Large Inductor

sandy.bridge

The other day one of our power labs we were using rather large inductors; that is, approximately 10"x4" in the shape of a doughnut. There were 4 connections into the inductor. We connected a wire from one to the other, and then used the other two connections as any other inductor. Does that merely mean there was two windings, or am I missing something here?

berkeman

Sounds like it's a transformer or maybe being used for a common-mode choke?

sandy.bridge

Decided to go in and ask what it was we were using. It was indeed two inductors in one, and we were essentially putting them both in series to get a greater inductance.

berkeman

Interesting. Thanks for the info.

Quiz Question -- what parameter do you need to check when doing something like that? There can be a problem when you double the amp-turns on an inductor...

sandy.bridge

We checked both the resistance and the inductance.

berkeman

What do you mean by "you checked the inductance"....?

sandy.bridge

By checked I meant measured!

berkeman

But that doesn't address my Quiz Question (which is optional of course). When you double up the amp-turns on an inductor, what should you be concerned about? What datasheet spec should you check? Or what experiment should you run to be sure you still have an inductor?

sandy.bridge

If the meter is reading an inductance, would that not indicate that it is indeed a inductor? I know that wires have some inductance, but not as much as what we measured. I'm not quite following, but I'd like to know for future reference.

berkeman

I was referring to inductor saturation. All inductors have a saturation current Isat, and if you exceed that Isat, the inductance value diminishes. Look at the B-H curve to see how there is a saturation flux density where the B versus H curve starts to bend over:

http://en.wikipedia.org/wiki/Saturation_(magnetic)

When the H field reaches a saturation value, the B value stops climbing linearly, so the effective inductance value goes down.

This is important in power electronics, and also in communication transformer design. When you mentioned that they were doubling up the two windings on the inductor to get 4x the inductance, that doubles the Amp*turns for the inductor. If the inductor was meant to be a transformer, for example, then using it in this way might exceed the Isat rating for the core. Just something to think about when using transformers and inductors...