336 ELECTRICAL MACHINERY 



How fast will the magnetic field revolve in such a motor 

 if it is supplied with 60-cycle power? As the motor has 

 two pairs of poles per phase, it requires two cycles of cur- 

 rent to make the field travel all the way around the stator. 

 As there are 60 cycles per second the field will make 

 60-^-2X60, or 1800 r.p.m. Suppose an 8-pole motor sup- 

 plied with 25-cycle power; it would require 4 cycles of 

 current to give the magnetic field one complete revolution, 

 hence the field would turn 25 -f- 4X60 or 375 r.p.m. 



Rotor Speed Less than that of the Field. The speed at 

 which the field of an induction motor turns is called syn- 

 chronous speed. The rotor never turns as fast as the 

 magnetic field, but perhaps 5-10% slower. 



Slip. The difference between rotor speed and field 

 speed is termed the slip. Slip is generally expressed as a 

 per cent of the synchronous speed. 



Thus slip (%) = { synchronous speed rotor speed) -r- syn- 

 chronous speed } X 100 (76) 



Suppose a 60-cycle, 16-pole, 3-phase motor has a slip 

 at full load of 6%; at what speed does the rotor turn at 

 full load? 



Synchronous speed = 450 r.p.m.; 



Slip = 450 X 6% = 27 r.p.m. ; 



Rotor speed = 450 - 27 = 423 r.p.m. 



92. Rotor Construction. There are two types of rotor, 

 one called the squirrel-cage rotor and the other the wound 

 rotor. The cores of both must, of course, be of laminated 

 iron; the distinction comes from the manner in which the 

 conductors of the rotor winding are connected together. 



Squirrel-cage Rotor. In the squirrel-cage rotor the wind- 

 ing consists of heavy copper bars imbedded in slots in the 

 periphery of the rotor and these copper bars are all short 

 circuited at both ends of the rotor by being soldered to 



