CHAPTER VI 



[ER MOTORS OPERATING SYNCHRONOUSLY WITHOUT DIRECT 

 CURRENT EXCITATION 



It is possible to devise other motors which, once brought to a suitable 

 speed, can continue to operate synchronously, without excitation 

 by a direct current or by a commutated pulsating current. These 

 motors, not being used industrially, constitute, properly speaking, 

 theoretical curiosities, but it is interesting, in order to give a general 

 idea of the whole subject, to indicate their principles. 



Mention can be made of two types of such motors; synchronous 

 motors without any excitation, known as " reaction-motors," and 

 synchronous motors with alternating-current excitation, or alternating- 

 field motors. 



Reaction Synchronous Motors. It is natural to suppose that the 

 constant armature-reaction-field of polyphase motors can be used 

 as an inducing field. It is sufficient, for this purpose, that the armature 

 should have a large number of windings to produce a large number 

 of ampere-turns, and that the current passing through it should be 

 much out of phase, in order that the magnetizing turns may coincide 

 with those of the field-poles. Moreover, the air-gaps should, obviously, 

 be very small, to reduce the reluctance of the inducing circuit. The 

 self-excitation of the latter is then only an exaggeration of the effect of 

 the lagging reactive current, which has already been mentioned several 

 times. The operation of such a motor is therefore quite possible, 

 even at light loads; but the inductance of the armature has been 

 increased at the same time as its reaction on the field; and the con- 

 sequence is that the stability and, moreover, the power-factor of the 

 current used, will be small, owing to the great lag of this current. Such 

 a motor would, therefore, be bad from a commercial point of view. 



The conditions of operation can be easily ascertained by making 

 use of the diagram on page 94, in which the E.M.F. due to external 

 excitation is supposed to be zero. 



The operation of motors on the reaction-principle is not limited 



141 



