I looked for some explanations online but I honestly dont understand anything at all 🙁
In: Engineering
The rotor, the spinning part is a bar magnet. It has a north pole and a south pole. Sometimes it goes NSNS, we call that a four pole. You can have more (even numbers) of poles too. They just make the motor spin slower.
The stator is the outside part that doesn’t move. This is where the coils of wire are. Passing an electric current through a coil of wire makes it an electromagnet, with a north and south pole.
So as you can imagine, passing a DC current through the stator makes the rotor lineup with it. North of the rotor magnet will match south of the stator electromagnet. Now, if you flip the DC current, the electromagnet will flip poles. The rotor will now rotate 180° to lineup again.
Now, if you fed the stator AC current, the current is always flipping. In North America, AC is 60 Hz. So the stator electromagnet will swap directions and back 60 times per second. This means a 2-pole rotor magnet will spin in a circle 60 timers per second (3600 RPM). The mechanical spinning frequency and the electrical AC frequency match, they are synchronous. It’s just two magnets chasing each other. One is flipping because of electricity, the other flips mechanically.
If you had a 4-pole motor, it would only spin at 30 Hz / 1800 RPM. This is because a full electromagnet flip only makes the rotor flip halfway, aligning the second north pole back to the position the first north pole started at. Needs two electrical cycles to get the first north pole back or its same starting position. This is still called synchronous, as it’s a nice whole factor of the electrical frequency.
Now, what I described is a permanent magnet synchronous motor. A lot are not a permanent magnet, but another DC fed electromagnet on the rotor. Works the same. DC electromagnet does the same thing a permanent bar magnet does.
The real complicated motor to understand is the asynchronous induction motor. If the electrical frequency is 60 Hz, they might spin at 58 Hz. That means the two magnets are still chasing, but they are slipping aparte every now and then. The rotor also has no permanent magnet or DC fed electromagnet, but is just some metal. It’s basically a spinning transformer, making it more complicated.
A synchronous motor is called that because its shaft/rotor turns at an integral multiple of the AC current coming in. If the current is 60 Hz, for example, a motor might spin at 60 Hz, or 30, or 120, etc. Depends on the coil and magnet configurations.
The easiest way to do this is to use permanent magnets on the rotor. They move due to the changing magnetic field from the stator’s coils, which changes with the line frequency.
Such motors can be handy for some applications, but their speed depends on the line frequency being stable, which it often isn’t quite. Years ago it was common for electric clocks to use synchronous motors with gear reduction. They would usually keep pretty good time, but not great. A crystal-stabilized clock is much better.
It is important to note that motors (and generators) operate by the interaction of two magnetic fields, one stationary and one rotating. Which is which is a matter of design.
In a typical synchronous machine the stationary field is on the rotor and the rotating field is on the stator. Rotor and stator are terms that refer to their mechanical positions, not their electromagnetic properties. An AC three phase winding does a good job generating a rotating magnetic field so that all the stationary field has to do is follow it around in lock step, which is the definition of a synchronous machine.
The speed of a synchronous machine is governed by only 2 things, number of pole pairs and frequency of the AC supply. RPM equals frequency times 60 divided by the number of pole pairs. The 60 is to compensate for RPM being in minutes and frequency in seconds. And since pole pairs is so uncommon, it is usually termed RPM=frequency x 120 / poles.
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