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ELECTRICAL ENGINEERING 



167. Phase Characteristics. If the field excitation of a motor 

 with constant output is varied, the armature current changes both 

 its value and its phase relation with the impressed e.m.f. For 

 each output there is a certain value of field excitation which makes 

 the current a minimum and brings it in phase with the e.m.f. ; as 

 the excitation is decreased below this value the current increases 

 and becomes lagging; as the excitation is increased the current 

 increases and becomes leading. 



In Fig. 256 are shown the phase characteristics for outputs, 

 P 2 = or at no load, P 2 = half load, P 2 = full load and P 2 = 150 

 per cent load. 



10 20 80 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 

 Amperes Exciting Current 



FIG. 256. Phase characteristics or "V" curves of a synchronous motor. 



For each curve the output P 2 = El cos < Pr constant 

 losses is kept constant; thus, 



+ 7V + constant losses 



cos <> = 



El 



(268) 



and EQ = V(E cos <j> - /r) 2 + (E sin - 7r ) 2 . 



As I varies the corresponding value of cos < is found from equation 

 268 and by substituting J, cos <f> and sin <f> in equation 264, the 

 values of E are found. These are replaced by the corresponding 

 values of field current // obtained from the no-load saturation 

 curve. 



The lowest point on each curve represents the smallest current 

 input for the given output and thus represents the condition of 

 unity power factor. The curve joining these lowest points is the 



