eli5 how small motors and rotating things work? Where does the rotating start?


Take something like a drill.

I understand its electric, I understand that in some way the electricity makes sothing move and then that movement is turned into spinning through gears and engineering magic.

But where does the initial movement come from? All i can picture is something is spinning or rotating because something else is rotating. But it has to start somewhere right?

In: 0

Angry pixies move along copper wire inside motor, there is a magnetic field around the copper wire when power runs through.

Run the cooper correct way around a magnet(or magnetized rotor) you get movement, magnets attracting and repelling eachother

a very simple model to get you started, imagine you’ve got a rod with 3 magnets sticking out of it, the N of the magnet points outwards.

Now imagine there’s 4 electro magnets equally spaced around it. electro magnets can be turned on and off at will. Right now, the topmost electromagnet is on and attracting the topmost permanent magnet. The 4 electro magnets on the outside and 3 permanent magnets on the inside are critical to visualise here, as it means only 1 permanent magnet lines up with an electro magnet at any one time.

The magnet on the right is now fairly close, but not lined up with the permanent magnet near it. So turn the topmost electromagnet off, and the one on the right on. It will attract the magnet towards it, spinning the rod. Now the permanent magnet is aligned with the spot on the right and the bottom electromagnet is near the next permanent magnet.

Turn off the magnet on the right, turn on the magnet on the bottom, and you’re attracting the new magnet towards it again. keep doing this, energising each electromagnet in turn, and the inner rotor spins.

That’s just a starter for 10, now you can look up the actual designs of different motors and see how they achieve much the same thing.

While i think many of the explanations here are reasonable, you wanted to know where the rotating _starts_, and none of these would explain why a homopolar motor spins.

It comes down to the forces exerted against the magnetic field by the flowing of electric current in the wires that make up the motor, this force can be understood through the “right hand rule” which explains that a force is generated tangental to flowing current and its magnetic field and even helps you understand which way rotation will start.



Most of the motors you’re probably thinking of are DC motors. Here’s a video that introduces current and electromagnetism in the context of DC motors and how they work.


It basically has to do with a careful alignment of permanent magnets and electromagnetism produced by switching polarity (like positive and negative terminals of a battery).

For the motors in many small tools and toys, you have a set of permanent magnets, and a set of electromagnets. One set is mounted on the shaft (the “rotor”) and the other on the stationary bit (the “stator”). The electromagnets are “soft” iron, wrapped with coils of copper wire. The electromagnet “becomes” a magnet when a current flows through the wire; the stronger the current, the stronger the magnet (up to a point).

(NB “Soft iron” = alloys and preparations of iron which will stick to a magnet, but will not become a permanent magnet themselves)

Crucially also, the *polarit*y of the electromagnet (which “end” is the north or south pole) depends on the *polarity* of the current.

To make a motor *keep going*, rather than just jump a bit when power is applied, you need to periodically reverse the polarity of the electromagnets as the motor rotates, so the electromagnets keep pulling (and pushing) the rotor by attraction/replusion to the permanent magnets.

Until 20-odd years ago, most common small motors used stationary permanent magnets on the stator (the outside, stationary bit), and electromagnets on the rotor shaft. A “split commutator” was used to connect the power to the rotor, this cleverly switched the polarity to the electromagnet coils as the rotor rotates.

Modern high specification motors use electronics to control and switch the current to the electromagnets, and can also vary the exact strength of the electromagnet with time (and angle of the shaft). This allows precise speed and torque control, and in particular allows for precise constant torque – which means that even really big motors can run super-smoothly and silently with practically no hum or vibrations. They can also be more efficient, which means they can deliver more power, be smaller, and run cooler.

(I cant really believe that every kid didn’t take their Hornby trains to pieces and have a look at the innards of the motors… (and put a drop of oil on them) Not everyone had engineering parents, I guess)