Why should you trust me

What is an ESC

An electronic speed controller (ESC) is a device used to control BLDC motors.

What is a BLDC motor

A brushless direct-current motor (BLDC motor) is a motor that does not contain physical brushes between the bell/core and stator, which are the rotating/stationary portions of a motor respectively. However, the name is a misnomer. These motors are not DC; far from it. The name comes from the fact that ESCs take DC current. The motors themselves operate on 3-phase alternating current.

What is 3-Phase Alternating Current

The name is a little scary. But it's really not that complicated. Let's break it down into 3 fundamental concepts.

Concept 1: Electromagnets

When current flows through a wire, it creates a magnetic field. And when combined with something ferromagnetic, like an iron nail, the field strength can be significantly increased. Inside a brushless motor stator, you'll find several electromagnets just like this.

Concept 2: Pushing and Pulling Permanent Magnets

Concept 3: Using Magnets to Spin Something

Pulling it all together

Let's review.

1. You can make a magnet that turns on and off with electricity.

2. You can change that magnet's poles by changing which direction electricity flows; pushing or pulling a permanent magnet.

3. If you mount those electromagnets in a circle with a permanent magnet in the center, you can push or pull on that magnet to make it spin by alternating the current in the coils.

Ok - so what is 3-phase alternating current?

It's a fancy way of saying we're alternating the current in 3 electromagnets. Or, in modern brushless motors, 3 groups of electromagnets. Even though some motors have more than 3 electromagnets - they always have multiples of 3, because they are always tied together into 3 "phases" or groups of magnets.

Fundamentally, energizing these coils one way or another at the right time is all an ESC needs to do. That is literally it.

The circuit

With some clever wiring, we can make a circuit comprised of 6 on-off switches that can push current through those 3 phases in either direction. Let's take a look at it.

Let the 3 colored loops, Phase A, Phase B, and Phase C represent the electromagnetic coils in the motor.

Let the 6 switches, 1,2,3,4,5,6 represent... switches.

Finally, note that the top of the circuit is positive and the bottom is negative.

Let's see what happens to the phases when we turn on and off some of these switches.

What Not to Do

Energizing a Phase Properly

This is fundamentally what an ESC is. 6 on-off switches. In the most basic case, they toggle like so, which steps a brushed motor through one rotation. The speed at which you toggle them corresponds to your motor speed.

The not-so-basics

What Are the Switches on Real ESCs?

They are N-Channel Mosfets 99% of the time. Sometimes, the switches closer to positive, or the high-side switches, are P-Channel Mosfets. Mosfets are basically solid-state relays. They are beefy on-off switches that can be triggered with a lower voltage signal, very quickly. If you want to learn more about N channel and P channel mosfets, this is a good resource, but the basic thing you need to know here is that it is challenging to use N channel mosfets for the high-side switches, and using P channel mosfets, while easier, results in a significantly worse ESC.

Mosfets and High/Low side Switches

This explanation pertains only to N-Channel mosfets. P-Channel mosfets are a little different.

Mosfets have 3 terminals, and here's what they do.

1. Source: beefy switch input/output

2. Gate: use this to turn on/off the beefy switch.

3. Drain: beefy switch input/output

If you google mosfet, you'll see a lot of people talking about VGS. This stands for voltage from gate to source. On an N channel mosfet, VGS must be beyond a certain threshold for the mosfet to "activate" or "turn on" (allow current to flow through source and drain). 

Shawty got Low Low Low

On the low side this is a piece of cake. Stick the source pin on ground, and as long as the gate is sufficiently higher than ground, the mosfet opens. On logic level mosfets, VGS can be quite low; 5 volts sometimes. So, a standard arduino can turn the mosfet on and off no problem.

Shawty got High High High (Which is bad - because drugs, are bad.)

On the high side this is difficult. That's because you have no choice but to wire the source pin to vbat. And that means that to turn the mosfet on, you need the gate voltage to be HIGHER than the source voltage; which is vbat. So, uh, how the heck do you even do that??? Well... you're gonna need a gate driver. They're kinda complex; and I won't explain them here. But... just think of them as relatively complicated additions to your circuit.

Dead Timing and Timing in General

Remember above how I told you what not to do? Well, you really don't want to do that. And if you're spinning a motor really quickly, your switches are turning on and off thousands of times per second. And if for even one god damn microsecond you close the wrong switch, your ESC instantly explodes and probably also your battery.

To avoid this shoot through, you need dead time. This is time where both the high and low side switch of a given phase are off so you don't accidentally make a short circuit. Inserting this dead time, as well as switching the switches quickly in the first place, very quickly becomes nontrivial at high motor speeds. This means you need extremely advanced microprocessors, like the STM32G4, which has hardware timers that are designed specifically to solve this problem.

Sometimes, gate-driver ICs have programmable deadtime as well, which is how my only working ESC works. It uses a teensy 4.0, which is not nearly as good as an STM32 for this purpose, but gets its deadtime from gate driver IC's that control both the low and high side switches per-phase.

Conclusion

Like most engineering, ESCs are simple in principal but hard in practice. In essence, they are 6 on-off switches. In practice, they're a masterclass in precision hardware timing, EMI reduction, mosfet feathering, BEMF detection, and so much more.

Don't let this disuade you though! They are a very fun learning exercise. Just know - making a good one will not be easy. Ask me how I know :)