Using Common-mode choke as inductor

Question

I am going to build a low voltage and low power Modular Multilevel Converter (MMC) for research purposes. In each phase of MMC, there are two inductors where low frequency (50 Hz) currents pass through. On the other hand, I need inductors with few miliHenry inductace that can tolerate low frequency currents of a few amps (e.g. 5A). I am having hard time finding such inductors on digikey or RS or Farnell. The only one that I've found is this common-mode choke, but I am not sure if it can be used as an inductor for the mentioned application or not. Any ideas?

Answer 1

This is just a stab in the dark pointer that hopefully indicates the size of inductor core needed....

A good permeability ferrite core might have an inductance of 10 uH for one turn and, to get 1 mH you need 31 turns because inductance is proportional to turns squared.

So, with 5A RMS the peak current is about 7.1 amps and that's an MMF (magneto motive force) of 219 amp-turns. Given that the mean length around a fairly sizable ferrite is about 40 mm, the H field (amp-turns per metre) is going to be 5480.

If the relative permeability of the core is about 3000 and remembering that µ0 = 4π × 10^−7, the resultant flux density will be 5480 x 3000 x µ0 = 21 tesla and too much for a ferrite (and also iron) core of the mean length mentioned above. Ferrite starts to saturate generally at about 300 mT.

So, get looking for something that is quite large - nothing ferrite will work as far as I know - it will probably need to be laminated iron and the size of a small transformer. Something you could comfortably hold in the palm of your hand and would weigh about half a kg (maybe a tad less).

Answer 2

Current mode chokes work differently to single inductors. Normally there is opposing current in each coil. This is critical as the magnetic flux from each winding opposes the other and even under high current there is no saturation. But the specified inductance is for a single winding.

If you put the 5A through just one winding alone the core will saturate and the inductance will collapse. This is because to get the super high millihenry levels of inductance with a few turns the cores are un-gapped.

You could be clever and feed your current through one winding then the other to keep the opposite current idea going. Saturation will be avoided but this will severely reduce the inductance. The resultant inductance will be referred to by the manufacturers as the differential inductance. What you get is more due to imperfection than design in this mode.

Inductors actually store energy and to do this the magnetic path needs an air gap or a low permeability material like powdered iron (where the gap is between all the microscopic particles). Oddly no magnetic materials are good at energy storage, they just provide a low reluctance path so the energy can be stored in the air gap without a crazy number of turns.

Powered iron cores are often used for the low freq filtering and are available in rather large sizes and may be a good start.

Try Magnetics Inc or Micrometals, they also have calculators to help with the design. For toroidal cores these can be custom wound by companies that do transformer winding. They can be hand wound but I suspect this will be quite a few turns.

One last idea is inductors for speaker crossovers. they are typically in the mH range and many are air cored, so can handle any current without saturation. You would need to check current rating from a heat point of view and provided adequate insulation.