Making a bigger transfomer core by stacking multiple smaller cores

Question

Does using a number of stacked EI or EE cores in the place of a bigger one (respecting the know magnetic characteristics) would pose any substantial difference?¹

There's even manufactures that sell glued cores, but I think 4 stacked cores is the maximum I have seen (ferrite cores not laminated ones).

UPDATE:

The idea is not to "miniaturize" the core, in fact they may be heavier and bigger than a single core, but to have access to high power cores if your local market is not selling larger cores.

In fact, the above manufacturer shows that even it sells some bigger cores as glued/united small cores and the specified magnetic characteristics from then

Single core:

Same core stacked:

Both images from Thornton products catalog.

Answer 1

Stacking multiple cores as you describe would effectively increase the cross-sectional area of the core. The question is then, what difference does increasing the cross-sectional area have?

Firstly, a larger core can dissipate heat without damage. This is because it has more surface area. If a smaller core would overheat (due to eddy currents, hysteresis losses, etc), then a sufficiently bigger core would not.

A fatter core also increases the inductance compared to a coil with the same magnetic path length and same number of turns, but on a thinner core. Why? Recall the definition of inductance: it is the ratio of magnetic flux to current:

$$ L = \frac{\phi}{i} $$

One ampere through a turn of any wire of any size produces a MMF of one ampere. As the area of the turn increases, this MMF is applied over a larger space, and thus, there is more flux, and thus more inductance. That is, we get more flux \$\phi\$ per current \$i\$ as we make the turn bigger. However, this larger flux is spread over a proportionately larger area, so the flux density remains the same.

Given that the flux density remains the same, the saturation current of a fatter core is the same as that of a thinner core. However, the fatter core has a higher inductance. We could then reduce the number of turns on the fatter core such that it has the same inductance as the thinner core. Being there now fewer turns, the flux density decreases, so saturation current increases if inductance is held constant.

Thus, advantages of a fatter core:

1. increased heat dissipation
2. increased inductance
3. (or) increased saturation current

Answer 2

As long as there is no significant mechanical differences from the magnetic viewpoint it should be much the same. Core laminations exist to limit eddy currents, and stacking cores is just separating some of the lamination groups.

Considerations:

It's likely that there will be slightly more gap at such boundaries if care is not taken to compress them together well.

Glueing core groups may allow a small gap due to glue thickness - and an air gape is vastly larger magnetically than the same distance in core material, so every effort must be taken to eliminate such possibilities.

Noise and vibration can occur if the core groups are able to move relative to each other more freely than can tight packed laminations.

Answer 3

Does using a number of stacked EI or EE cores in the place of a bigger one (respecting the know magnetic characteristics) would pose any substantial difference?

Stacking two cores (for instance) so that the cross sectional effective area of the core doubles doesn't give you any benefit magnetically (see edit section below) over the single core so I'm wondering the reason behind the question. If you double the area, the magnetic reluctance halves, allowing twice as much flux to be produced but, it's spread over the same area therefore, flux density remains the same.

As an example: if a single core was on the verge of saturation, with two cores in parallel and the same current flowing and same number of turns, the double-core will be at the same point of saturation. Nothing gained.

If, on the other hand you replaced an I (from an EI core set) with another "E" part, the length of the magnetic field through the core will increase and this increases magnetic reluctance and therefore decreases flux meaning, the core won't saturate as much for the same ampere-turns.

The answer is - it depends how you stack them.

EDIT

The question is about transformers and a lot of applications of transformers have DC running in the windings. If DC is the dominant cause of saturation then there would be no benefit in paralleling cores to obtain better saturation figures except when you can reduce the number of turns because the AC inductance/impedance will have automatically increased when the core area increased. If on the other hand, the transformer is voltage driven and the dominant cause of saturation is the AC current, doubling the area also doubles the inductance: -

\$A_L = \dfrac{\mu_0 \mu_e}{core factor}\$

Where core factor is length of magnetic field divided by core area, hence inductance doubles if area doubles. If inductance doubles and the transformer is voltage driven then current reduces and flux density also reduces and this can prevent saturation when it is the AC that causes saturation problems.

Answer 4

The multicored transformer when compared to the big single core can operate at basicly the same flux density because the type of ferite is the same .Now the big single core looks more like a sphere than say a lanky transformer that had say 4 cores .So the ratio of surface area to volume is better for getting rid of heat than the big single core .So the multicore cools better so you should get more power out of it because most SMPS transformers are thermally limited.I teach this surface area to volume ratio to the non mathematicly minded by saying there aint any eskimos that look like me .