484 ALTERNATING CURRENT* 



87. A 25-cycle, 25-kv-a., 550-volt, single-phase alternator has an effective- 

 armature resistance of 0.20 ohm and a synchronous impedance of 1.8 ohms. 

 What is its synchronous reactance and what is the regulation of this alter- 

 nator at unity power-factor and at 0.7 power-factor, lagging current? 



88. A 2,000-kv-a., 3-phase, 2,300-volt, 60-cycle alternator has an effective 

 armature resistance of 0.0354 ohm per phase and a synchronous reactance 

 of 0.62 ohm per phase. The machine is Y-connected. Determine its 

 regulation at unity power-factor and at 0.8 power-factor, lagging current. 



89. Following are the constants of a 50-kv-a., 220-volt, three-phase alter- 

 nator: 



Average ohmic resistance between terminals, 0.0233 ohm. 



Field current adjusted to 6.2 am])., open circuit terminal volts = 125 

 volts. 



Field current adjusted to 6.2 amp., generator short-circuited, three line 

 currents each equal to 164 amp. 



Ratio of effective to ohmic resistance = 1.5. 



Calculate the regulation of the machine when the power-factor is 0.8 

 lagging and 0.8 leading current. Assume that the machine is Y-connected. 



90. Repeat problem 89, assuming that the machine is delta-connected. 



91. Below are given open-circuit and short-circuit data of a 200-kv-a. 

 500-volt, 60-cycle, three-phase alternator.- The machine is Y-connected 

 and has an ohmic resistance of 0.012 ohm per coil. The ratio of effective 

 to ohmic resistance is 1.5. Find the regulation of the machine by the synchro- 

 nous impedance method at 0.7 power-factor for both leading and lagging 

 current. 



OPEN-CIRCUIT SHORT-CIRCUIT 



FIELD CURRENT TERMINAL ARMATUF.E 



VOLTS CURRENT 



10 245 300 



15 365 450 



20 460 



25 525 



35 623 



40 665 



50 725 



92. Determine the regulation of the alternator of problem 91 by the 

 magnetomotive force method, at unity and 0.7 power-factor for both leading 

 and Jagging current. 



93. A 1,000 kv-a., three-phase, 2,200-volt, 60-cycle alternator is tested 

 for its regulation by the A. I. E. E. method. A load of practically /ero- 

 power-factor lagging current is applied, this load being adjusted until the 

 rated current of the machine is flowing. The field current is then adjusted 

 until the machine terminal voltage is 2,200 volts. Under these conditions 

 the field current is 250 amp. When the field current is 250 amp. on open 

 circuit the terminal voltage is 2,700 volts. The machine is delta-connected 

 and has an effective resistance of 0.3 ohm per coil. Find the synchronous 



