What is it about having multiple coils that enables the magnetic field? Why can't I just have one big wire or threaded wire on a motor? Sorry, it's kind of a baby question but I couldn't find the answer.
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7\$\begingroup\$ inductance depends on number of turns and magnetic permeability of the core \$\endgroup\$– IndraneelFeb 26, 2019 at 7:09
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\$\begingroup\$ MMF is \$NI\$, where \$I\$ is current and \$N\$ is the number of turns. In a stranded conductor, each strand carries the same fraction of the total current. \$\endgroup\$– ChuFeb 26, 2019 at 7:10
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\$\begingroup\$ The fundamental "law" here is : "A single turn produces a magnetic field proportional to the current in it. " || Fields from turns add. If one turn carrying current I produces field F then N turns produce (simplistically) N x F. You CAN have "one big turn" nit it's "ONE" turn. Applying the 'fundamental law" above shows that one big medium small or teensy-tiny turn carrying current I has the same effect. Wires are made larger or smaller for other reasons. eg large wires have lower resistance . Small wires allow more turns in a given space. \$\endgroup\$– Russell McMahon ♦Feb 26, 2019 at 13:12
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4 Answers
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It is true that it's only the volume and the power fed to the winding that matters for magnetic field, in electromagnets and motors. Therefore, you could have a single turn winding.
Unfortunately, a single turn would (generally) require a very high current and a very low voltage. This is true on the scales we tend to work at, and the values that physical constants happen to have.
Practical electromagnets use a relatively cheap trick to increase the voltage and decrease the current, by splitting the short fat wire of a single turn into a long thin wire, wound round several times. As each turn has a different voltage, they need to be insulated from each other.
A huge advantage of thin wire in the winding is that connection wires can be a reasonable thickness, and still be much lower resistance than the working winding.
A disadvantage of this trick is that circular wire does not fill 100% of the available area, and the insulation consumes some space as well, so we lose some copper area compared with a single turn. However the trick is so cheap and useful that this inefficiency in area is a small price to pay for the benefits, for almost all applications (in some very big machines, square cross section wire or bar is used for windings to improve the packing density).
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2\$\begingroup\$ There is another advantage of square coil wire next to a bigger copper area (thus lower resistance). At high frequencies, current tends to flow at the surface of a wire (which is known as the skin effect), and compared with a round wire of same size, the square wire much better. \$\endgroup\$– HuismanFeb 26, 2019 at 8:05
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\$\begingroup\$ Isolation is not only needed to prevent each turn is shorted. At high voltages, the isolation is also used to seperate the turns enough to prevent breakdown. \$\endgroup\$– HuismanFeb 26, 2019 at 8:10
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\$\begingroup\$ An example of using bars instead of wires can be seen in this EEVblog video. IIRC the component in the video is an inductor. \$\endgroup\$ Feb 26, 2019 at 21:08
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1\$\begingroup\$ My first reaction upon reading your answer was to disagree, since Ampère's law factors the number of turns, but now I see what you did there. Ampère's law equally factors the current, doesn't it? Yours is a clever way to explain it. I shall remember this cleverness. If I had been answering, I would have started explaining in terms of Faraday's law, which, actually, you implicitly did, but in a way that did not name Faraday and therefore was not confusing at OP's level. Nicely done. \$\endgroup\$– thbFeb 27, 2019 at 14:39
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1\$\begingroup\$ @thb thanks. When I'm comparing numbers of turns, usually in transformers, another way I word it is first assume two identical windings, each under exactly the same voltage current power H field conditions, then connect them in series, then connect them in parallel. All that changes is the impedance, the voltage/current scaling, but volume, field, power dissipation, cost all remain the same. To first order anyway, SRF and voltage breakdown might well change a bit. \$\endgroup\$– Neil_UKFeb 27, 2019 at 17:59
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Why can't I just have one big wire or threaded wire on a motor?
No problem with this - check out the rotor on most induction motors:
There is no insulation on the aluminium (squirrel) cage and it is, in effect, one shorted turn.
What is it about having multiple coils that enables the magnetic field?
A magnetic field is produced by current AND turns so you can trade turns for current and vice versa. However, if you are interested in making an inductor with particular characteristics then, you need to engineer it by using multiple turns to optimize the inductance for the intended circuit given that there will be limitations on the availability of magnetic core materials.
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The reason you need it to be insulated is to ensure that the current goes around each loop when you coil it. If it weren't it could just go "straight". You can have one big wire indeed, but you would need more current to produce the same results.
That is what the number of turns N actually gives in all the magnetic field formulas. It actually lets you have multiples of the current in a given space.
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You can do it with a single loop and I have seen this done. However the wires are enormous and must be fabricated in a special way. For instance the wire (more: busbar) is extruded as a wedge cross section, and then rolled in a helix to yield a rectangle cross section.
But the current will be massive. If your input does not lend itself to deliver that kind of current, it won't work.
Magnetic force is amps x the number of turns. You have to carefully calibrate the number of turns and wire size so that it matches your circuit's ability to drive it. Doing it in one turn would require a fairly extreme amount of bucking to get the voltage very low and amps very high.
answered Feb 26, 2019 at 19:31