I've constructed a rotating valve. It contains a magnet. I can rotate the magnet from outside the pipe where the valve may be immersed in a corrosive liquid. It is a kind of an unipolar motor.
Obviously I want to spent the least amount of copper in the coil to exercise a moment onto the magnet. What is more favourable: N windings on one side N/2 windings on either side.
Or doesn't it make a difference, to a first approximation?
Groetjes Albert
The magnet has length. I'd put my money on two saddle coils, one on either side of the pipe. What you win on the pole closest to the nearer coil is probably worth having.
windings on either side gets you something like a Helmholtz coil which means a more uniform magnetic field, but if your field is not saturating the magnet I don't think it makes a difference.
Thanks. Actually the magnet I use is a 10 mm disk with a thickness of .75 mm. My coil is at least 10 mm removed from the near side of the disk. In that case your answer implies that not much improvement is to be expected, if any. Correct?
I started from the model of a minute magnetic dipole in a homogenous magnet field. You draw attention to the fact that with one coil and a finite magnet the force on the two sides is not symmetric. With two coils the field may not be homogeneous, but -- being symmetric -- at least it will result in a pure torque, with no net force to be compensated by the valve seat. That means less friction, which could result in saving copper, and probably better reliability as well.
Groetjes Albert
Beats me. Field-plotting software would tell you, but that costs money.
The force probably decreases as the square of the distance. If the magnet is flat disk, with the magnetic field through the disk, rather than across the disk, when it is across the pipe the bit farthest away from a single sided coil is 20mm away from the coil so then would gets a quarter of the force acting on the nearest bit.
Putting coils on either side of the pipe would even that out.
We can hope.
It might. An ounce of experiment beats a ton of speculation.
Can you post a sketch of the geometry?
If the guts are symmetric, I'd vote for two coils and an outer ferrous path to reduce air gap, like an overall section of iron pipe.
If you want the 'valve' to rotate only on command, consider using an external motor to rotate a permanent magnet; that will allow a bit of friction to hold the position with no power applied.
As long as you don't saturate the core parts (which direct and contain the field), it's only different in effective use of available winding space; N/2 on either side means lessened winding diameter, which is usually more efficient.
A field strength improvement will result from adding a core to the windings; otherwise the magnetic field will stray far from the space it is intended to affect.
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