thanks for ur reply , i have some more doubts in that design 1) how to calculate saturation level 2) till what order of harmonics will be controlled in the choke 3) how to calculate total harmonic destortion (THD) 4) how to design 3 phase reactors 5) difference in designing input and output choke for variable frequency drives(drives) 6) can we have very small air gap in chokes ive bought a choke 3 phase 415v 250A 0.028mh (for110kw vfd) it is giving heavy noise & heat , the choke has very small air gap(almost no gap, top core virtually sits on bottom core ) , mechanically every thing is right ,is that because of the small air gap the choke is giving problems another choke of different manufacturer is working fine , or what may the problem in choke , please guide me.