492 ALTERNATING CURRENTS 



How is the load of the induction general or controlled? (d) From where 

 does the machine obtain its exciting current? (e) What detormincs its 

 frequency and voltage? 



20. State the advantages and the disadvantages of the induction generator 

 as compared with the synchronous generator. Why is the machine power- 

 factor determined by the machine itself and not by the load? Illustrate 

 with a vector diagram. To what type of work is the induction generator 

 particularly well adapted? 



21. What measurements are necessary in order to obtain data for the 

 construction of the circle diagram? Why is reduced voltage used in the 

 blocked run? How is the diameter of the semi-circle determined? What 

 construction is necessary in order to separate the primary and secondary 

 copper losses? 



22. How are the following factors determined for any given value of 

 primary current: (a) Secondary current; (6) power input; (c) core and 

 friction losses; (d) primary copper loss; (e) secondary copper loss; (/) 

 output; (g) efficiency; (h) torque; (i) slip; (j) power-factor. 



23. Why is it inaccurate to determine the slip by measuring the rotor 

 and synchronous speeds and then subtracting? Describe the principle 

 of the stroboscope method. How may slip be measured mechanically? 



24. What types of common alternating-current machinery does the 

 induction regulator resemble? What windings has the regulator and where 

 are they placed? Why is a tertiary winding necessary in the single-phase 

 regulator and where is it placed? 



25. How is the regulator operated? How is it connected to the circuit? 

 Compare the three-phase regulator with the single-phase regulator. 



PROBLEMS ON CHAPTER VIII 



118. A three-phase, 60-cycle induction motor has 10 poles. Through 

 how many space-degrees will the rotating field advance during one cycle? 

 What is the speed of the rotating field in revolutions per second? In 

 revolutions per minute? 



119. Repeat problem 118 for a 25-cycle motor of the same number of 

 poles. 



120. It is desired to obtain an induction motor which shall have a speed 

 in the neighborhood of 400 r.p.m. How many poles should this motor have 

 if 60-cycle power is available? 25-cycle power? 



121. The rotor of a six-pole, three-phase, induction motor rotates at 

 1,160 r.p.m. when the motor is connected across 60-cycle mains. What is 

 the slip of the motor in per cent? What is the frequency of the currents 

 in the rotor when it is running at this speed? 



122. A certain squirrel-cage induction motor develops a starting torque 

 of 40 lb.-ft., when it is connected to the 40 per cent, taps of a three-phase 

 starting compensator, and the line current is 80 amp. What is the approxi- 

 mate starting torque and the line current when it is connected to the 60 

 per cent, taps? 



