496 ALTERNATING CURRENTS 



When the synchronous motor is over-excited what must be the phase re- 

 lation between its current and its terminal voltage? Illustrate by a vector 

 diagram. 



7. What effect is noted when the field of a direct-current shunt motor is 

 weakened? Why cannot these reactions occur when the field of the syn- 

 chronous motor is weakened? 



8. What is the effect of a lagging current upon the field of a synchronous 

 motor? Upon the relation of the induced to terminal voltage? Make 

 a vector diagram for the motor when operating under-excited. 



9. Why is a synchronous motor able to operate, even without direct- 

 current excitation? Whence does it obtain its excitation under these 

 conditions? 



10. Sketch a synchronous motor V-curve. Show the point of unity power- 

 factor, the region. of lagging current, and the region of leading current. 

 Sketch another V-curve in which the power is twice that of the original 

 curve. How is the position of this curve determined? What is meant 

 by "normal" excitation and how does this vary with the motor load? 



11. Give two reasons for building squirrel-cage or "amortisseur" 

 windings around the poles of a synchronous motor. Analyze the reactions 

 by which an amortisseur winding stabilizes the operation of the synchronous 

 motor. 



12. Describe the method of starting a synchronous motor by means of an 

 auxiliary motor. What types of motor are used for this purpose? What 

 are the objections to their use? 



13. What is the sequence of operations in starting a synchronous motor 

 by means of its direct-current generator? What objection is there to 

 starting a motor in this way? 



14. By what process may the synchronous motor start of itself? Why is 

 a compensator used? Of what order of magnitude is the starting torque? 

 When should the direct-current field be closed? 



16. Analyze closely the method by which the synchronous motor, when 

 starting as an induction motor, is able to pull into synchronism even 

 without direct-current excitation. 



16. What happens at the time of closing the field switch if the direct- 

 current excitation opposes the field built up by armature reaction? What 

 should be the position of the starting device when the field switch is closed, 

 and why? 



17. How may correct polarity of the field poles be insured so that little 

 or no disturbance results when the field switch is closed? 



18. Why is it necessary to insulate the field coils of a synchronous motor 

 for a voltage several times the normal operating voltage? How may the 

 electromotive force induced in the field be reduced? 



19. Why are synchronous motors, running without load, often installed 

 at various points of power systems? What is the motor called when oper- 

 ating under these conditions? 



20. What is the distinct advantage of using a synchronous motor drive in 

 certain instances? 



