3 oo 



INDUCTION MACHINES. 



[Exp. 



open. The induction machine is driven at about normal speed 

 and is then connected (by the switch S 2 ) in parallel with a syn- 

 chronous generator or supply line. Note that the switch S 2 may 

 be closed when the induction generator is running either above or 

 below synchronous speed. After the switch is closed, the induc- 

 tion machine continues to run as a motor below synchronous 

 speed or as a generator somewhat above synchronous speed. 



12. Vary the speed of the induction machine, by varying the 

 speed of the driving motor, and note the wattmeter W. 



Below synchronous speed, the in- 

 duction machine takes power from the 

 line as a motor. At synchronous speed, 

 corresponding to the point A in Fig. I 

 or 3, the wattmeter shows only the no- 

 load losses which are all supplied elec- 

 trically. 



A little above synchronism (between 

 A and a) the wattmeter reading de- 

 creases, some of the losses being sup- 

 plied mechanically by the pulley. At 

 the point a all the losses are supplied 

 mechanically and the wattmeter read- 

 * n & Becomes zero. Above this speed, 

 as generator the wattmeter reverses and shows elec- 

 tric power given to the line by the in- 

 duction machine as a generator. 



Note that the frequency and voltage 

 in all cases are determined by the syn- 

 chronous alternator or supply circuit. 



13. Take readings of volts, watts and amperes through the 

 full range between no load and full load with the machine oper- 

 ating as a motor and as a generator. (It is instructive also to 

 measure speed or slip and to plot slip positive and negative for 

 different amounts of power.) 



FIG. 3. Current taken in by an 

 induction machine as a motor 

 and given out as a generator. 



