How to Control Brushless Motors (Part 3): Commutation

Why It Matters

Choosing the right commutation strategy directly influences motor efficiency, torque smoothness, and suitability for low‑speed or high‑precision applications, affecting product cost and performance across robotics, automation, and consumer devices.

Key Takeaways

• Trapezoidal commutation uses Hall sensors, 60° resolution
• Sinusoidal commutation relies on high‑resolution encoders for smooth torque
• Sensorless commutation derives angle from back‑EMF, unsuitable at low speed
• Torque ripple occurs at Hall‑state transitions in six‑step control
• Phase‑init algorithms eliminate Hall sensors for linear BLDC designs

Pulse Analysis

Accurate commutation is the linchpin of modern brushless DC motor performance. By continuously aligning the stator’s magnetic vector with the rotor’s position, controllers maximize the useful Q‑force while suppressing the opposing D‑force. This alignment is achieved through three distinct approaches. Trapezoidal, or six‑step, commutation relies on Hall‑effect sensors that generate six discrete states per electrical cycle, offering a simple, low‑cost solution but limiting angular resolution to 60 degrees. The resulting torque ripple and modest efficiency loss become noticeable in high‑precision or high‑torque applications.

Sinusoidal commutation upgrades the process with encoder‑derived position feedback, delivering near‑continuous angle updates. The resulting sinusoidal drive waveforms produce smooth torque, eliminate the stair‑step ripple of six‑step control, and improve overall efficiency. While many designs retain Hall sensors for initial phase detection, algorithmic phase‑initialization techniques can forgo them entirely, a practice gaining traction in linear BLDC systems where embedding Hall sensors is costly. This encoder‑centric model, however, adds sensor and processing overhead, raising component cost and firmware complexity.

Sensorless commutation removes external position sensors altogether, estimating rotor angle from back‑EMF voltage on the undriven winding. It excels at moderate to high speeds where back‑EMF is strong, making it ideal for applications like hard‑disk spindles or ceiling fans that operate primarily in velocity‑control mode. The method falters at low speeds or during start‑up, and its susceptibility to electrical noise demands careful filtering. Engineers must weigh these trade‑offs—cost, performance, and application demands—when selecting a commutation strategy, a decision that will shape the next generation of motion‑control products. The upcoming Part 4 will explore how current‑control loops and power‑stage amplifiers close the loop on these commutation techniques.