9-B] 



INDUCTION GENERATOR. 



295 



EXPERIMENT Q-B. Operation and Test of an Induction Gen- 

 erator (Primary Generator). 



PART I. INTRODUCTORY. 



i. Principle of Operation. As the speed of an induction 

 motor approaches synchronism, the slip decreases so that there is 

 less cutting of magnetic flux by the secondary conductors. There 

 is accordingly less electromotive force induced in the secondary, 

 less secondary current and less torque. At synchronous speed 

 the slip becomes zero, and there is no secondary current and no 

 torque. When the rotor is driven above synchronous speed, the 

 slip becomes negative ; the secondary current and the torque are 

 now reversed* and mechanical power is required to drive the 

 rotor. The machine has become a generator and supplies elec- 

 trical power to the line. (This condition occurs when an electric 

 train, equipped with induction motors, runs down hill.) As the 

 induction generator does not operate at synchronism, it is fre- 

 quently described as non-synchronous or asynchronous. 



2. When the secondary current is reversed, the primary 

 "added" current / (2) due to the secondary is also reversed (see 

 la), but it still follows the circle locusf as shown in Fig. I. The 

 point P follows the upper semi-circle (as PM) when the machine 

 is operating as an induction motor, the lower semi-circle (as PG) 

 when it is operating as an induction generator. 



In Fig. i, the impressed or line electromotive force is repre- 

 sented as E ; the generated or counter electromotive force, as E r . 



3. As a motor, the primary current is I = OPw, consisting of 

 the exciting current, I = OA, and the added current / (2) =^P M . 

 ( The lines for a motor are not all shown in Fig. i ; see also Fig. I, 

 Exp. 8-B.) The primary current is always lagging with respect 



* (ia). Strictly speaking only the power component of current is re- 

 versed, as will be seen later. 



t It will be understood that for an induction generator, as for an induc- 

 tion motor, the circle locus is approximate and not exact. 



