340 



ELECTRICAL MACHINERY 



ductor in a direction at right angles to itself. By applica- 

 tion of the rule given in paragraph 11 it is seen that the 

 top conductors (Fig. 222 have a force urging them toward 

 the left and the bottom ones have a force urging them 

 toward the right. Hence the rotor develops a turning 

 effort which tends to make it revolve in the same direction 

 as that in which the magnetic field is turning; in other words, 

 if the rotor can revolve it follows the magnetic field. 



94. Rotor Speed. We have just shown that the rotor 

 is urged in the same direction as the magnetic field and now 

 the question arises, how fast will the rotor turn? Suppose, 



FIG. 222. A Revolving Magnetic Field Must Cut the Rotor Con- 

 ductors, unless the Rotor is Turning as Fast as the Field. 



first, that there is no load on the motor and that the friction 

 is negligible so that the rotor is perfectly free to revolve. 

 As long as the rotor is turning at a speed less than that 

 of the magnetic field, e.m.fs. are induced in its conductors, 

 currents flow in the conductors, and a torque is developed 

 which tends to make the rotor " catch up " with the field. 

 The Rotor will not Revolve Faster than the Field. 

 When the rotor has caught up with the field so that field 

 and rotor are turning at the same speed there is no cutting 

 of the magnetic field by the rotor conductors, and hence no 

 currents flow in the rotor circuits. But when there is no 

 current in the rotor there can be no torque developed, 



