CHOKES AND TRANSFORMERS 



In the c.g.s. system of units, K^ is equal to 10"^, and 5max comes out in gauss. 



108 F 

 ^'"^'^ - 4-45 NAF^n ^^'''' 



where A is the cross-sectional area of the core in cm^. In a practical core A 

 is the area of the centre limb. Work out 5max for the core chosen : the 

 answer ought to be substantially less than 2,000, otherwise the Ferroxcube 

 will come too near saturation. If 5max is greater than 2,000, try again with a 

 bigger core. 



If -Smax is much less than 2,000 and R comes out to much less than 7?max 

 it may be worthwhile to try again with a smaller core. The object is usually 

 to arrive at a design using the smallest and cheapest core which will satisfy 

 requirements. 



Transformers without d.c. polarization — It is required to produce a 1 to « 

 transformer to work between a generator of internal resistance r and a load 

 Rj^, down to a turnover frequency i^min- Maximum R.M.S. generator 

 vohage, Fmax- It is hoped that when the design is completed the winding 

 resistances will be small compared with the generator and load resistances 

 with which they are in series {Figure 22.7). If this is the case then the primary 

 inductance required is roughly given by arranging that at Fmm the reactance 

 of Lp equals the parallel resistance of r and i^^/n^, that is 



R 



n 





«2 



or L 



n^ 



p 



mm 



(cf. Chapter 4, expression for lower turn-over frequency of signal transformer.) 

 Work out Lp and choose a core. Find out the number of primary turns 

 required to secure Lp and thus a suitable (i.e. thickest possible) gauge of 

 primary wire, taking two-thirds of the window area as before and allowing 

 half for the primary winding and half for the secondary. Estimate the 

 resistance the primary winding is likely to have — as for chokes^ — and see 

 that it is less than, say, 10 per cent of r. If not, try a bigger core. Multiply 

 the primary turns by n to get the secondary turns, and thus compute a likely 

 secondary wire gauge and hence secondary resistance. Check that the 

 secondary resistance is less than about 10 per cent of the load resistance. 

 If not, begin all over again with a bigger core. If all seems to be weh, cornpute 

 Bm&x as for chokes (but bear in mind that the applied voltage is not V but 

 F [(^i«^)/(^i«^)+r] (Figure 22.7) and check that the value is below 2,000. 

 If ^max is safely within bounds and the estimated winding resistances are 

 much less than the external generator and load resistances with which they 

 are in series, try re-designing with a smaller core. 



Transformers and chokes with d.c. polarization — When the current through 



312 



