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Learn-Six Step Sensorless Brushless DC (BLDC) Motor Commutation

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

Introduction

Six‑step sensorless control is one of the most widely used techniques for driving Brushless DC (BLDC) motors without mechanical position sensors. Instead of Hall effect sensors or encoders, the controller estimates rotor position by observing the electrical behavior of the motor itself—specifically the back Electromotive Force (back‑EMF)  generated by the spinning rotor. This lesson explains how back EMF zero‑crossing detection is used for commutation in a six‑step sensorless BLDC controller, discusses practical issues such as noise and inductive ringing, and introduces the concept of phase advance for extending speed range.

Principle of Sensorless Six‑Step Commutation

Brushless DC Motor Sensorless Six-Step Commutation

In a three‑phase BLDC motor, six‑step commutation divides each electrical revolution into six sectors of 60 electrical degrees. At any instant, two of the three motor phases are actively driven—one connected to the positive DC bus and one to ground—while the third phase is left electrically floating.

In a sensorless system, this floating phase becomes the key to rotor position estimation. As the rotor’s permanent magnets pass the stator windings, they induce a voltage in the unpowered phase. This voltage is known as back-EMF. The back-EMF is proportional to motor speed and has a predictable phase relationship with rotor position.

By monitoring the voltage of the floating phase, the controller can determine when the back-EMF crosses zero volts. This event, called a zero crossing, indicates that the rotor has reached the midpoint of the current commutation sector.

Zero‑Crossing Detection and Commutation Timing

Brushless DC Motor Zero-Crossing Detection and Commutation Timing

Every 60 electrical degrees, a different motor phase is left floating, and its back-EMF is monitored. When the back-EMF of that phase crosses zero, the controller detects the event using either an analog comparator or an Analog‑to‑Digital Converter (ADC) with digital processing.

The zero crossing does not occur at the optimal commutation point. Instead, there is a known phase offset—typically 30 electrical degrees—between the zero crossing and the ideal moment to switch phases. To compensate, the controller waits for a fixed delay after detecting the zero crossing before advancing to the next commutation step. This delay is usually generated using a timer and is adjusted based on motor speed.

Conceptually, the zero crossing marks the center of the commutation interval, while the actual commutation occurs at the boundary between sectors to ensure maximum torque production.

Practical Signal Considerations

In real systems, the back-EMF signal is not a clean waveform. At the beginning and end of each commutation interval, inductive ringing can occur due to the motor winding inductance and inverter switching. In addition, Pulse Width Modulation (PWM) switching and electromagnetic noise from the motor introduce significant disturbances into the measured back EMF signal.

To ensure reliable zero‑crossing detection, these signals must be filtered. Microchip Technology provides digital filtering techniques, such as majority‑function filters and synchronized ADC sampling, to suppress noise and improve detection accuracy without requiring external analog hardware.

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Phase Advance and High‑Speed Operation

Brushless DC Motor Phase Advance and High Speed Operation

As motor speed increases, the inductance of the windings and switching delays in the inverter cause the phase current to lag behind the applied voltage. If commutation occurs exactly at the zero‑crossing‑based timing, the current peak arrives too late, reducing torque and efficiency.

To compensate, controllers use phase advance, also known as commutation advance or lead angle. Phase advance intentionally commutates the motor earlier than the nominal zero‑crossing‑based timing. In practice, this means scheduling the next commutation using a timer before the expected position transition would normally occur.

Phase advance is typically proportional to speed—little or no advance at low speed, increasing advance at higher speeds. This technique substantially increases the usable speed range of a BLDC motor, improves high‑speed torque, and can even compensate for minor misalignments in Hall‑effect sensors in sensored systems.

Microchip’s dsPIC® Digital Signal Controllers (DSCs) support phase advance through flexible timer peripherals and high‑resolution PWM modules. Application notes such as Using the dsPIC30F for Sensorless BLDC Control demonstrate up to 30 electrical degrees of programmable phase advance for improved efficiency and extended speed range.

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Summary

In a six‑step sensorless BLDC controller, rotor position is estimated by monitoring the back-EMF generated in the floating motor phase. Zero‑crossing detection provides a reliable reference for commutation timing, with a fixed delay—typically 30 electrical degrees—used to align switching with optimal torque production. Practical challenges such as inductive ringing and PWM noise require careful filtering. At higher speeds, phase advance further improves performance by compensating for current lag. Microchip Technology supports these techniques with dsPIC DSCs, advanced PWM and ADC peripherals, and proven reference designs that simplify the implementation of robust sensorless BLDC motor control.

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