264 INDUCTION MOTORS. [Exp. 



under opposite poles will flow in opposite directions so that the 

 conductors under both poles will tend to give rotation in the same 

 direction. 



After a certain speed is reached, a centrifugal device is com- 

 monly used to lift the brushes, thus saving friction and wear, and 

 to short-circuit the rotor windings so that the motor runs as an 

 induction motor. It then has the characteristics of an induction 

 motor and not the characteristics of a repulsion motor, which are 

 similar to those of a series motor. 



19. Phase Splitters. A single-phase motor may be made to 

 start as a 2-phase or 3-phase motor by means of polyphase cur- 

 rents temporarily derived from a single-phase circuit. Polyphase 

 currents sufficient for this purpose can be obtained from a single- 

 phase line by various arrangements of resistances and reactances ; 

 such devices, termed phase-splitters, are in common use and will 

 be discussed later (26-32). Polyphase currents so derived can 

 never be balanced (1-3, Exp. 7~A) and are only used for 

 starting. 



PART II. PRELIMINARY STUDY. 



20. Structure and Rating. Study the general structure and 

 windings* of the motor ; note the rated full-load speed and output 

 and the frequency, voltage and kind of circuit (single-phase, 

 2-phase 3-wire, 2-phase 4-wire, 3-phase, etc.) for which the motor 

 is designed. 



21. Compute the watts input and the current per line at full 

 load, assuming a certain efficiency (say 80 per cent.) and a certain 

 power factor (say 80 per cent.). For a polyphase motor, com- 



*(2oa). A detailed study of the windings may be made when the 

 necessary data are obtainable. The study may include a diagram of 

 winding, and the following data : number of poles or groups of coils per 

 phase; number of coils per group and turns per coil; size wire; current 

 density for full-load current; total resistance; length of wire (per coil 

 and total) computed from dimensions and from resistance measurement; 

 type of secondary ; number and size of conductors ; number of slots, etc. 



