can any tell me why do we need minimum flux density in a transformer core??

 

I wasn't aware the you did.
Where did you read that?

 

on net... i too wasnt aware of this..
but i think it must be to avoid core saturation.

 

Certainly you need to avoid core saturation but I don't see how that's related to a minimum flux density.

 

Maybe he means Minimum Flux (Density) Leakage to be ensured in transformer design for good regulation .....

 

Generally, the product of winding window and core cross section tells you about the transformer size. It is a transformer with an iron core transferring the power defined by U1 and I1 at a given frequency f. You are free to make the flux density B very small, but the transformer will grow in size, cost and winding losses.

 

Generally, the product of winding window and core cross section tells you about the transformer size. It is a transformer with an iron core transferring the power defined by U1 and I1 at a given frequency f. You are free to make the flux density B very small, but the transformer will grow in size, cost and winding losses.
View attachment 84545

Hi,

When you post a formula like that you should show what every variable stands for, and what dimensions it is in such as inches, meters, etc., Hertz, etc.

 

Sorry, you are right:

Af, Aw ... core cross section [m2], windings window area [m2]
kw .... windings filling factor --> describes insulation , no dimension, typical values often 0.4..0.7
Srms ... current density in wire [A/m2] (natural convection 1e6...3e6A/m2, forced convection cooling 3e6...5e6A/m2, oil transformer 5e6...7e6A/m2)
B ... peak flux density [T]; has to be below saturation (iron core B<1.7T, nanocristalline B<1T, ferrite B<0.35T, amorpeous core B<1.4T)
U1 ... rms of primary voltage [V]
I1 ... rms of primary current [A]
f ... operating frequency [Hz]

Actually, I put the equation to show that the transformer size is inverse proportional to the flux density.