66 THE MAGNETIC CIRCUIT [ART. 27 



the stator and rotor windings. The formulae of this chapter also 

 apply without change to the synchronous motor, because the con- 

 st i action and the operation of the latter are identical with those of 

 an alternator; the only difference being that an alternator trans- 

 forms mechanical energy into electrical energy, while a synchro- 

 nous motor transforms energy in the reverse direction. In all 

 cases the induced voltage is understood and not the line voltage. 

 The latter may differ considerably from the former, due to the 

 impedance drop in the stator winding. 



Prob. 15. A delta-connected, 2300-v., 60-cycle, 128.5-r.p.m. alternator 

 is estimated to have a useful flux of about 3.9 megalines per pole. If the 

 machine has one slot per pole per phase how many conductors per slot are 

 needed? Ans. 8. 



Prob. 16. A 100,000-cycle alternator for wireless work has one conduc- 

 tor per pole and 600 poles. The rated voltage at no load is 110 v. What 

 is the flux per pole and the speed of the machine? 



Ans. 82.5 maxwells; 20,000 r.p.m. 



Prob. 17. It is desired to design a line of induction motors for a per- 

 ipheral speed of 50 met. per sec., the maximum density in the air-gap to be 

 about 6 kilolines per sq.cm. What will be the maximum voltage induced 

 per meter of active length of the stator conductors? Hint: Use formula 

 (27). Ans. 30 volt. 



Prob. 18. Formula (31) is deduced under the assumption that each 

 armature conductor is subjected to the " cutting " action of the whole 

 flux. In reality, almost the whole flux passes through the teeth between 

 the conductors, so that it may seem upon a superficial inspection that 

 little voltage could be induced in the conductors which are embedded in 

 slots. Show that such is not the case, and that the same average voltage 

 is induced in the conductors placed in completely closed slots, as in the 

 conductors placed on the surface of a smooth-body armature. Hint: 

 When the flux moves, the same amount of magnetic disturbance must 

 pass in the tangential direction through the slots as through the teeth. 



Prob. 19. Deduce eq. (31) directly from eq. (27). Can eq (31) be 

 derived under the case of the transformer action? 



27. The Breadth Factor. Armature conductors are usually 

 placed in more than one slot per pole per phase, for the following 

 reasons : 



(a) The distribution of the magnetic field is more uniform, 

 there being less bunching of the flux under the teeth ; 



(b) The induced e.m.f. has a better wave form; 



(c) The leakage reactance of the winding is reduced; 



(d) The same armature punching can be used for machines with 

 different numbers of poles and phases; 



