Solenoid questions...

[AE35]

Hi all!

I'm new here! Just stumbled upon this forum

I'm trying to learn about solenoids. The reason is that heard about taking a
solenoid and removing windings to increase force. This sounds unsure to me, let
me try an example:

Let's say we have a solenoid of 10ohm and 25V. Using Ohms Law
we would get 25 / 10 = 2.5A. If we then remove windings and get 9ohm, then 25 / 9 = 2.8A.

That makes sense, as numbers, but does it really apply to solenoids? I mean, removing
windings can only go so far before resistance is zero, so the coil should be the strongest
when there's nothing left What do you think of removing turns to make the solenoid
stronger? Does that work?

So I tried to find other answers, and found this:

B = Uo * (NI/l)

Example:

B = 1.257*10^-6 * (750 * 2.1A / 0,8M) = 2.5 mT

But this also confuses me. If I increase the windings to 1500, I double the B...the same
with ampere, and lowering l

But if I double the windings doesn't that double the resistance? or is it 'understood'
that that means also increasing the diameter of the wire? or does the resistance not
matter?

Thanks for your time!!

 

[ronsimpson]

You are using the wire resistance to limit the current. If you remove 1/2 the wire, R=1/2, current=2x.

Force is related to (AMP * TURNs).

I think the force is the same.

 

[alec_t]

For a DC-energised solenoid I'd agree with Ronsimpson. However, for an AC-energised solenoid the coil current (hence the force) will depend not only on the coil resistance but also on its inductance and the AC frequency. Inductance varies as the square of the number of turns, so the force will no longer be the same if turns are removed.

 

[AE35]

The solenoids in question are DC

Let's say that we instead of I(ampere) insert volt / resistance, as 25 / 10

B = 1.257*10^-6 * (750 * (25/10) / 0,8M) = 0,0029 T

Then if I remove windings until there's only 1 left(!) and insert the resistance after this(probably close to zero),

B = 1.257*10^-6 * (1 * (25/0,1) / 0,8M) = 0,00039 T

It seems to get a lot weaker, yes?

 

[alec_t]

No. Your '0.1' value for resistance is wrong. It should be 10/750 = 0.01333....

 

[AE35]

Alec, can you expand on that, please? What does 10/750 mean?

Thanks!

 

[alec_t]

If I understand you correctly you proposed reducing the number of turns from 750 to 1. Therefore the resistance will be reduced by a factor of 750 also; hence R = 10 divided by 750.