The Ossie motor

[gyulasun]

Hi Augen,

My question is that out of the two coils you connected in series, the one with half the length (2.5mm) has got the same number of turns like the other one?  Also, are the DC resistances the same?

rgds,  Gyula

[Augen]

Hi Gyula,

No. By cutting the length of the coil to half (from 5mm to 2.5mm), I decreased the length of the wire to half -> so both the number of the turns and the resistance were cut to half. It is like cutting the 5mm long coil in two 2.5mm long peaces. If you put them together, you would get the original coil. Like these below.

Augen

[gyulasun]

Once your longer coil has twice the number of turns than your 2.5mm long coil has,  then why wonder the longer coil is stronger?

EDIT: if Amperturns differ, so does B,  no?

[Augen]

If Amperturns matter that much, then you are right. But what about "B"? "u" is the same. U=3V and B=N*I/l (without "u"). So:

Coil 1:
N=660 turns
l=5 mm
R=14 Ohms
I=215 mA
142 Amperturns
B=u*28380 T

Coil 2:
N=330 turns
l=2.5 mm
R=7 Ohms
I=430 mA
142 Amperturns
B=u*56760 T (using 430 mA)
71 Amerturns
B=u*28380 T (using 215 mA) In that case, both coils have the same current.

So what about this? The math seems right. Then what is wrong? In parallel the amperturns are the same and B is twice, but in series amperturns are half and B is the same. So what matters? Amperturns or B? It is important, because the key to this project is to make the most powerful coil, using the list current. Right?

I did some experiments and I experienced the same as before. In series the bigger coil was stronger, and in parallel the smaller coil was stronger (I had twice the current. I guess that is the reason.)

[gyulasun]

Hi Augen,

So what matters? Amperturns or B? 

Well, in the meantime I realized we are dealing with an electromagnet (lol) and as such its attract (or here the repel) force depends on B squared, B² ok?   (see some more here http://info.ee.surrey.ac.uk/Workshop/advice/coils/force.html )

So when you have them in parallel, the current is being higher in the thinner 2.5mm coil and the resulting B is higher, hence its squared value is even higher and this gives a stronger force.
And when you have them in series, the current is being the same but the number of turns are different, (the smaller coil has just half number of turns than the big coil) so the same current can only create less H field, hence the B² value is also less.

The "strange" thing is the attract (or repel) force of an electromagnet is proportional to B², and B² is proportional to the current squared, I².

You can also consider this as follows: an air cored coil that has a higher number of turns than another coil, can store higher magnetic energy because its stored energy is proportional to its inductance L, and normally a coil with higher number of turns has got a higher inductance.  (E_L=L*I²/2   I=current)

And when you reduced the length of one coil to half wrt the other one, you reduced its inductance too.  If you want to compare coils with differing lengths, you ought to use identical inductance coils with the same current flowing through,  this would involve higher OD for the shorter coil to maintain the same inductance. 

rgds,  Gyula