Tin-: SYNCHRONOUS MoToR 315 



as an induction motor, as has already been pointed out. That 

 is. if sufficient excitation is not supplied l>y the direct current in 

 the field winding, the motor will take lagging exciting current 

 from the alternating-current line to make up the deficit. 



The power-factor of the induction motor for a given load 

 cannot be altered without changing the motor design and the 

 ordinary induction motor always takes a lagging current. The 

 power-factor of the synchronous motor can be altered at will, 

 and the current can be changed from lagging to leading by 

 simply changing the field excitation. 



128. Synchronous Motor V-curves. If the power, P, de- 

 livered to a three-phase synchronous motor be kept constant and 

 the field current // varied, the power-factor of the motor will 

 change. The power for a 3-phase motor is 



p = -/3FJ cos 6 



where V is the terminal voltage, / the line current and cos the 

 power-factor of the motor. As both P and V are constant, any 

 decrease in the power-factor (cos 0) must be accompanied by a 



corresponding increase in the current /. Likewise, any increase 

 in the power-factor must be accompanied by a decrease in the 

 current /. 



Therefore,a change in the field current at constant load changes 

 the line or armature current 7. In order to determine the rela- 

 tion between the field current and the armature current and also 

 the characteristics of a synchronous motor as regards its ability 

 to correct the power-factor of a system, the so-called V-curves 

 of the motor are obtained. These Y-curves show the relation 

 which exists between the armature cm-rent and the field current 

 for different constant-power inputs. Several curves are usually 

 obtained, each curve representing a constant value of power 

 input. 



The connections for making such a test an 4 shown in Fig. 202. 



field current i< varied by means of the field rheostat. For 



each value of field current , as read on the direct -current ammeter, 



ding value of the alternating line current is noted. 



The electrical power delivered to the motor i- kep Mf by 



adju-ting the load applied to the motor shaft. A polyphase 

 able for thi> experiment, afl it eliminates the 



