CHAPTER IX 



68. Alternator used as motor. Synchronism 69. Stability of synchronous motor 

 70. Magnitude and phase of current for various conditions of load. Overload 

 capacity 71. V curves of synchronous motor 72. Condenser action of over- 

 excited synchronous motor. Use of synchronous motor as compensator 73. 

 Hunting of synchronous motor 74. Prevention of hunting 75. Starting of 

 synchronous motor 78. Paralleling or synchronization of alternators 77. 

 Everett-Edgcumbe rotary synchronizer 78. Siemens and Halske three-phase 

 synchronizer 79. Parallel running of alternators. Starting of new machines 



68. Alternator used as Motor. Synchronism 



A SINGLE-PHASE alternator is, like a continuous-current dynamo, a 

 reversible machine i.e. it is capable of being driven as a motor when 

 supplied with alternating currents. The possibility of using a single- 

 phase alternator as a motor is immediately obvious. For, during the 

 rotation, each armature conductor comes alternately under cover of 

 poles of north and south polarity. If, then, we send current impulses 

 through the armature winding so timed that they always give rise 

 to a driving torque, a series of impulses will be communicated to the 

 rotor, and the effect will be the same as that of a steady driving 

 torque whose value is equal to the mean value of the fluctuating 

 torque due to the current impulses. Since these current impulses 

 must obviously alternate in direction, a reversal of current taking 

 place in an armature conductor as it passes from a field of one 

 polarity into a field of opposite polarity, it is evident that they will 

 constitute an alternating current. We thus see the possibility of 

 communicating a driving torque to the machine by sending an alter- 

 nating current of suitable frequency through its armature. The 

 frequency of this current must obviously be the same as that of the 

 e.m.f. generated in the armature coils. Hence the frequency of 



the alternating current which drives the machine is equal to P X , 



where P = number of pairs of poles and m = revs, per min. The 

 corresponding speed of the alternator is known as the synchronous 

 speed, or speed of synchronism, and it is evident that an alternator 

 is only capable of running as a motor at this particular speed. 



A polyphase alternator may also be used as a motor. This is 



