DESIGN OF TRANSFORMERS. 113 



Let A be the cross-sectional area of the core in square centi- 

 metres, and L the mean length of the magnetic path, so that 



AL = approximate volume of iron core in cubic centimetres 



The iron losses are due to hysteresis and eddy currents. If 

 the core is built up of soft annealed iron stampings, the hysteresis 

 loss can be found from Table II., Chap. X., and 62. The 

 maximum induction was taken to be 2500 lines per square centi- 

 metre ; hence the ergs lost per cycle per cubic centimetre are 600. 



With a frequency of 50 complete cycles per second, this gives 

 the loss in watts per cubic centimetre to be 



600 x 50 



-Tffr- 



= 0-003 watts ...... (11) 



The watts lost per cubic centimetre due to eddy currents are 

 calculated from the formula (see Chap. XL 61) 



k' = VW& X 10~ 16 



where b is the thickness of the stampings in mils. 



Suppose that & = 15 mils., then in the present case 



li = 15 2 x 50 2 x 2500 2 x 10~ 16 



= 0-00035 watts ........ (12) 



The total iron loss in watts per cubic centimetre is, therefore, 

 given by 



H=h + h' 



= 0-00335 watts ..... (13) 



Since the total iron loss is 150 watts, the volume of the core 

 is given by 



0-00335 



= 44776 cubic centimetres 

 that is 



AL = 44776 ...... (14) 



where A is the cross-sectional area of the core, and L is the mean 

 length of the magnetic path. 



As the insulation between the stampings will occupy 10 or 15 



I 



