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Introduction to Six-Step Sensored Brushless DC (BLDC) Motor Commutation

Last modified by Microchip on 2026/05/11 15:59

Introduction

Six‑step commutation using Hall effect sensors is one of the most common and reliable methods for controlling Brushless DC (BLDC) motors. This approach combines simple digital rotor position sensing with a deterministic commutation sequence that is easy to implement in embedded controllers. In this lesson, we explain how Hall effect sensors determine rotor position (also called the commutation sector), how those signals drive the six‑step firing order, and how Microchip Technology’s dsPIC® Digital Signal Controllers (DSCs) efficiently implement this control method in real hardware.

Rotor Position Detection with Hall Effect Sensors

A Hall effect sensor is a magnetic field sensor that produces a digital output when exposed to a magnetic field. In a BLDC motor, three Hall effect sensors—commonly labeled H1, H2, and H3—are mounted inside the motor and spaced 120 electrical degrees apart. As the permanent‑magnet rotor rotates, the magnetic field alternately activates and deactivates each sensor.

Each Hall sensor outputs either a logic HIGH (1) or logic LOW (0) depending on the rotor’s position. The three outputs together form a 3‑bit digital code that uniquely identifies which of the six valid electrical sectors the rotor is currently in. These sectors divide one electrical revolution into six regions of 60 electrical degrees each.

The controller continuously reads the Hall sensor signals to know the rotor position, often referred to as the sector. Based on this sector, the controller determines which motor phases should be energized.

Six‑Step BLDC Controller Firing Order

Brushless DC Motor Six Step BLDC Controller Firing Order Sector 1 of 3

Brushless DC Motor Six Step BLDC Controller Firing Order Sector 2 of 3

Brushless DC Motor Six Step BLDC Controller Firing Order Sector 3 of 3

A six‑step BLDC controller energizes the motor windings in a fixed sequence known as the firing order. At any given time, two of the three motor phases are energized—one connected to the positive supply and one connected to ground—while the third phase is left floating. This produces a stator magnetic field that pulls the rotor forward.

As the rotor moves, the Hall sensor states change. Each new Hall code corresponds to the next commutation step. The controller uses a commutation table (often implemented as a lookup table) that maps each Hall sensor combination to a specific pair of energized phases. The commutation sequence repeats every electrical cycle, ensuring the stator magnetic field rotates synchronously with the rotor.

This method provides inherently synchronized commutation, meaning the electrical field always aligns correctly with the rotor magnets. Because of this, Hall‑based six‑step control delivers reliable startup, stable low‑speed operation, and predictable torque behavior.

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Practical Implementation With dsPIC® DSCs

Microchip Technology’s dsPIC DSCs are well-suited for Hall‑sensored BLDC control. These devices include dedicated Motor Control Pulse Width Modulation (PWM) modules that can directly drive the six Metal‑Oxide‑Semiconductor Field‑Effect Transistors (MOSFETs) of a three‑phase inverter. Each inverter leg is controlled by a high‑side and low‑side PWM output, with built‑in dead‑time insertion to prevent shoot‑through.

Hall sensor inputs are commonly connected to Change Notification (CN) or external interrupt pins. Whenever a Hall signal changes, an interrupt is generated. Inside the interrupt service routine, the controller reads the Hall sensor state, determines the current sector, and updates the PWM outputs according to the commutation table.

Microchip application notes (such as the application note "Sensored BLDC Motor Control Using dsPIC Digital Signal Controllers (DSCs)") provide complete reference implementations of this approach.

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Efficient DSP Execution in Commutation and Control Loops

Efficient DSP Execution in Commutation and Control Loops

While basic six‑step commutation does not require heavy computation, many systems also perform current measurement, filtering, or torque control. dsPIC DSCs include a Digital Signal Processing (DSP) engine optimized for these tasks.

In a single instruction cycle, the DSP engine can perform multiple operations, including two data fetches, pointer updates, a multiply‑accumulate (MAC) operation, and a write‑back to memory. For example, a MAC instruction multiplies two 16‑bit values, aligns them properly in a 40‑bit accumulator, and preserves the sign and fractional format (commonly 1.15 fixed‑point). This architecture enables fast, deterministic control loops without sacrificing commutation timing.

These features allow Hall‑based six‑step BLDC control to scale easily from simple on/off commutation to more advanced schemes such as current‑regulated trapezoidal control.

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Summary

Hall effect sensor‑based six‑step commutation is a proven and widely used method for BLDC motor control. Three digital Hall sensors provide rotor position information in the form of six valid sectors per electrical cycle. The controller uses this information to energize the correct pair of motor phases, producing a rotating magnetic field that drives the rotor smoothly and reliably. This approach is easy to implement, robust at startup and low speeds, and well supported by Microchip Technology’s dsPIC DSCs, which offer dedicated PWM hardware, fast interrupt handling, and powerful DSP capabilities for efficient motor control applications.

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