Transformer Riddle No.32 - Flux deflection in transformers
I have created a model of a transformer comprising a figure 8 core with face 1 of the primary and face 1 of the secondary coils in the first 'hole' of the figure 8 and face 2 of the primary and face 2 of the secondary in the second 'hole' with an air gap between the coils and the upper face of the core. I have flux deflection and don't understand why.
ok, here goes! If you can imagine a rectangular 'figure 8' core and inside the two holes i have placed a primary and secondary coil. The first face of the primary coil is on the left hand side of the left 'hole' and the first face of the secondary coil is on the right hand side of the left 'hole' and the second face of the secondary coil is on the left side of the right hand 'hole' and the second face of the primary is on the right side of the right hand 'hole'. I have an air gap between the upper of the coils and the lower of the core face of 5mm. I have measured in this gap in both 'holes' and in the left hand hole air gap the flux is barely negative and in the right hand hole the flux is barely positive, can anyone tell me why?

#1
Wed, October 28th, 2009 - 10:53
Flux deflection or flux fringing effects is appeared when locally saturation in core limbs is possible. In below figures which have drawn according your explanation, we front to different flux distribution in core limbs. In figure 1, the turning direction of face 1 primary winding is similar to face 2 of same winding and as you see we front to a good flux distribution in all limbs of flux path in this case. In figure 2, that mentioned direction are opposite and we face to saturation probability in some zone of flux path can be caused flux deflection or fringing effects generally.

#2
Wed, October 28th, 2009 - 13:57
Thanks for that answer, it has helped quite a bit. However, i neglected to mention that the core of my transformer is a solid figure 8 and the 2 coils are inside the core holes and not wrapped around the outside of it or round the centre limb, they are inside the 2 'holes' with a small air gap down the centre of the 2 coils in each hole and a small air gap between the top of the coils and the lower face of the core. I'm sure this makes a great difference in the calculations but unfortunately i cannot upload a picture of it onto this website but can email a copy of it to you. Thanks again

#3
Wed, October 28th, 2009 - 15:55
You can upload picture with gif  format (max. 500 kb) by click on foreseeing tree picture in tool bar.

#4
Wed, October 28th, 2009 - 16:58

This is the transformer that I have designed and as can be seen I have measured in the air gap between the upper face of the coils and the lower face of the core. The coils are copper with 1000 turns each and a current of 4A is being applied to the primary coil only. In that air gap is hardly any flux, on the left side is just below zero and on the right side it is just above zero. Is this due to mutual induction and an opposing force being set up in the secondary coil and also due to the air gap reluctance which is higher than that of the core? I am only interested in the air gap between the coils and the core and not the air gap between the coils. I have also noticed that when I apply a current of 2A the flux in the core is quite low, but if I apply 4A it is quite high which would follow, but here’s a thinker! When I apply a current of 6A, the flux reverts back to a low state, why would this be? Also when I apply a current of 8A or 10A, the flux is high again. Why would the flux be low for 6A and 2A but high for 4A, 8A and 10A? I just found this strange is all.

#5
Wed, October 28th, 2009 - 19:44
Ok, obviously you front to a lot of leakage flux for magnetizing current bigger than 2 A due to core saturation because what you can be measure in mentioned air gap cannot be linkage magnetizing flux. Leakage flux is a part of whole created flux which cannot link all turn of windings. The behavior and accurate path of leakage magnetizing flux is not simple and it shall be calculated by special finite element software.