348 PROBLEMS. [CHAP. 



27. Given a transformer with an open magnetic circuit, as the now 

 obsolete " Hedgehog " transformer. Investigate the transformer with 

 a view to bringing out the characteristic differences between it and 

 a transformer with a closed magnetic circuit. Report on the relative 

 advantages and disadvantages of the two types. At one time this 

 subject was much debated. 



28. Required to find at what frequency, current and voltage a 

 given transformer will give the highest efficiency. The voltage should 

 not exceed a certain specified value; the temperature rise should not 

 exceed the limit set by A. I. E. E. Standardization Rules ; assume that 

 any frequency, from say 20 to 125 cycles, is available. Outline com- 

 pletely the method of procedure before making tests. 



29. Make a comparison of loading-back methods for testing trans- 

 formers. 



30. Make a general study of a series current transformer; make 

 a particular study of the accuracy of its ratio at different frequencies 

 and of phase errors when used with a wattmeter. 



31. Determine the effect of different wave forms upon transformer 

 losses, regulation and magnetizing current. 



32. Study the effect of wave form upon circle diagrams and other 

 vector diagrams. 



33. Given a 2-phase 4-wire supply from two independent trans- 

 formers. Insert between the two neutrals an additional source of 

 electromotive force of different frequency (or, direct current). Meas- 

 ure all voltages and construct a vector model. What must be the value 

 of the inserted electromotive force to cause all voltages between line 

 wires to be equal (see 93, Exp. 7~A). 



34. Determine the variation in the starting torque of an induction 

 motor for different values of secondary resistance. 



35. An induction motor was purchased for a certain frequency. 

 The central station equipment has been changed to a different fre- 

 quency. Can this motor be used? If so what voltage will be best? 

 Will any benefit come by changing its 3-phase primary from star to 

 delta or vice versa? 



36. Test an induction generator with condenser excitation. 



37. Test the method of obtaining polyphase from single-phase 

 current by means of an induction motor (3, Exp. 7~A) and a similar 

 method using a synchronous motor. 





