Tin-: i.\i>rcno\ MOTOR 259 



number of poles which determine the speed of the induction 

 motor. In order to change the speed, it is necessary to change 

 at least one of these factors. 



Changing the Slip. The slip may be changed by introducing 

 ince into the rotor circuit. This has already been dis- 

 cussed in connection with the wound-rotor type of motor. At 

 a iri\ '< -n slip, any value of torque up to the break-down torque may 

 be obtained by this method. Its disadvantages are lowered 

 efficiency and poor speed regulation. 



These disadvantages may be avoided by introducing counter 

 (mi-, instead of resistance into the rotor circuit, either at line 

 frequency, which requires that the rotor have a commutator, or 

 by means of an auxiliary commutating machine which introduces 

 counter emfs. at rotor frequency through slip-rings. This last 

 method necessitates the use of a commutating type of machine 

 which produces emfs. at rotor, or slip frequency. It must 

 therefore be excited by the rotor currents themselves. The 

 S her hi i is 1 method of speed control is the most common example 

 of this counter-electromotive force method. 



When a current Hows against a counter-electromotive force in 

 a rotat ing machine, mechanical power is developed. This occurs, 

 for example, when the current Hows against the counter-electro- 

 motive force in a direct-current motor. The countor-electro- 

 motive force machine accordingly develops mechanical jxnver, 

 which is available for various purposes. Unlike the volt air- 

 drop in a resistance, the counter-electromotive force is practi- 

 cally independent of the current (see "Counter-electromotive 

 ( Jells," Vol. I, page 401). Tin T< -fore, a motor employing 

 tin- method of speed control has good speed regulation and 

 efficiency. 



..' ifi shows t he conned ion- employed in ohtaininir speed 



control hy the i'on^< ,j M jr method. A is the main induction motor 



havinir a slip-rim: rotor. H is a three-phase commutator inotonm 



hunt -held windings F } , F 2 , and F 8 , spaced 120 



>peed ami I'mvrr-fartor ( 'nntrnl <>f Lar^c Induction V 

 by Neutrah/rd I'nl\ pha>c Alternating-current ( 'omniutator Machine-,' 



'... I. Hull. Juu'r,,nl ,,>!>,, .1 / / I 



Also. i.f Obtaining A<lju-tal>le SjM-rd with Klert rirally 



Driven Hollini: Mill-." l.y K. A. I'auly. General Electric Review, May. L921, 

 page 422. 



