INDUCTION MOTOR 



331 



impedances. These currents reacting on the magnetic field 

 produce torque and the rotor revolves in the direction of the field. 

 At no load the rotor runs almost or fast as the field and very small 

 e.m.fs. and currents are induced in its conductors. When the 

 motor is loaded the rotor lags behind the field in speed and large 

 currents are induced to give the required torque. 



FIG. 314. Squirrel-cage rotor. 



204. Slip. The difference between the synchronous speed or 

 speed of the stator field and the speed of the rotor is called the 

 slip and is expressed as a per cent of synchronous speed. 



A 6-pole, 60-cycle motor has a synchronous speed 



N = 



120 X 60 



= 1200 r.p.m. 



If the speed of the rotor at full load is 1176 r.p.m., the slip is 

 1200 - 1176 



1200 



X 100 per cent = 2 per cent. 



The slip at full load varies from 2 per cent to 5 per cent in motors 

 designed for constant speed. 



The rotor speed may be expressed as 



S=(l-s)N r.p.m (301) 



205. Magnetomotive Force of the Rotor. The frequency of the 

 e.m.fs. and currents induced in the rotor windings at a slip s is sf 

 if / is the frequency of the e.m.fs. impressed on the stator. 



The polyphase currents in the rotor windings produce a re- 

 sultant m.m.f. revolving relative to the rotor at a speed 



