96 ELECTRICAL ENGINEERING PROBLEMS 



would be the maximum e.m.f. of the motor with which it could 

 run as a motor? (20 min.) 



23. Two 1100-volt alternators, A and B, each having a resist- 

 ance of 0.21 ohm and a reactance of 1.2 ohms are in parallel and 

 are supplying a circuit having a resistance of 9.79 and a reactance 

 of 3.8 ohms. Draw the parallelogram diagram for the following 

 cases obtaining the terminal e.m.f. in each case; (a) the e.m.f. 's 

 are equal and opposite; (b) owing to decrease in the driving power 

 machine B lags 2 degrees behind opposition to machine A. Find 

 the synchronizing current, the power it gives to B, the total 

 current on each machine, and the power given by each. Neglect 

 the synchronizing component in getting the terminal e.m.f. Note 

 that the currents, while combined in the outside circuit, are in 

 opposition in the local circuit. (25 min.} 



24. A 5500-volt, 1500-kv-a., three-phase, Y-connected alter- 

 nator, 83 r.p.m. 50-cycle, is to be used as a synchronous motor. 

 The resistance is 0.3 ohm per phase, that is to the neutral. The 

 reactance is 2.7 ohms per phase. Construct the parallelogram 

 diagram, and find the motor voltage necessary to give unity power 

 factor on a 5500-volt line, with an input of 1500 kw. into the 

 motor. Find also the current and power factor with an excitation 

 of 5500 volts. (30 min.) 



25. Check the results of problem 24 by the symbolic method. 

 (30 min.) 



26. Construct the circle diagram for the alternator of problem 

 24 and determine the maximum load it will carry when excited 

 to 5456 volts. (10 min.) 



