SYNCHRONOUS MOTORS 61 



load current over what range should it be possible to vary the 

 field current when the motor is delivering a constant load of 

 2000 h.p.? Use the magnetomotive-force method for calculat- 

 ing the field current. 



25. A 3-phase, 230- volt, 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. The 

 rotational losses are 750 watts and may be assumed constant. 

 The motor delivers a constant load of 20 h.p. What is the 

 least excitation voltage with which the motor will run? What 

 is the armature current at the instant of breakdown'.' 



26. A 1340-h.p., 3-phase synchronous motor receives a con- 

 stant power of 860 kw. from an 11,000-volt circuit. The arma- 

 ture windings, which are connected in Y, have an effective 

 resistance of 0.94 ohm and a synchronous reactance of 56 ohms 

 per phase. 



Over what range can the excitation voltage be varied so that 

 the current will not exceed 135 per cent, of its full-load value, 

 which is 62.5 amperes? 



27. A 3-phase, 5000-volt synchronous motor, whose armature 

 windings are connected in Y, has an effective resistance of 0.40 

 ohm and a synchronous reactance of 15.4 ohms per phase. The 

 rotational losses are 30 kw. and may be assumed constant. The 

 greatest excitation voltage that can be obtained is 3520 volts per 

 phase. 



When the motor is delivering its full load of 1100 h.p. over 

 what range can the power factor be varied? What is the arma- 

 ture current at each of the limiting conditions? Compare the>e 

 with the full-load armature current. 



28. NVglrrting the field copper loss, the efficiency of a 2200- 

 h.p.. 3-phase synchronous motor is 96.0 per cent, at full load 

 when the motor is operating at unit power factor. Assume that 

 the rotational losses are constant. The armature windings, 

 which are connected in Y, have an effective resistance of I..VJ 

 ohms and a synchronous reactance of 37. -I ohms per phase. 

 With the motnr delivering 'J(MM) h.p. tin- lidd current is adjusted 

 ><> that the motor takes a leading current of 85 amperes from a 

 13,500-volt constant potential circuit. 



(a) At what power factor is the motor operating? What is 



the -\rit;itl(>n Yolt.'iL 



If this load is thrown off what current will the motor take, 



