Advantages of Circular vs Cylindrical Inductor Core?

[WorldWiz]

All the inductor components I’ve see are made with a circular core instead of a cylindrical core. Are there any advantages to this design in terms of field strength relative to input current (assuming the same number of turns of wire)?

In the field strength equation, is “coil length” always perpendicular to the wire wrapping, and therefore would be longer in the circular design than in a cylindrical design of the same thickness?

Thanks.

 

[berkeman]

All the inductor components I’ve see are made with a circular core instead of a cylindrical core.

Sorry, what the heck is a cicular core? Can you post some links and images?

 

[WorldWiz]

Sorry, what the heck is a cicular core? Can you post some links and images?

Sorry for my lack of terminology. Basically the two wrapping styles below:

 

[berkeman]

Ah, thanks. First image is a ferrite toroid (closed magnetic path), and second image is a ferrite bar (open magnetic path).

Can you tell us your thoughts? What kind of differences would you expect in the charactistics like inductance, saturation current, and external B-field coupling for these two different construction techniques?

 

[WorldWiz]

Ah, thanks. First image is a ferrite toroid (closed magnetic path), and second image is a ferrite bar (open magnetic path).

Can you tell us your thoughts? What kind of differences would you expect in the charactistics like inductance, saturation current, and external B-field coupling for these two different construction techniques?

My guess is that the coil length, l, is measured perpendicularly to the wrap direction, so therefore would be greater in the toroid, but the bar allows for more layers of wire, and so would ultimately have a higher possible maximum for achievable field strength?

 

[berkeman]

My guess is that the coil length, l, is measured perpendicularly to the wrap direction, so therefore would be greater in the toroid, but the bar allows for more layers of wire, and so would ultimately have a higher possible maximum for achievable field strength?

Those aren't really the considerations for using a toroid core or a bar core to build an inductor. It is true that the bar core is much easier to wrap versus the toroid, whether you use a bobbin or not on the Bar. Winding a toroid generally takes a pretty specialized winding machine, especially if you are putting on a lot of turns.

The more important distinctions are that the toroid is a closed magnetic path, so there are generally no external B-fields generated by the currents flowing in the coil on the toroidal core. That is important in devices where you want to minimize the B-field coupling out of your magnetic components like this.

If you use a full-circumferential wind on the toroid like you show in your picture, the inductor (or transformer) is also much less susceptible to external B-fields coupling interference voltages into the toroidal device.

You have to be more careful about saturation currents in the toroidal devices generally, since there is no "air gap" to increase the Reluctance of the magnetic path.

Lots of good Google search terms in my reply above...

 

[WorldWiz]

Those aren't really the considerations for using a toroid core or a bar core to build an inductor. It is true that the bar core is much easier to wrap versus the toroid, whether you use a bobbin or not on the Bar. Winding a toroid generally takes a pretty specialized winding machine, especially if you are putting on a lot of turns.

The more important distinctions are that the toroid is a closed magnetic path, so there are generally no external B-fields generated by the currents flowing in the coil on the toroidal core. That is important in devices where you want to minimize the B-field coupling out of your magnetic components like this.

If you use a full-circumferential wind on the toroid like you show in your picture, the inductor (or transformer) is also much less susceptible to external B-fields coupling interference voltages into the toroidal device.

You have to be more careful about saturation currents in the toroidal devices generally, since there is no "air gap" to increast the Reluctance of the magnetic path.

Lots of good Google search terms in my reply above...

Ah, sorry, I didn’t clarify that I only intend to make an inductor coil to accelerate a ball bearing along a wire track (not a coil gun, btw) in my personal project, and I just want to maximize power use efficiency.

I will look into the terms you provided though. Thanks.

 

[berkeman]

Ah, sorry, I didn’t clarify that I only intend to make an inductor coil to accelerate a ball bearing along a wire track (not a coil gun, btw) in my personal project, and I just want to maximize power use efficiency.

I will look into the terms you provided though. Thanks.

Okay, that is completely different.

Do you want to pull or push the ball bearing? To pull it in, you would use a configuration like a Solenoid. Pushing it away is more complicated, since if the ball bearing is unmagnetized, you can pretty much only attract it with a diverging magnetic field.

 

[WorldWiz]

Okay, that is completely different.

Do you want to pull or push the ball bearing? To pull it in, you would use a configuration like a Solenoid. Pushing it away is more complicated, since if the ball bearing is unmagnetized, you can pretty much only attract it with a diverging magnetic field.

I want to pull it in. My initial plan was a solenoid-like coil wrap with the tracks running through the coil, but then I started thinking it might better to place the coil beneath the track to ensure that there’s some force holding the ball on the tracks. Either way, I expect to be testing about 24v with 10-15a, which I think (?) should be fine with 20 gauge copper wire given the sort activation duration of the coil (activated by an n-channel MOSFET triggered by a PNP inductive proximity sensor).

 

[berkeman]

I want to pull it in.

Yes, a solenoid to pull in a ferrous object (rod or ball) is an open air core coil, like the one shown below. There is no value to a core inside the coil, since you just want to have a big diverging magnetic field to attract the metal ball into the coil area.

https://www.arborsci.com/products/solenoid-air-core

 

[Tom.G]

20 gauge Copper wire is rated for 1.47Amps continuous and will melt around 58Amps in free air.

Since the wire will be wound in a solenoid, there will be some heat buildup in the inner layers. This may be enough to damage the insulation and cause some shorted turns. Wire with high temperature insulation is available, common hi-temp insulations are Glass cloth, Teflon and Asbestos.

Both Teflon and Asbestos have their own potential drawbacks though.
If Teflon gets hot enough to degrade, the fumes (hydrogen fluoride, carbonyl fluoride, and others) are highly toxic (https://pubchem.ncbi.nlm.nih.gov/compound/8301#section=Toxicity-Summary).

Less dangerous in small doses are Asbestos fibers released in handling that can be inhaled and eventually cause lung damage (Asbestosis).
https://www.mayoclinic.org/diseases-conditions/asbestosis/symptoms-causes/syc-20354637

Cheers,
Tom

p.s. Please let us know how the project turns out!

 

[renormalize]

Ah, sorry, I didn’t clarify that I only intend to make an inductor coil to accelerate a ball bearing along a wire track (not a coil gun, btw) in my personal project, and I just want to maximize power use efficiency.

How about buying a pre-made kit to experiment with and modify to your specific use?