494 ALTERNATING CURRENTS 



QUESTIONS ON CHAPTER IX 



1. What suggests that both the shunt and the series direct-current motors 

 might possibly be used with alternating current? Why is it not possible 

 to use the shunt motor effectively with alternating current? What char- 

 acteristic of the series motor makes it possible for this type of motor 

 to operate effectively with alternating current? 



2. In what way does the field structure of an alternating-current series 

 motor differ from that of the direct-current series motor? How does the 

 number of series turns of the alternating-current motor compare with 

 the number ordinarily used with the direct-current motor of corresponding 

 rating? Why are the poles short and of comparatively large cross-section? 

 Why is the air-gap short? Why is low frequency necessary? 



3. Why does the alternating-current series motor have a large number 

 of armature turns? Give two reasons why armature reaction must be 

 compensated. Show two methods of compensating. 



4. What commutating difficulty exists in the alternating-current motor, 

 which is not present in the direct-current motor? How is this difficulty met? 

 Why is there a large number of commutator segments? 



6. Sketch a vector diagram of the motor? How is the speed controlled? 

 Where is this type of motor used? 



6. What is the nature of the induced electromotive forces in a gramme-ring 

 armature having a commutator, when the armature is placed in a single- 

 phase alternating-current field? What occurs when the brushes are short- 

 circuited and placed in the geometrical neutral? When these brushes are 

 placed parallel to the pole axis? Why is no torque developed in either 



case 



7. Why is torque developed when the brush axis makes some angle 

 greater than zero and less than 90 with the pole axis? How is the direction 

 of rotation controlled? How may the field structure be wound so that the 

 brushes may be left in the geometrical neutral? 



8. Why are repulsion motors made with uniform air-gaps rather than with 

 salient poles? What is the nature of the speed and torque curves of the 

 repulsion motor? 



9. Show that a single-phase alternating-current field can be replaced by 

 two fields rotating around the air-gap in opposite directions. Sketch the 

 slip-torque curve due to each of these two fields. How may the fact that 

 the single-phase induction motor has no starting torque be explained by 

 these curves? How do they explain the fact of the motor accelerating in 

 the direction in which it is started? 



10. By means of a sketch show the position of the rotor ampere-turns of 

 an induction motor when the transformer currents alone are considered. 

 Show the direction of the magnetic field which these ampere-turns produce. 



11. Show that a speed electromotive force in time-phase with the single- 

 phase flux is produced by the rotation of the armature conductors. What 

 flux is due to the current produced by this speed electromotive force and 

 what is its space position? Why do the combined effects of this field and of 



