# FLUX DENSITY IN TRANSFORMER CORE

Discussion in 'General Electronics Chat' started by ujjwal agarwal, Apr 23, 2015.

1. ### ujjwal agarwal Thread Starter New Member

Apr 23, 2015
6
0
can any tell me why do we need minimum flux density in a transformer core??

2. ### crutschow Expert

Mar 14, 2008
13,505
3,378
I wasn't aware the you did.

3. ### ujjwal agarwal Thread Starter New Member

Apr 23, 2015
6
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on net... i too wasnt aware of this..
but i think it must be to avoid core saturation.

4. ### crutschow Expert

Mar 14, 2008
13,505
3,378
Certainly you need to avoid core saturation but I don't see how that's related to a minimum flux density.

5. ### Ramussons Active Member

May 3, 2013
562
92
Maybe he means Minimum Flux (Density) Leakage to be ensured in transformer design for good regulation .....

6. ### uwed Member

Mar 16, 2015
64
17
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.

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7. ### MrAl Distinguished Member

Jun 17, 2014
2,562
519
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.

8. ### uwed Member

Mar 16, 2015
64
17
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.

Last edited: Apr 24, 2015