the Synchronous Motor. 199 



This explains the condenser action of an over-excited 

 synchronous motor noticed by Professor S. P. Thompson and 

 others. 



20. It is known that a leading current strengthens the 

 field of a generator and weakens that of a motor, while with 

 a lagging current the reverse is the case. We therefore con- 

 clude that when the excitation of the motor field is small, 

 armature reaction weakens the fields of both generator and 

 motor, and when the motor is over-excited both machines 

 have their fields strengthened. When working at minimum 

 current, armature reaction strengthens the motor field and 

 does not affect the field of the generator. 



When the motor field is unaffected, the generator field is 

 weakened. 



21. Now the field of the motor is, under ordinary working 

 conditions, excited to a somewhat greater extent than is 

 required to obtain minimum current ; for, though the c 2 R 

 losses are a minimum and the efficiency a maximum when the 

 current is a minimum, it is advisable to increase the counter 

 E.M.F. to a certain extent in order to cope with accidental 

 variations of the load. Under ordinary working conditions, 

 therefore, the effect of armature reaction is to strengthen the 

 field of the motor and also of the generator, but to a less 

 extent. 



22. We now proceed to obtain an expression for the altera- 

 tion, in ampere turns, of the field excitation due to armature 

 reaction. 



Let cj) be the displacement of phase of the current over 

 the E.M.F. of the machine ; n the number of turns of wire 

 in one section of the armature; i the virtual current, and 

 i sin pt the instantaneous value of the current, so that i 

 is its maximum value. 



It has hitherto been customary to assume that the alteration 

 in the excitation is given by the expression 



in<f> (10) 



where <f> is expressed in circular measure. 



Now it is not the virtual current to which the armature 

 reaction is due, but the mean value of the current through an 

 angle <£ on each side of its maximum value ; that is, the 

 proper value of the current is given by 



T p r^ } p . . 



±=£t\ , i ft smpt at 



'0 • i 



= -sin 9. 

 9 



