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Introduction to Six Step Timed Brushless DC (BLDC) Motor Commutation

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

Introduction

Six‑step commutation, also known as trapezoidal control, is one of the simplest and most widely used techniques for driving a three‑phase Brushless DC (BLDC) motor. It is commonly used in low‑ to medium‑cost applications where simplicity and robustness are more important than ultra‑smooth torque. This lesson explains how timer‑based six‑step commutation works, how the inverter hardware energizes the motor windings, and how a revolving electrical field is created to rotate the rotor.

Concept of Timer‑Based Six‑Step Commutation

In timer‑based commutation, the controller does not measure rotor position directly. Instead, it advances the motor commutation states at fixed time intervals using a timer. Each commutation step energizes a specific pair of motor windings, creating a rotating magnetic field in the stator. If the timing is approximately correct, the rotor magnets will follow this rotating field and the motor will spin.

A three‑phase BLDC motor has six electrical sectors per electrical revolution, with each sector spanning 60 electrical degrees. The controller repeatedly cycles through these six sectors. Between each sector transition, a delay is introduced using a timer. Increasing or decreasing this delay changes the electrical frequency, which directly controls motor speed.

Winding States and Electrical Field Rotation

Brushless DC Motor Winding States and Electrical Field Rotation

In each commutation sector, only two of the three motor windings are energized. This energization pattern creates a current path from the high‑side winding to the low‑side winding, producing a magnetic field in the stator. The third, unpowered winding is not actively driven. The sequence is repeated for all six sectors, with the role of each winding changing from sector to sector.

The voltages often shown in six‑step commutation diagrams represent the back electromotive force (back‑EMF) generated by the motor as it spins. These are not the actual voltages applied by the controller. In practice, the inverter applies square‑wave voltages to the motor phases using power transistors.

As the controller advances through the six commutation states, the stator’s magnetic field appears to rotate. This revolving electrical field interacts with the permanent magnets on the rotor, causing the rotor to rotate and produce mechanical motion.

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Inverter Hardware and dsPIC® DSC Implementation

A typical BLDC inverter uses six Metal‑Oxide‑Semiconductor Field‑Effect Transistors (MOSFETs), arranged as three half‑bridges—one per motor phase. Each half‑bridge consists of a high‑side and a low‑side transistor. By turning these transistors on and off in specific combinations, the controller energizes the motor windings.

Using six Pulse Width Modulation (PWM) outputs from a dsPIC® Digital Signal Controller (DSC), each MOSFET can be individually controlled. This effectively implements a commutation state machine in software.

For example, starting from one commutation sector:

Brushless DC Motor MOSFET Diagram Sector 1 of 3

One step turns ON PWM2H (Phase 2 high‑side) and PWM3L (Phase 3 low‑side), energizing two windings while the third remains disconnected.

Brushless DC Motor MOSFET Diagram Sector 2 of 3

The next step turns ON PWM1H and PWM3L.

Brushless DC Motor MOSFET Diagram Sector 2 of 3

The following step turns ON PWM1H and PWM2L. The sequence continues until all six steps are completed, then repeats.

In every step, exactly two transistors are active—one high‑side and one low‑side—ensuring current flows through two windings while the third winding is floating. Dead time is typically inserted between switching events to prevent shoot‑through in the inverter.

Microchip Technology’s dsPIC33 motor‑control‑focused devices include high‑resolution PWM modules, built‑in dead‑time control, and timer peripherals that make implementing six‑step commutation straightforward and reliable. Reference designs and application notes from Microchip demonstrate this exact commutation sequence in real inverter hardware.

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Practical Considerations

Timer‑based commutation is simple and does not require position sensors or complex estimation algorithms. However, because commutation timing is not synchronized to the actual rotor position, efficiency and torque are lower than in sensored or sensorless back‑EMF‑based systems. This method is most commonly used for startup, demonstration, or low‑performance applications.

Despite its limitations, six‑step timer‑based control is an excellent learning tool and a foundation for understanding more advanced BLDC control techniques such as back‑EMF‑based sensorless control and Field‑Oriented Control (FOC).

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Summary

Timer‑based six‑step commutation drives a BLDC motor by energizing two of the three windings in a fixed six‑state sequence, using square‑wave voltages generated by an inverter. By advancing these states with a timer, a rotating stator magnetic field is created, causing the rotor to spin. Although simple and robust, this approach lacks precise rotor synchronization and is best suited for basic or low‑cost applications. Microchip Technology’s dsPIC DSCs provide dedicated PWM, timer, and safety features that make six‑step BLDC commutation easy to implement and an ideal stepping stone toward more advanced motor control strategies.

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