BLDC structure: electronic switches instead of brushes
A motor in which the commutator of a brushed DC motor is replaced by an inverter and sensors. Inrunner and outrunner, pole count, electrical and mechanical angle.
Difference from brushed DC
In a brushed DC motor, the brushes and commutator mechanically switch the coil current direction as it turns. That brings friction, sparks and wear. A BLDC puts the coils on the stator and the magnets on the rotor, and the inverter switches the current direction electronically.
| Brushed DC | BLDC | |
|---|---|---|
| Commutation | Brushes + commutator | Inverter + position information |
| Life | Brush wear | Bearing life |
| Efficiency | Low | High |
| Driving | Turns when voltage is applied | A controller is required |
Structure
- Stator: iron core (laminated thin steel sheets) and three-phase coils (U, V, W)
- Rotor: permanent magnets
- Inrunner: rotor on the inside (industrial, high speed)
- Outrunner: rotor on the outside (drones, gimbals, in-wheel, high torque)
Electrical and mechanical angle
With p magnet pole pairs, electrical angle = mechanical angle × p. A 14-pole (7 pole pair) motor goes round electrically seven times per revolution (360° mechanical). Hall sensors, commutation tables and FOC calculations are all in electrical angle.
BLDC and PMSM
The structure is nearly the same; the back-EMF waveform differs. It is common to call a motor with trapezoidal back-EMF a BLDC (suited to six-step) and one with sinusoidal back-EMF a PMSM (suited to FOC).