320 ELECTRICAL MACHINERY 



is varied. There are likely to be upper harmonics in the 

 current wave form at light loads and when such is the case 

 the watt meter reading is never equal to the products of 

 volts X amperes, and hence the power factor, as calculated 



from cos <]> = T cannot be equal to unity. This is 



shown in the power factor curves of Fig. 205. 



83. Effect of Changing Load and Excitation. We have 

 just seen that the overexcited synchronous motor takes a 

 leading current from its supply line and an underexcited 

 motor a lagging current. And of course the motor always 

 takes current in phase with its supply voltage sufficient to 

 supply the power required by the motor to overcome its 

 losses and to furnish power to its load. 



Fig. 208 shows the possible current and voltage relations 

 in an overexcited synchronous motor. The line voltage 

 is given by the vector OE and the motor current, for certain 

 conditions, by the vector 01. Now this current 01 may 

 be regarded as made up of two components, OA, in phase 

 with OE, and OB, leading the voltage OE by 90. The 

 product OEXOA represents the electrical power required 

 by the motor to supply its load and losses. 



Increase of Load with Fixed Excitation. Now suppose 

 that the load on the motor is increased and its excitation 

 left at its previous value. The reactive component, OB, 

 will remain as before but the product OEXOA must increase 

 to supply the increased load. As OE is assumed constant, 

 OA must increase to some value OA' and the total motor 

 current is therefore given by 01' . The effect of this 

 increased load is evidently to increase the power factor of 

 the motor, as cos ' is greater than cos . 



Fixed Load and Increased Excitation. Now suppose the 

 load on the motor had remained at its first value but the 

 field excitation had been increased; the active component 

 OA would remain constant but the reactive component 

 OB would have been increased from OB to OB'. The motor 



