SYNCHRONOUS MOTORS 59 



At what power factor is this motor operating? What is the 

 excitation voltage (a) if the excitation is greater than normal, 

 (b) if the excitation is less than normal ? 



18. A 5000-h.p. synchronous motor is operated from a 2200- 

 volt, 2-phase circuit. The effective resistance of the armature 

 is 0.0254 ohm per phase. The field winding has 60 turns per pole 

 and a total resistance of 0.37 ohm. 



At no load this motor takes 67.5 kw. at 2200 volts when the 

 excitation is adjusted so that it operates at 0.86 power factor. 

 How much must the excitation be increased in order that the 

 motor will deliver its rated load and operate at this same power 

 factor? Use the magnetomotive-force method for calculating 

 the field current. 



19. A 3-phase synchronous motor, whose armature windings 

 are connected in A, has an effective resistance of 0.302 ohm and 

 a synchronous reactance of 4.36 ohms per phase. This motor 

 receives a line current of 40 amperes at 230 volts and the field 

 current is adjusted so that the excitation voltage is 310 volts. 



What power does the motor receive? At what power factor 

 is it operating? 



20. A 2400-volt, 3-phase, synchronous motor has a full-load 

 capacity of 1340 h.p. The effective resistance of the armature 

 is 0.067 ohm per phase and the synchronous reactance is 2.64 

 ohms per phase. The armature windings are connected in V. 

 The resistance of the field circuit is 0.427 ohm. The rotational 

 losses at normal voltage are 2<> <> k\\ . 



This motor is operated from a 2400-volt circuit at 0.82 power 

 factor and is delivering 1220 h.p. What is the line current, 

 ami what is the necessary excitation voltage? Calculate the 

 excitation voltage for both under- and overexcitatiun. 



21. A 135-h.p. synchronous motor is operated from a 480-volt, 

 2-phase circuit. The effective resistance of the armatun ifl 

 0.135 ohm per phase at normal running tempi -ratim-. Tin- 

 rotational losses at normal voltage are 5.7 kw. 



