A brushless or BLDC motor is a motor that does away with brushes and uses electronic commutation to generate rotation.
In the article on the DC motor, we saw its major weakness: the mechanical commutator. The brushes generate friction, heat, and sparks, and they wear out with use.
Brushless motors eliminate the brushes, at the cost of requiring a more complex controller (an ESC).
Comparison with DC Motors
| Characteristic | DC Motor (Brushed) | Brushless Motor (BLDC) |
|---|---|---|
| Wires | 2 (+ / -) | 3 (Phases U, V, W) |
| Controller | Optional (Direct to battery) | Mandatory (ESC) |
| Efficiency | 50% - 70% | 85% - 95% |
| Maintenance | Brush replacement | None (bearings only) |
| Noise | High (friction) | Quiet (electrical hum) |
| Cost | Cheap | Expensive (requires ESC) |
Anatomy: The Total Inversion
If you open a Brushless motor (for example, one from a drone), you’ll see it’s the exact opposite of what you might expect:
- Stator (Stationary): The copper coils. They are fixed in the center. They do not move.
- Rotor (Rotating): The outer casing (the bell) is filled with glued permanent magnets. It is the casing that rotates.
With no electrical physical contact between the stationary and rotating parts, there are no sparks. The only friction is from the shaft bearings (balls).
The 3 Wires: Three-Phase Current
If you look at a Brushless motor, you’ll see it has 3 wires (usually yellow, red, black, or three black ones). This confuses many people: “If it’s a DC motor (Direct Current), why does it have 3 wires and not 2?”
Because in reality, a Brushless motor is a Three-Phase Alternating Current motor. The thing is, we power it with DC batteries.
The internal coils are connected in 3 groups (Phases A, B, and C). For the motor to spin, we must activate the coils in a very precise sequence to create a Rotating Magnetic Field. The rotor’s magnet will try to chase that magnetic field, causing the rotation.
The ESC (Electronic Speed Controller)
Do not connect a brushless motor directly to a battery. Applying fixed DC between phases locks the rotor and can produce a current capable of damaging the windings or the battery.
Since we have removed the “mechanical commutator” that performed automatic switching in DC motors, we need an Electronic Commutator. That is the ESC.
The ESC is a three-phase inverter. It takes the direct current from the battery and generates three sinusoidal (or trapezoidal) waves phase-shifted 120° from each other.
How does the ESC know when to switch phases?
For the motor to spin smoothly, the ESC needs to know exactly where the rotor is at every millisecond. It has two ways of knowing this:
- Sensored: The motor has internal Hall Sensors that tell the ESC the position. Used in RC cars and scooters for high torque from a standstill.
- Sensorless: This is most common in drones. The ESC reads the Back-EMF (the voltage generated by the spinning motor) on the phase that is off to figure out where the magnet is.
- Problem: At startup, since the motor is stationary, there is no Back-EMF. This is why drone motors sometimes “stutter” when starting until they gain speed.
The Key Data: KV (Velocity Constant)
When choosing a brushless motor, one of the important parameters is
KV = RPM per Volt. It is a physical constant that tells us the theoretical speed the motor will spin at for every Volt applied to it (with no load).
Practical Example
You have a 2300KV motor and a 3S LiPo battery (11.1V).
The Basic Rule of KV
- High KV (e.g., 2300KV): Spins very fast, but has less Torque. Used with small propellers (racing drones).
- Low KV (e.g., 900KV): Spins slowly, but has a lot of torque. Used with large propellers (photography drones) or for moving wheels.
Want to lift heavy loads? Use low KV and high voltage (6S batteries). Want to go very fast? Use high KV.
Types: Inrunner vs Outrunner
Depending on how they are built, there are two families:
- Outrunner (External Rotor):
- What spins is the outer casing.
- Advantage: Having the magnets far from the center gives them more leverage. They have a lot of Torque.
- Usage: Drones, airplanes, electric scooters.
- Inrunner (Internal Rotor):
- What spins is the internal shaft (like a normal DC motor). The coils are on the outside.
- Advantage: They spin extremely fast (can reach 60,000 RPM).
- Usage: High-speed RC cars, ducted fans (EDF).
Regenerative Braking and “Active Braking”
An incredible advantage of modern Brushless systems is digital control. In the ESC firmware (like BLHeli_S or Bluejay), we can activate “Damped Light” or Active Braking.
When you throttle down a DC motor, it continues spinning due to inertia. In a Brushless motor with active braking, the ESC inverts the magnetic field to brake the motor instantly. Additionally, when braking, the motor acts as a generator and recharges the battery a little (Regenerative Braking). This is key for the stability of acrobatic drones.
