PARALLEL RUNNING 



149 



ascertain whether the speed is too low or too high. In order to 

 make this observation as easy as possible, the lamps are in practice 

 mounted at the corners of an equilateral triangle, as shown in Fig. 

 111. The light will appear to travel round the triangle, counter- 

 clockwise if the speed is too low, clockwise if it is too high. 



In addition to the lamps, two voltmeters are generally provided : 

 one for adjusting the e.m.f. of the incoming machine to the required 

 value, and the other for ascertaining the exact instant of phase 

 opposition with greater accuracy than is afforded by the darkening of 

 the top lamp. 



79. Parallel Running of Alternators. Starting 

 of New Machines 



Let us suppose that an alternator has been duly synchronized and 

 switched into parallel with other machines. By increasing the 

 supply of power to the engine driving the alter- 

 nator, it may be made to take part of the load. 

 The phase relation of the alternator e.m.f. to 

 the current and the 'bus bar p.d. is determined 

 by (1) the mechanical power supplied to the 

 alternator, and (2) the excitation. 



In Fig. 112 is given a vector diagram 

 similar to that of Fig. 97 showing the connec- 

 tion between the e.m.f., p.d., and current, and 

 their phase relations. 0V, as before, denotes 

 the p.d., V ; OE the e.m.f., E ; OR the impe- 

 dance drop in the alternator armature, and 

 01 the current, which lags behind OR by an 



T 



angle tan- 1 - . The total electrical power 



developed by the alternator is given by E x 01", 

 while the useful power transmitted to the 'bus 

 bars is V X 01'. 



By assuming different values for the angle 6, 

 and supposing E to be constant, we can find the 

 relation connecting 6 with the useful or the 

 total electrical power. This relation may be 

 plotted in the form of a curve. Corresponding 

 to each value of the exciting current, or to 

 each value of E, we can find such a power curve. By this means a 

 series of power curves similar to those shown in Fig. 99 for a syn- 

 chronous motor may be obtained. From these curves we see that, 

 within the limits of stability, an increase of S results in an increase of 



Fio. 112. Vector Dia- 

 gram of Parallel- 

 oooncoted Alternator. 



