60 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



Terminal voltage on open Armature current on 

 field current . . 



circuit short circuit 



With the greatest allowable excitation the motor takes a 

 current of 135 amperes when delivering its rated load. At what 

 power factor does it then operate? What is the field current? 

 Use what you consider the best method for calculating the field 

 current. 



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

 connected in Y, has an effective resistance of 0.172 ohm and a 

 synchronous reactance of 2.12 ohms per phase. This motor 

 receives 750 kw. at a line potential difference of 2200 volts, 

 and the field current is adjusted so that the excitation voltage 

 is 2800 volts. 



What is the line current? At what power factor is the motor 

 operating? 



23. A 2200-volt, 2-phase synchronous motor has a full-load 

 capacity of 5000 h.p. The full-load efficiency of the armature, 

 with the field current adjusted for unit power factor, is 96.1 per 

 cent. The armature winding has an effective resistance of 

 0.025 ohm and a synchronous reactance of 1.12 ohms per phase. 

 The motor receives a constant power of 3000 kw. at 2200 volts. 

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

 what is the greatest allowable excitation voltage? At what 

 power factor would the motor be operating? 



24. A 3-phase, 13,500-volt synchronous motor is rated to 

 deliver 2200 h.p. when operating at unit power factor. The 

 armature has an effective resistance of 1.52 ohms per phase. 

 The armature windings are connected in Y. 



The rotational losses at normal voltage are 68 h.p. 



If the maximum allowable current is 125 per cent, of the full- 



