THE INDUCTION MOTOR. 269 



There are therefore two feasable methods for controlling the 

 speed of an induction motor which is supplied with alternating 

 currents at a given frequency, namely, (a) To arrange the stator 

 windings so that they may be changed quickly from, say, a four- 

 pole winding to an eight-pole winding by means of switches, and 

 () To bring out the rotor windings to collector rings, as shown 

 in Fig. 225 and short-circuit the rotor through external adjustable 

 resistances. 



The difficulty with the first method of speed control is that the 

 arrangement of the stator windings becomes very complicated if 

 more than two speeds are provided for ; and the difficulties with 

 the second method of speed control are (a) that the motor is very 

 inefficient at low speeds, and () that the speed varies greatly with 

 the load when the rotor resistance is great enough to give a low 

 speed with a given load. 



130. Use of the induction motor as a frequency changer. If 

 we consider that the effect of the stator windings of an induction 

 motor is to produce a rotating state of magnetism which acts 

 upon the rotor exactly as would a mechanically revolving field 

 magnet, it is evident that the electromotive forces in the rotor 

 windings are zero in value and zero in frequency when the rotor 

 speed is equal to the speed of the stator magnetism, that the 

 rotor electromotive forces increase in value and in frequency as 

 the rotor speed decreases, that when the rotor stands still the 

 frequency of the rotor electromotive forces is the same as the 

 frequency of the alternating currents supplied to the stator wind- 

 ings, and that the rotor electromotive forces are still further in- 

 creased both in value and in frequency if the rotor is driven 

 backwards by an external source of power. 



Under the ordinary operating conditions of an induction motor 

 the rotor speed n f is nearly equal to the speed n of the sta- 

 tor magnetism. In this case the rotor electromotive forces are 

 small, their frequency is very low, and the amount of electrical 

 power developed in the rotor windings is but a small fraction 

 [( ;*')/'] of the mechanical power developed in the rotor. 



