SYNCHRONOUS MOTOKS. 127 



On starting such a motor, the armature current is not switched 

 on until the speed corresponding to synchronism is attained by 

 some other means. 



82. Any piece of apparatus which is used to indicate when 

 synchronism is attained is called a Synchronizer. 



Let M (Fig. 36) be the motor, and D the distant alternator ; 

 let V be a hot-wire or electrostatic voltmeter, and, when working 

 at high voltages, let R be a high resistance ; and let the connec- 

 tions be as represented in the 



diagram before the alternating / > v / 



current is sent round the motor 

 armature. When the motor 

 and generator are not in syn- 

 chronism, the P.D. between 

 the terminals of the voltmeter 

 will oscillate with a rapidity 

 equal to the difference of the frequencies of the E.M.F.s of 

 generator and motor ; and the voltmeter needle will oscillate 

 accordingly. The oscillations will become slower and slower 

 as the speeds approach synchronism ; and when synchronism 

 is exactly attained, the needle will be steady, and will have a 

 minimum deflection. Incandescent lamps, or a gold-leaf electro- 

 scope, make good synchronizers, and might replace the voltmeter. 

 The above method of obtaining synchronism is obviously applic- 

 able to the general case of any two alternating-current machines, 

 whether both generators, or one a generator and the other a motor. 



83. We now pass on to the following theory of the synchro- 

 nous motor, due to the author. 1 



We consider the case of an alternating- current machine whose 

 field is separately excited by a direct current, while a simple 

 alternating current passes round the armature. 



Let w = output of motor in watts. 



i = E.M.S. value of armature current. 



r = resistance of armature. 

 E = E.M.S. P.D. impressed between motor terminals. 



e = E.M.S. Counter E.M.F. of motor. 

 L = coefficient of self-induction of armature. 



n = frequency of armature current. 



1 Philosophical Magazine and Electrical Review, vol. 30, Nos. 905, 913; vol. 37, 

 Nos. 919, 924, 926, 929, 934, 938 ; and vol. 38, Nos. 949, 974. 



