400 ELEMENTS OF ELECTRICAL ENGINEERING. 



12.6 square centimeters is to be magnetized by alternating cur- 

 rent taken from 133-cycle no- volt mains so that the maximum 

 flux density in the core may be 4,500 lines per square centimeter. 

 (a) How many turns of wire are required ? (&) How many turns 

 of wire would be required at 60 cycles per second, other things 

 remaining the same? Ans. (a) 328 turns, (b) 728 turns. 



107. A transformer has 60 turns of wire in its primary coil, 

 and it takes 750 amperes from an alternator at no volts and 

 steps up to 20,000 volts, (a) How many turns of wire are there 

 in the secondary coil ? A usual allowance in transformers is 500 

 circular mils of sectional area of wire for each ampere of current. 

 (fr) Find size of primary wire and of secondary wire of the above 

 transformer. Ans. (a) 10,909 turns, (b) 375,000 circular mils, 

 2,062 circular mils. 



108. The sectional area of the core of the transformer in prob- 

 lem 107 is 161 square centimeters. Find the maximum core 

 flux and maximum flux density at a frequency of 60 cycles per 

 second. Ans. 3> = 687,700, cB = 4,272. 



109. A transformer has 800 turns of wire on the primary coil, 

 40 mils in diameter, and 80 turns on the secondary coil, second- 

 ary wire being ten times as large in sectional area. The sectional 

 area of the magnetic circuit of the iron core is 1 2 square inches. 

 Allowing 500 circular mils of conductor per ampere, and allow- 

 ing a maximum flux density of 4,000 lines per square centimeter, 

 calculate the electromotive force, current and power ratings of the 

 transformer at 60 cycles per second. Ans. 660.4 volts, 3.2 am- 

 peres, 2. 1 1 3 kilowatts. 



110. A given transformer is rated at 5 kilowatts and is designed 

 to take current from i,ioo-volt mains at a frequency of 60 cycles 

 per second. Under these conditions P h , P e , and P e will be 

 called normal. 



(a) The transformer gives 6 kilowatts output at the rated 

 electromotive force and frequency. Find P e in terms of normal. 



(b) The transformer is used at rated electromotive force but at 

 a frequency of 75 cycles per second. Find P h and P e each 

 in terms of normal. 



