62 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



and at what power factor will it be operating? The excitation 

 is unchanged. 



29. The rotational losses of a 1340-h.p., 2400-volt, 3-phase 

 synchronous motor are 43 h.p., and may be assumed constant. 

 The armature windings, which are connected in Y, have an 

 effective resistance of 0.067 ohm and a synchronous reactance 

 of 2.64 ohms per phase. 



(a) What is the least power factor at which the motor can be 

 operated at no load so that the current will not exceed 135 per 

 cent, of its full-load value? If the motor is overexcited what is 

 the excitation voltage? 



30. When operating at unit power factor the full-load losses 

 of a 1100-h.p., 5000-volt, 3-phase synchronous motor are: 

 Armature copper loss=13.9 kw: Field copper loss=18.6 kw: 

 Rotational losses = 27. 8 kw. Assume that the rotational losses 

 are constant. The armature windings, which are connected in 

 Y, have a synchronous impedance of 15.4 ohms per phase. 



If the current is limited to 130 per cent, of its full-load value 

 what is the least power factor at which this motor can be oper- 

 ated when it is delivering full load? What is the necessary 

 excitation voltage if the motor is overexcited? 



31. The synchronous motor described in problem 14 is rated 

 to deliver 6500 h.p. when operating at unit power factor. If 

 the maximum allowable current is 130 per cent, of its full-load 

 value what is the least power factor at which it can operate when 

 delivering its rated load? What is the greatest allowable value 

 of the field current at full load under this condition? 



32. A 20-h.p., 230-volt, 3-phase synchronous motor has an 

 effective resistance of 0.302 ohm and a synchronous reactance of 

 4.36 ohms per phase. The armature windings are connected 

 in A. The rotational losses are 750 watts and may be assumed 

 constant. 



With the maximum excitation voltage of 315 volts at what 

 load will this motor break down? Compare the current at 

 breakdown with the full-load current for normal excitation. 



33. If the ratio of resistance to synchronous reactance is 

 increased to 0.30 by inserting equal resistances in series with 

 each phase of the synchronous motor what will be the results 

 called for (a) in problem 25; (b) in problem 26; (c) in problem 

 27; (d) in problem 29; (e) in problem 30; (f) in problem 32. 



34. A 3-phase synchronous motor receives line currents of 



