THE INDUCTION MOTOR. 271 



second, and the voltage between collector rings is 528 volts 

 [= (n + n')ln x 220 volts]. Suppose that the rotor delivers a 

 total electrical output of 100 kilowatts, then, since electrical 

 output of rotor : electrical intake of stator : mechanical intake 

 of rotor as n + n f : n : n f , it follows that 41^3 kilowatts of 

 power is delivered directly to the stator windings from the 25 

 cycle mains, and that 58^ kilowatts of mechanical power is de- 

 livered to the rotor by the motor which drives it. 



131, The induction generator. When the rotor of an induc- 

 tion motor is driven at a speed n' which is greater than the speed 

 n of the stator magnetism, a torque T is developed which opposes 

 the motion of the rotor, so that mechanical power amounting to 

 27rn r T must be used to drive the rotor. A portion of this me- 

 chanical power appears as electrical power developed in the rotor 

 windings, and the remainder 2irnT is delivered to the stator, 

 where, ignoring losses, it is delivered to the alternating-current 

 supply mains in the form of alternating-current output from the 

 stator windings. When used in this way the induction motor is 

 called an induction generator. To operate as a generator the in- 

 duction motor must be connected in parallel with an ordinary 

 alternator. 



132. The starting of the polyphase synchronous motor (or con- 

 verter) by inherent induction motor action. The windings of a 

 polyphase alternator armature are like the stator windings of an 

 induction motor, and the polyphase current in such an armature 

 produces a rotating state of magnetism in the armature core 

 exactly as the stator windings of an induction motor produce a 

 rotating state of magnetism in the stator iron. 



When an alternator is in operation either as a generator or as a 

 synchronous motor or converter, the magnetism due to the arma- 

 ture currents rotates with reference to the armature structure in 

 a direction opposite to that in which the armature rotates,* and at 

 the same speed as the armature. Therefore the armature mag- 



* This statement is made to refer to the machine with stationary field and rotating 

 armature for the sake of simplicity. 



