Most of the times you want to move something, you need some kind of motor, but not all motors work the same way. There are several families of motors and each of them shine in a different aspect, so being able to choose the correct motor for each project is not just an skill but a real need.

The first thing to take in account is that motors can be classified in different ways, mostly because they are designed having different goals in mind. But before getting into that, we need to talk a little about how do motors work and why and how to control them.

Induction AC motor

The induction AC motor was invented by various people at the same time, it’s basically a device that gets three power AC signals, in three-phase, each of them in a different coil. The coils are located in a triangle with a permanent magnet fixed in the shaft in the middle.

As the voltage on the coils change the magnet is attracted or repelled from them, and as the phases are well distributed they make the magnet rotate smoothly.

This is an extremely simplified model of the motor where you can see the magnet in the middle will rotate and align itself according to the currents in the coils:

AC signal control

In order to change the direction of the motor, the coils have to be activated in the opposite direction.

DC Motors

DC motors are just an evolution of that principle but having power lines that are easier to control. 3-Phase signals make sense only on high power applications, but in smaller devices we don’t have that kind of signals available.

Brushed DC motors

Both of AC motors above need changing currents to create magnetic fields so if we want to create DC motors we need to do some weird trickery!

Brushed motors are easy to understand in their simplest form. Imagine a shaft with an inductor in it and a couple of magnets on an enclosure around it, one facing the motor with the north pole and the other facing the motor with the south pole.

If you put a constant current on the inductor, it gets aligned with the magnets and stays that position, forced by the magnetic field. But if you change its polarity it is aligned the other way around, due to the same effect.

But we are talking about DC motors here, how do they change the direction of the current?

It’s just a mechanical solution called commutator: the shaft has a couple of contacts (brushes) that rotate with it, so when the shaft turns they alternate the contact with the positive and negative pole.

As always, there’s a great visual explanation of that in the Wikipedia: https://en.wikipedia.org/wiki/Brushed_DC_electric_motor#Simple_two-pole_DC_motor

The best thing of brushed motors is they are simple, as you saw, so they are cheap.

The worst part of the brushed motors is the concept itself: the shaft needs to have electrical contact with the exterior. The commutator is the weakest point of this device, because it wears out due to the friction and the high-temperature electric arcs that appear when the contact switches.

In summary, brushed DC motors have a mechanical DC-AC converter (the commutator), and they are just a simplification of the induction AC motor we introduced first.

DC signal control

The easiest way to think about a brushed DC motor is the following: when we apply a current they turn one way, and if we change the direction of the current they turn the other way around. Also: if you make them turn by hand they generate the current.

That’s probably all you know about DC motors¹, right?

But you know we didn’t came here to play around.

https://www.youtube.com/watch?v=yO9xIVv8ryc

TODO: Inrush current, shutdown currents, PWM, speed control, half drivers, full drivers (h-bridge)

Brushless DC

The existence of a brushed motor implies that brushless motors exist.

The brushless version is an improved idea on top of the previous. Instead of putting the magnets in the enclosure and putting the inductor inside, the brushless motors make the permanent magnets move so they don’t need to have contacts on the moving part and they electronically control the activation of the inductors, which now are in a fixed position.

This version doesn’t suffer from the friction on the commutator because there’s no physical commutator, but an electronic one. Of course, it means they need some control unit that increases the price of the motor.

But we already knew about that, right? That’s an induction AC motor.

Control

They are basically three phase AC motors that have an external controller that is driven with an DC supply signal and some control signals. The control signal activates the coils in the needed order.

They are called DC motors but what the motor gets is not a DC signal. They need an external controller. That’s cheating.

Motors by use

Now you understand the very basics of DC motors, it’s time to classify them by their goal.

AC motors

These were already explained, they are more efficient, specially the induction motors, and all that so they are cool for things like electric cars industrial environments and the like.

DC motors

They are the kinds already explained, they are designed for smooth multiple-turn operation: drone propellers, toy electric cars…

Stepper motors

They are a type of brushless motor designed for replicable movement based on steps and high torque.

According to their internal structure they can be unipolar or bipolar.

Servo motors

Both brushed and brushless, they can be multiple-turn or one-turn (or just an angle).

The famous SG-90 has 180 degrees of range.

https://www.latex-tutorial.com/tutorials/more-circuitikz/#Labels