The diagram is taken from a textbook called “Elektronik 1” from the following authors: Helmut Röder, Heinz Ruckriegel, Willi Schleer, Dieter Schnell, Dietmar Schmid, Werner Zieß, Heinz Häberle. All of them are either PhDs or electrical engineers. The diagram refers to a synchronous motor (the word "FALSE" is added by me).

The true diagram should look like this:

What does a DC motor have to do with this? A DC motor works exactly on the same principle as the motor above.

How so?

Please look at the figure below:

There are three permanent magnets, two stationary and one placed on an axle so that it can rotate freely. When the middle magnet comes in line with the other two, then it stops moving. Now, let’s imagine that in that moment the polarity of the middle magnet by some magic becomes reversed. Then the magnet continues to rotate for another half cycle. Then the magic works again and so on. The magnet keeps rotating forever.

So, at which moments should the “polarity reversal magic” occur? At the moments when the middle magnet is exactly in line with the other magnets.

Now imagine that instead of the rotating magnet, a coil of wire is placed in the middle. The coil is connected to a DC source. Same as the permanent magnet, the magnetic field of the coil gets in line with the magnetic field of the two other magnets. If we find a way to reverse the polarity of the battery, then the current in the coil is also reversed, its magnetic field, too. Thus the coil will keep rotating.

How is this achieved? By a commutator. It looks like this:

Two metal half rings are connected via metal wires to a battery. The half rings are additionally connected to the terminals of the coil. The plane of the gap between the half rings must be the same as the plane of the magnetic field of the coil as shown in the screenshot below:

The screenshot is taken from this video (please watch it):

If the author of the video had glued the half metal rings otherwise (let’s say turned for 90 degrees), the motor would not have worked. Why? Because the polarity reversal would not happen at the right moment. What is the right moment? It is when the magnetic field of the coil is in line with the magnetic field of the permanent magnets.

Through the coil actually flows an alternating current, i.e., a square wave AC.

That’s why it is actually an AC motor, although it is fed by a DC source. And there is no essential difference between this AC motor and the one from the beginning of this post. There we had a sine wave AC, here we have a square wave AC.

P.S. I have written several times about this and also other misconceptions regarding the electric motors (such as “magnetic locking”, “rotating magnetic field” etc.). I will cite here only my last article:

All the theory about AC generators and motors is wrong!