[OTORS OPERATING SYNCHRONOUSLY WITHOUT D. C. 145 



This power is necessarily very low, since ^ is never more than a 

 iction of the mean self-induction /', and since the E.M.F. which is 



itilized, , is itself only a small fraction of the E.M.F. of self-induction 



the alternator, which latter is always lower than the E.M.F. that 

 rould be generated if the alternator were excited by its fields. It 

 :an be easily seen that the power-factor always remains very low. 

 [oreover, this motor has the same disadvantage as polyphase reaction- 

 lotors. For this reason, this method of using alternators does not 

 ippear susceptible of practical application. 



Synchronous Motor with Alternating Fields. The invention and 

 the theory of a synchronous motor with alternating fields are due to 

 Galileo Ferraris (see Memoir presented to the Academy of Sciences of 

 Turin, Dec. 1893, and also Industrie Electrique, 10 Juin, 1894). In 

 this apparatus the fields and armature are excited in series or in shunt 

 by alternating currents taken from the same source, and the armature 

 is previously brought to a speed double that which would be suitable 

 for a synchronous motor with constant fields. The motor then runs 

 synchronously, and it can be lightly loaded. 



The operation of such a motor is easily explained by the aid 

 of the theorem of Leblanc. Let zp equal the number of poles and 

 0} the speed of pulsation of the currents. The alternating field 

 (the field winding being supposed stationary) can be expressed ficti- 

 tiously by two fields, HI, H 2 , one rotating to the right, the other to the 



0} (JL) 2CO . 



left, with the contrary speeds + and . A speed equal to is 



P P P 



given to the armature in one direction ; the alternating field which the 

 armature produces is itself also equivalent to two revolving fields 



having the velocities H and - - relatively to the armature. The 



P P 



corresponding absolute speeds will be 



First field, MI, += tQ the rf ht 



P P P 



Second field, M 2 , = to the right. 



P P P 



The field MI will have no action on the fields HI and H 2 , whose 

 speeds are either different or contrary. Likewise, M 2 will have no 



