QUESTIONS AND PROHl.K.MS 497 



21. Show by a vector diagram how it is possible to control the voltage 

 at some point on a system by means of a synchronous motor or synchronous 

 condenser. What condition is necessary in order that tin- voltage at the 

 motor be raised to a value higher than that of the rest of the system '.' What 

 degree of excitation is necessary in order that the voltage may be raised? 

 Sketch the connections of a motor, together with the necessary instruments 

 for making tests when the motor is used as a voltage-controlling de\ 



22. Why an- single-phase synchronous motors not common? 



23. What are the advantages of the polyphase synchronous motor over 

 the polyphase induction motor? What are its disadvantages? I'nder 

 what conditions should it be used? 



24. Describe two simple types of synchronous motor which are of very 

 small size. Upon what property of the magnetic circuit do they depend for 

 their operation? From where do they obtain their field excitation? For 

 what purposes are such motors used? 



PROBLEMS ON CHAPTER X 



129. A 100-kv-a., 600-volt, Y-connected, three-phase synchronous motor 

 has the following constants. 



Armature resistance per coil = 0.08 ohm. 

 Synchronous reactance per coil = 1.0 ohm. 



Determine the hack electromotive force of the motor for a current of 

 100 amp. 



(a) When the po\v< i-t.i< t..r of the motor is unity 

 (6) When the current leads the terminal volts by approximately 90. 

 (c) When the current lags the terminal volts by approximately 90. 

 >te: In (6) and (c) the resistance drop may le neglected.) 



130. The motor of problem 129 requires a field current of 30 amp. at 

 1 10 volts and the core and friction losses are 3,000 watts when the motor 

 is operating at unity power-factor at its rated load. What is its output in 

 hp. and what is its efficiency at this load? 



131. Figure 131 (A) shows four V-curves for a 200-kv-a., 600-volt, syn- 

 chronous motor. 



(a) Indicate the kilowatt input at which each curve was obtained. 

 (6) Draw lines through jxiints of unity power-factor and of 0.75 power- 

 factor, leading ami lagging current. 



132. Tin- -\ nehronoiif motor of problem l:: .-ted in parallel with 

 a load of \:A) kilowatts at 600 volts and <> po\\. 'egging current. 

 The motor runs light without load. To what vain.' mwl III evii-ition In- 

 adjusted in order to bring the system power-factor up to unity? (Use 

 lowe 



133. roblem 1 ih.-il the pOWei ight up to 

 many k\ ach case? 



134. \\ 1 .um JM.V. . .vhidi the system can have in 

 problem 131 if :ies a load of 150 kw.? It must not be over- 

 loaded. What is the value of the field current? 



32 



