How Sensorless Brushless DC (BLDC) Motors Work

Introduction

Sensorless control of Brushless DC (BLDC) motors is a widely adopted technique in applications where reliability, cost and physical constraints are paramount. By eliminating physical position sensors, sensorless BLDC motor systems reduce complexity, improve durability and enable operation in environments where sensors would be impractical or prone to failure. This lesson explores the motivations for sensorless BLDC motor control, explains the principle of back electromotive force (back-EMF) sensing and compares sensorless and sensored approaches.

Advantages

• Lower system cost
  • Eliminating Hall sensors, wiring, connectors and mounting hardware reduces the bill of materials. Even in high‑volume designs, position sensors can add several dollars to the total system cost. Sensorless control minimizes this expense.
• Improved reliability
  • Fewer components mean fewer potential failure points. This is especially important in aerospace, military, automotive and industrial applications where long‑term reliability is critical. Removing sensors also eliminates failures caused by vibration, thermal stress, or connector fatigue.
• Simpler mechanical design
  • Sensorless control removes the need for precise sensor alignment relative to the rotor. This improves manufacturability and reduces tolerance and calibration requirements during production.
• Better suitability for harsh or sealed environments
  • Sensorless BLDC motors are well-suited for applications where the motor is sealed, immersed in fluid, or exposed to dust and contaminants (for example, pumps, fans and fuel systems). Sealing windings is straightforward, while sealing electronic sensors is more challenging.
• Reduced axial length and smaller form factor
  • Position sensors and their electronics add length along the motor shaft. Sensorless designs are preferred where flat or compact motors are required.
• Good performance at medium to high speeds
  • At higher speeds, back‑EMF signals are strong and well defined, allowing accurate position and speed estimation.

Disadvantages

• Poor or no position information at standstill and low speed
  • Back Electromotive Force (Back‑EMF) is proportional to motor speed and is zero at standstill. As a result, sensorless systems cannot determine rotor position at startup and at very low speeds. Special startup techniques such as open‑loop or blind commutation are required.
• More complex control algorithms
  • Sensorless control requires mathematical models, observers, or estimators (such as Phase‑Locked Loop (PLL) estimators). These algorithms demand more processing power and careful tuning compared to simple Hall‑based systems.
• Sensitivity to noise and parameter variation
  • Back‑EMF signals can be affected by Pulse Width Modulation (PWM) switching noise, motor parameter variation and temperature changes. Poor filtering or inaccurate motor models can degrade performance.
• Reduced low‑speed torque control
  • Because position estimation is unreliable at low speeds, torque ripple and control instability may occur during slow operation or under rapidly changing loads.
• Startup complexity
  • Sensorless motors require a controlled startup sequence to accelerate the rotor until sufficient back‑EMF is generated. Incorrect startup tuning can cause failed starts or reverse rotation.

Why Sensorless?

Sensorless BLDC motor control is preferred in many demanding applications, such as aerospace, military and industrial systems, where reliability is critical. Removing sensors reduces the number of components, thereby minimizing potential failure points and increasing overall system reliability. Physical space restrictions, especially in applications requiring flat or compact motors, make sensorless control advantageous, as motor detection circuits and sensors add bulk to the motor axis.

Manufacturability is also improved, as sensor alignment and calibration are eliminated, reducing production complexity and tolerance issues. In environments where the motor operates flooded (immersed in fluid, such as fuel pumps), sensorless control avoids sealing challenges associated with electronic components. Additionally, sensorless systems are more cost-effective, particularly in high-volume or low-power applications, as position-sensing electronics can add several dollars to the cost of each motor.

What is Back Electromotive Force (Back‑EMF)?

Back-EMF is a voltage generated in the stator windings of a BLDC motor as the Permanent Magnet (PM) rotor spins past them. This induced voltage is directly proportional to the motor’s speed and is a key parameter for sensorless position detection. Back-EMF is absent when the motor is stationary, which means sensorless BLDC motors require a special startup procedure, often called “blind commutation,” until sufficient back-EMF is generated for reliable position estimation.

During operation, every commutation sequence energizes two of the three motor windings, leaving the third winding open. The back-EMF in this unpowered winding is measured and compared to the motor’s neutral point. The zero crossing of the back-EMF (when it equals half the supply voltage, Vbus/2) indicates the optimal moment for commutation. Once the motor is spinning and back-EMF is detectable, commutation timing is synchronized to these zero crossings.

Microchip’s motor control solutions provide the necessary hardware and software to monitor back-EMF, filter noise and implement observer algorithms for accurate sensorless control.

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Back-EMF vs. Hall Sensors

Sensorless BLDC control using back-EMF detection offers several advantages: it eliminates the need for physical sensors, reduces system cost and increases reliability in harsh environments. It also simplifies mechanical design and is ideal for applications where sealing and space constraints are significant. However, sensorless control cannot detect rotor position at standstill or very low speeds, as back-EMF is only present when the motor is moving. This limitation requires complex signal processing and filtering, and performance may degrade in noisy environments.

In contrast, sensored BLDC control uses three Hall effect sensors mounted 120° apart in the motor to provide digital signals indicating rotor position every 60 electrical degrees. Hall sensors enable reliable position detection from zero speed to high speed, making startup and low-speed operation straightforward. They are simple, robust and easy to implement, but add cost and complexity due to sensors, wiring and mounting. Hall sensors can also be affected by Electromagnetic Interference (EMI) and provide only coarse position information.

A key difference in commutation timing is the 30-degree shift between back-EMF zero crossings and Hall sensor outputs, requiring sensorless commutation to be advanced by 30 degrees for optimal performance.

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Summary

Sensorless BLDC motor control is a powerful approach for applications demanding reliability, compactness and cost efficiency. By leveraging back-EMF sensing, engineers can eliminate physical sensors and simplify motor design, though careful attention must be paid to startup and low-speed operation. Sensored control remains valuable for applications requiring reliable position detection at all speeds. Microchip Technology provides comprehensive support for both sensorless and sensored BLDC motor control, enabling robust and efficient solutions for diverse industries.

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Learn More

• AN885 - Brushless DC (BLDC) Motor Fundamentals
• AN901 - Using the dsPIC30F for Sensorless BLDC Control
• AN1160 - Sensorless BLDC Control with Back-EMF Filtering Using a Majority Function
• Sensorless Field Oriented Control (FOC) for a Permanent Magnet Synchronous Motor (PMSM) Using a PLL Estimator and Equation-based Flux Weakening (FW)
• AN957 - Sensored BLDC Motor Control Using dsPIC® Digital Signal Controllers (DSCs)
• Microchip Motor Control and Drive
• Microchip Supported Motor Types
• Motor Control Application Framework (MCAF)
• motorBench® Development Suite
• Proportional Integral Derivative (PID) Compensator
• Motor Control Terminology