CHAPTER VIII. 

 INDUCTION MOTORS. 



EXPERIMENT 8- A. Preliminary Study of an Induction 

 Motor and the Determination of its Performance by Loading. 



PART I. INTRODUCTORY. 



I. Use. The induction motor is the form of alternating cur- 

 rent motor in most general use. It is practically a constant speed 

 motor, the speed at full load being a few per cent, less than the 

 speed at no load, and in this respect it resembles the shunt motor. 

 On account of its simplicity and nearly constant speed, it is well 

 adapted for operating machinery, but is not so well adapted for 

 use in traction,* or similar service, where there is frequent accel- 

 eration. (For variable speed motors, see 59.) Induction motors 

 may be single-phase or polyphase, large motors being generally 

 3-phase or 2-phase. 



2. Structure. An induction motor consists of two members, 

 a stationary member, or stator, and a revolving member, or 

 rotor, corresponding in a way to the field and armature of a 

 direct current motor. (The terms field and armature should not 

 be used, however, in referring to an induction motor, since they 

 are inexact and confusing.) 



*(ia). For traction purposes the induction motor is best suited to 

 service permitting long runs at uniform speed. The first such use in 

 America, at the Cascade Tunnel of the Great Northern Railway Company, 

 is described in a paper by C. T. Hutchinson read before A. I. E. E., Nov. 

 12, 1909 (see Transactions, Vol. XXVIII., p. 1281), in which the follow- 

 ing advantages of the 3-phase induction motor are stated : simplicity, 

 greater output for given space, uniform torque, constant speed, impossi- 

 bility of excessive speed (either on down grade or when the wheels slip), 

 regeneration on down grades, impossibility of excessive current, possl 

 bility of using 2.5 cycles. (See also discussion following the paper.) If 

 Europe the 3-phase induction motor for traction is not uncommon. 

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