop torque because of the 

 induced currents, acting in conjunction with the rotating mag- 



netie field. 



As has already been pointed out, the armature can never 

 attain the speed of the rotating field, for if it did. the cutting of 

 conductors by flux would cease, there would be no rotor current 

 and, therefore, no torque. 



The difference between the speed of the rotating field and that 

 of the rotor is called the rcrolulion^ .s7//> of the motor. For 

 example, if the rotor of a four-|><>i ck motor has a speed 



of 1.7HO r.p.m., its revolutions slip is 1 ,S(K) - 1,730 = 70 r.p.m.. 

 where l.SOO r.p.m. is its synchronous speed. 



It is more convenient to express the slip as a fraction of the 

 synehronous speed. Denote the speed <>f the rotor by NI and 

 the synchroiiou- -p-ed by N. Then t he slip 



(64) 



