356 ELECTRICAL ENGINEERING 



A single-phase induction motor is usually either a two-phase 

 or three-phase motor operated on a single-phase circuit using 

 only part of the stator winding. 



If one phase of a two-phase motor is opened at light load, the 

 magnetizing current of the other phase is doubled and the motor 

 runs as a single-phase motor. If two phases of a three-phase 

 motor are opened the motor runs as a single-phase motor with 

 the magnetizing current in the third phase trebled. In both cases 

 the flux distribution and flux densities remain approximately the 

 same as before. 



226. Induction Generator. If the stator of an induction 

 motor is connected to the supply lines and its rotor is driven 

 above synchronous speed, the machine will develop electrical 

 power and supply it to the system. 



The stator flux is not affected by the increase in the speed of 

 the rotor, but revolves in the same direction as when the machine 

 operates as a motor. The slip is, however, reversed and the 

 e.m.fs. and currents induced in the rotor are reversed. Thus 

 the direction of torque and power is reversed and the mechanical 

 power supplied to drive the rotor is transformed into electrical 

 power and supplied over the lines to the load. 



The power transferred from the rotor to the stator depends on 

 the slip just as in the induction motor. Using the same notation 

 as in Art. 211, the power transferred to the stator is 



P = nE 2 Iz cos 6 2 



v r 2 2 + s 2 x z 2 v r 2 2 



r 2 2 



Thus to increase the power delivered by the generator its speed 

 must be increased. If therefore an induction generator is con- 

 nected to a prime mover of variable speed, it will supply power 

 almost in proportion to the increase of its speed above synchro- 

 nous speed. 



The frequency of the stator induced e.m.f. is the frequency of 

 the exciting current and does not depend in any way on the speed 

 at which the rotor is driven. 



The induction generator has two very serious disadvantages, it 

 is not self-exciting and it cannot supply wattless currents to an 



