Tin- INDUCTION MOTOR 237 



The slip 



1,200 - 1,164 



= - = 0.03 



1,200 



/, = 0.03 X 60 = 1 .8 cycles per second. Ans. 



The rotor frequency has a very important bearing on the 

 operating characteristics of the induction motor. 



The induction motor can be used as a frequency changer, 

 provided the rotor is driven mechanically at the proper speed. 

 Current is taken from the rotor, or secondary, through slip-rings. 

 Tudor these conditions, some of the power is supplied electrically 

 and some mechanically. 



103. Alternating-current Torque. It has already been pointed 

 out. in connection with the -direct-current motor, that the torque 

 is proportional to the current and to the density of the magnetic 

 field in which the current finds itself. This same law holds for 

 alternating-current motors, provided the instantaneous values 

 of current and flux are considered. 



Figure 227 (a) shows the space distribution of flux from one 

 north pole as it glides along the air-gap of an induction motor. 

 This flux is distributed sinusoidally along the air-gap, as is shown 

 by the flux-distribution curve, </>, Fig. 227 (6). 



If the slip be small, the reactance of the rotor conductors is low 

 because / 2 = s/andz' 2 = 2r/sLj, where /is the stator frequency, 

 z'j is the rotor reactance at slip s, and L 2 is the rotor inductance. 

 Because of the rotor reactance the rotor current lairs the induced 

 enif. of the rotor by an angle a. At low values of slip, this angle 

 a is very small, since tan = 2irfsL*/R*, where # 2 is the rotor 

 resistance. 



The indue.-, 1 emf. in any single conductor, I centimeters in 

 length, in a field having a density of B gausses, the conductor 

 moving at a velocity of v centimeters per second, is c = Blv 

 10~ 8 volts, the flux, the conductor and the velocity being 

 mutually perpendicular (see Vol. 1, pane iM7. e.piatin. 

 Therefore, when a conductor is cut t inn flux at a uniform velo.-it \ . 



the flux being sinusoidally or otherwise distributed in space, the 



emf. in the conductor i<? /em when it i- ni.>\ iim in a region where 



B, the flux den : the emf . is a maximum when th< 



ductoi- is moving j n where B, the flux density, is a 



maximum. As the emf. e, is proportional to B at every instant, 



