QUESTIONS AND PROBLEMS 491 



6. Describe the construction of a squirrel-cage rotor, indicating the 

 various methods of making the end-ring connections. 



6. Why does the rotor slip increase with increased load? What is the 

 order of magnitude of the slip in commercial motors? 



7. What direct-current motor characteristics do the squirrel-cage motor 

 charac Mi'? Why do the power-factor and the efficiency of 

 the induction motor increase rapidly with load? 



8. State one very serious objection to the squirrel-cage motor for certain 

 types of service. Analyze carefully the reasons why this type of motor 

 develops but little torque at starting although it takes an unusually large 

 current. Under what conditions is the torque a maximum when the 

 current and flux are fixed in magnitude? 



9. Sketch a typical slip-torque curve of an induction motor. What is 



by the break-down torque? Upon what three factors does it depend? 



10. Name several industrial applications to which the squirrel-cage motor 

 is particularly well adapted. 



11. Sketch the special connections used for starting squirrel-cage motors 

 when no starting-compensator is necessary. Why are starting compen- 

 sators necessary? Make a diagram of two different types. 



12. What, is the effect upon the slip of an induction motor of introducing 

 resistance into the rotor circuit? Explain. What is the distinct disadvan- 

 tage of controlling the speed of the motor by inserting resistance into the 

 rotor circuit? 



13. Why are wound-rotor induction motors often necessary? Compare 

 their starting characteristics, their operating characteristics and their 



t h those of squirrel-cage motors. State a few of the industrial appli- 

 cations of the wound-rotor motor. 



14. What is the effect upon the operation of the induction motor; (a) 

 of increasing the length of the air-gap; (6) of using open slots; (c) of u>ing 

 semi-closed stator slots; (d) of using semi-closed rotor slots? Discuss the 

 mechanical construction of the motor with special reference to air-pap 

 requirements. 



16. What three quantities determine the speed of the induction n 

 State briefly the underlying principle of a method of speed control which 

 does away with some of the disadvantages of introducing < mto 



the rotor circuit. Make a diagram of connection- and discuss the dl 

 of this method. When- would such methods of speed control be used? 



16. (live an example where speed may be controlled by change of fre- 

 quency, Stating the limits ,,f >U ch speed control. 



17. State how the number of poles of an induction motor may be changed 



in order to give different s|M'<-d>. \\ hat are the limitation- <>f tin- method? 



18. What is meant by eoncat en:, t mi, ' Dfeoilfl tin- method ,.f speed 



!. irivirm a din. when t w<> similar motors are used. 



To what din-, -t-rurr.-nt method of speed control does this correspo 



19. I'nder what conditions will an induction m .chine develop ele< 



-p the reversal 

 iose reactions upon the stator. (c) 



