5-A] 



STUDY AND OPERATION. 



141 



ferent values for different parts of the magnetic circuit. From 

 these computations it can be seen whether B will be more or less, 

 if a transformer is operated at a higher or lower frequency than 

 rated and at the same voltage. (Note in what manner E should 

 be changed to maintain B the same at different frequencies.) 

 Practically, transformers are run at different frequencies without 

 changing E, if the frequency is not too far below the frequency 

 for which the transformer is designed. For a discussion of the 

 effect of frequency upon core loss, see 8-14, Exp. 5-B. 



In transformer design,* B is given a wide range (4,000-14,000 

 gausses at 60 cycles), being sometimes greater in small than in 

 large transformers and greater in transformers designed for low 

 than in those for high frequency. In design, E and n being 

 given, B may be assumed and the product A X S determined. 

 This product being fixed, the designer may adjust the values of 

 A and 5" to suit his purpose, increasing A and decreasing S to use 

 more iron and less copper, or vice versa. 



26. From the formula for flux density, it will be seen that 

 the electromotive force of any coil of a transformer is propor- 

 tional to the number of turns in the coil, a fact already noted. 

 The volts-per-turn should be computed as a constant for the 

 transformer. For small transformers this may be one third or 

 one half, being greater for large transformers, perhaps 2 to 4 

 for transformers above 30 K.W. The reciprocal gives the turns- 

 per-volt. The volts-per-turn, when known for a certain type and 

 size of transformer, may be used as a design constant. 



27. Other data of interest to the designer (which may be de- 

 termined when worth while) are the weight of copper and of iron, 

 total and per K.W. This may range from 5 to 25 Ibs. per K.W. 

 for either copper or iron. The space factor for copper is the 



* (2Sa). For more complete design data, see handbooks, etc. As mag- 

 netic material is improved, higher magnetic densities are possible for the 

 same loss. While densities of 4,000-8,000 were used with ordinary grades 

 of iron, densities of 6,000-12,000 are now common with alloy steel. 



