GENERAL PRINCIPLES OF SYNCHRONOUS MOTORS 9 



An over-excited synchronous motor, connected to the terminals of an 

 alternator having excessive armature-reaction, can even replace the 

 excitation of the latter. It is observed, indeed, that on suppressing 

 this excitation, the generator continues to run the motor, and furnishes 

 the normal voltage at its terminals; but it can develop only little power. 

 An over-excited motor thus produces an indirect self -excitation which is 

 equivalent to that obtainable from a condenser. There is, in other re- 

 spects, a complete analogy of effects between the two forms of apparatus. 



These experimental results are much too complex to be studied 

 more in detail here. They can be discussed more satisfactorily later, 

 in connection with the theory of these motors and their applications. 



Elementary Explanation of Polyphase Synchronous Motors. If 

 we turn our attention, first, to polyphase synchronous motors, the 

 explanation of the phenomena just described is made easy by the con- 

 sideration of revolving magnetic fields. 



For the sake of brevity we will adopt the terms " rotor " and " stator " 

 to designate the movable and fixed portions, respectively, of the motor, 

 in accordance with the terminology of Professor S. P. Thompson. 



Let us take, as an example, a motor having two pairs of pole-pieces, 

 in which the inductive circuit is of movable form (rotor) and the 

 induced is circuit of stationary form (stator). 



In the ordinary form of polyphase alternators, the rotor will 

 consist of a crown of iron cores, with protruding poles excited by 

 coils receiving direct current from a separate exciter. The stator, 

 on the other hand, will consist of a circular core of laminated iron 

 having some induced windings disposed in notches or slots in such 

 a manner that the wires in the successive slots shall have alternating 

 currents of different phase passing through them. If we suppose, 

 for example, that we have a winding for four poles and for six phases 

 (three slots per pole) such as is shown in Fig. 7, the wires in the six 

 slots which cover two poles of the stator, as we follow along the periphery 

 of the latter, will have, passing through them, six currents * which are 

 out of phase with respect to each other by of a period, and which can 

 be represented by the equations 



i\ = /o sin a>t ; 



/ 27T 



i2 = Io sin cut 



\ 6 



1 In reality, the six phases are supplied by three-phase currents only; the 

 windings which are of exactly opposite phases being connected in series, with 

 reversed connections. 



