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# Solenoid design

## Solenoid design

(OP)
Hi all!
I would like an advise about how to choose the proper dimensions of two solenoids:
I would like to have them with air core, internal max diameter 4 mm, external max diameter 8mm, maximum length 2cm.

Now: these solenoids are going to be operated at 30kHz sinusoidal current so the diameter of the wire should be thin enough not to present a big skin effect but big enough to carry quite a lot of current.

The aim is endeed this:
I would like to put two solenoid (aligned on their axis) with a gap of 2.5cm between the two opposite faces (as they were a singol solenoid splitted in two parts with some distance in between)

I would like to have 50 gauss peak magnetic field in the gap between the two part of the solenoids..

Can you help me to understand if this is feasible and how to calculate the current I need?

### RE: Solenoid design

probably feasible.

First calculate the B along axis of one solenoid (easily google it to know how). For two solenoid (aligned wiht axes), the field B can be calculated by using half of distrance (i.e. 1.25cm).

vice versa, when you need B= 50 Gs, you can re-calculate the needed N.A

### RE: Solenoid design

keep in mind, the B field at a distance of 2.5 cm for two solenoids is not simply double the B field when only one solenoid exists. The max B at a distance can never be larger than B inside the solenoid regardless of how many solenoids aligned.

### RE: Solenoid design

(OP)
Thanks a lot for your help; while doing my simple calculation I had a curiosity: how does the B field in the air gap change using differ material in the core of the two solenoids? I can't understands the relation between the permeability of the core of the solenoid and the value of B in the air gap...probably it's a very stupid question, sorry about that.

### RE: Solenoid design

At 30kHz you would have a tough time finding a good core material. The AC losses may outweigh the benefit.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

### RE: Solenoid design

(OP)
I thought ferrite would be good but since I've trouble calculating the Inner B able to create the 50Gauss in the air gap I cannot evaluate if there is the risk that ferrite saturates..

### RE: Solenoid design

For two aligned (in series) solenoids (L=2cm, ID = 4mm), at the air gap of 2.5cm, you will need about 1 T field in the solenoid to generate a B of 50G. obviously any ferrite will be saturated for a B field of 1 T.
I recommend using powder core with tens of permeability even at a oprn circle. here is a link telling oyu how to choose powder core materials and how to design circle.

i meant circuit

### RE: Solenoid design

(OP)
...I'm not sure where the link is :(
And what do you mean with tens of permeability at open circuit??
So finally the ratio between core and gap field is really high..what about optimizing the geometry so that this ratio goes down? Do you think would be an improvement using the Helmoltz configuration? Not sure I ave enough space to build both of the solenoids with 1" inner diameter, but one for sure..

### RE: Solenoid design

tens of permeability: assuming permeability = 10, you just need H filed of H = 1000 Oe to generate a B field of 10,000Gs. If you use air coil (perm =1), you will need a H filed of 10,000 Oe to generate a B of 10,000Gs. That basically saves your N.A (ampere turns) 10 times.

### RE: Solenoid design

(OP)
Oh yes sure, I was not sure about the interpretation of 'open circuit' but maybe you meant air circuit.
I have probably already saw that link but I will double check it for sure, thanks!

Ps any comment about the geometry?

### RE: Solenoid design

open circuit: flux flows widely open in all directions. close circuit: there are some pole pieces to guard the flux from N to S with a min reluctance.

increasing diameter or length of solenoid can help. for example, For two solenoids with L=2cm, ID = 8 mm, at the air gap of 2.5cm, you just need about 2600 G field in the solenoid to generate a B of 50G. you may use ferrite now.

### RE: Solenoid design

(OP)
Oh thanks, didn't know the definition open/close circuit..

Wow, that's great, do you have an analytical relation between ID and ration between core and air gap or you're using a software? If I know what you're using I can perform a parametric study on that and find out the optimum ID or even try to understand how the solenoid length affects that ratio (stopping in this way to ask un million single questions eheheh)

### RE: Solenoid design

(OP)
At the end I manage to understand that to create 50G in the gap without a core one has to use several windings so I don't have enough space for the second coil.

So now the problem is to design a solenoid able to produce 50G on his axis at 1" distance.

I found that with 1mm diameter wire and 2A (hoping it's not too much for the wire (it's going to be in vacuum)) I need 20x20 turns-> a lot...

And since they're going to be operated at 30khz I'm worried about proximity effect (I'm going to have 20 layers of windings..) not sure if Litz wire can help to reduce this..and not sure if you suggest me using bigger section with more current and less turns or something else (to me it seems that with less layers the field is going to spread quicker outside of the solenoid..

I'm also tring to find a power supply able to give power at 30khz, probably 3A max, not sure about the Voltage, it seems to be the impedance is going to be quite high at that frequency..

At this point I may also need a matching network..

I tought it was easier to build and drive a solenoid at 30khz with a peak B field of 50G at 1" ....

### RE: Solenoid design

If this is going to be in a vacuum, how do you plan on removing the heat from the coil?

### RE: Solenoid design

(OP)
Only radiation (I agree that's dangerous for solenoid integrity but we usually run coils in vacuum, maybe I'll need a lower current density..)
Problem is also the impedance of this inductor, i'll probably need a matching network otherwise the power supply is going to be damaged or not going to work good...

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