134 ALTERNATING CURRENTS 



shown in Fig. 101. On account of their shape, these curves of a 

 synchronous motor are frequently spoken of as its V-curves. The 

 dotted branch represents conditions of instability. 



72. Condenser Action of Over-excited Synchro= 

 nous Motor. Use of Synchronous Motor as 

 Compensator 



A reference to the vector diagram of Fig. 97 shows at once that 

 when and are known, we can find the angle VOI = ^, since 

 ^ = TT (0 + 0). Now, i// is the angle by which the armature 

 current lags behind the p.d., so that cos ^ gives us the power factor. 

 By once more making use of the power curves of Fig. 99, and equations 

 (1), (3) and (4) of 70, we can determine the relation connecting motor 

 e.m.f. with the angle i// for a constant value of the load. We then 

 find that for small values of the e.m.f., $ is positive i.e. the current 

 lags behind the p.d. ; as the excitation is increased, i decreases, 

 reaching a zero value (corresponding to a power factor of unity) 

 when the armature current is at its minimum, and beyond this 

 point it assumes a negative value, corresponding to a leading current. 

 A strongly excited synchronous motor thus behaves as if it possessed 

 capacity. 



The fact that by sufficiently increasing the excitation of a 

 synchronous motor it may be made to take a large leading current, 

 has been practically applied in a number of instances, notably in the 

 United States. In cases of power transmission over long distances, 

 the load has not unfrequently a comparatively low power factor, so 

 that a large lagging current has to be transmitted along the line. 

 Not only does this reduce the efficiency of transmission, but a more 

 important matter it causes a large drop along the line, rendering 

 satisfactory regulation very difficult. The regulation might be im- 

 proved by using larger generators at the generating station. But a 

 cheaper solution of the problem has in some cases been found by 

 installing a synchronous motor at the receiving end of the line, the 

 excitation of the motor being adjusted so that the leading current 

 which is taken by it exactly balances the lagging current taken by 

 the load. The wattless current is thereby entirely confined to the 

 local circuit formed by the synchronous motor and the load, while 

 the line current is in phase with the p.d. at the receiving end of the 

 line. The synchronous motor here plays the part of a compensator 

 for the wattless current of the load. 



