306 SYNCHRONOUS MACHINES. [Exp. 



current which flows in the armature of a synchronous motor is 

 equal to this resultant divided by the synchronous impedance Z 

 of the armature; or, 



This current lags behind Ez by an angle Oz whose tangent is equal 

 to the synchronous reactance of the armature divided by the arma- 

 ture resistance. 



5. Synchronous Impedance. The synchronous impedance of 

 the armature is made up of two components : true ohmic resistance 

 and synchronous reactance, in quadrature. As in a generator, 

 synchronous reactance includes local reactance due to inductance 

 (as in any circuit) and the effect of armature reaction in strength- 

 ening or weakening the field, the two effects being so similar that 

 for most practical purposes they may be considered as one. The 

 electromotive force method of treating the synchronous motor, 

 based upon the conception of synchronous reactance and syn- 

 chronous impedance, is more fully discussed in Exp. IO-B, which 

 can be read to advantage in connection with the present experi- 

 ment. 



6. Armature Reaction. The physical effect of armature reac- 

 tion in a generator has already been discussed ( 1-15, 44-49, 

 Exp. 3~B) ; a lagging current weakens the field and a leading 

 current strengthens it. When a generator furnishes current to 

 a synchronous motor, a current which is lagging with respect to 

 the electromotive force of the generator is leading with respect 

 to the counter-electromotive force of the motor, and vice versa. 

 In the motor, therefore, a lagging current strengthens the field 

 and a leading current weakens it, the former being leading and 

 the latter lagging with respect to the counter-electromotive force 

 of the motor. 



An under-excited motor takes a lagging current ; this strength- 

 ens the field and increases the motor electromotive force until it 

 just balances the line electromotive force. On the other hand, 



