How to Control Brushless DC Motor Speed?

Speed Control - A Core Advantage of Brushless DC Motors

Brushless DC motors (BLDC) have become the preferred power source in numerous fields like industrial automation, home appliances, power tools, drones, and electric vehicles, thanks to their significant advantages: high efficiency, long lifespan, high torque density, and low noise. However, one of the keys to unlocking their full potential is precise speed control. Whether it's a fan application requiring constant RPM or a servo system demanding dynamic response, mastering effective speed control methods is crucial.

As a professional factory specializing in the R&D and production of high-quality brushless DC motors (BLDC) and universal motors, we understand the critical importance of precise speed control for your applications. This article will clearly explain the fundamental principles and main methods of BLDC motor speed control, helping you select the optimal solution for your needs.

• Step 1: The Core Principle of Speed Control

  The rotational speed (N) of a brushless DC motor is directly related to its input voltage (V) and the motor's internal back-EMF constant (Ke). The fundamental relationship can be approximated as:

  N ∝ (V - I * R) / Ke

  Where:

  • N = Rotational Speed (RPM)
  • V = Voltage applied to the motor terminals (Volts)
  • I = Armature current (Amps)
  • R = Armature resistance (Ohms)
  • Ke = Motor's back-EMF constant (Volts/RPM or Volts/rad/s)

  Key Takeaway: To change the motor speed, the most direct and effective method is to alter the average voltage applied to the motor terminals. The influence of current (I) and resistance (R) is relatively minor (especially under no-load or light-load conditions), and Ke is an inherent characteristic of the motor. Therefore, regulating the input voltage is the foundation of speed control.

• Step 2: Main Methods for Brushless DC Motor Speed Control

  Several mature and widely used techniques achieve voltage regulation and thus speed control:

  • 1.PWM (Pulse Width Modulation) Control:
    • Principle: This is the most common and efficient method. The controller rapidly switches (typically at frequencies ranging from several kHz to tens of kHz) the power voltage applied to the motor windings. By changing the ratio of "on" time (pulse width) to the total period within each switching cycle (duty cycle), it regulates the average voltage applied to the motor.
    • Advantages: High efficiency (low switching losses), high control precision, fast response, easy digital implementation, often incorporates current limiting.
    • Implementation: Requires a dedicated BLDC controller/driver, typically containing power switches (MOSFETs/IGBTs), control logic (microcontroller/DSP), and position/current feedback interfaces. Our factory can provide custom or recommend matched high-performance drivers.
  • 2.Voltage Regulation (Variable DC Link Voltage):
    • Principle: Directly changes the DC bus voltage (Vdc) supplying the BLDC driver. Higher voltage allows for a higher maximum speed (under the same load).
    • Implementation: Typically requires an adjustable DC power supply or a front-end AC-DC converter (like a controlled rectifier).
    • Advantages: Conceptually simple and direct.
    • Disadvantages: Lower efficiency (especially when reducing speed, as energy is dissipated in regulating devices), limited speed range (motor may stall or run unstably at very low voltages), slower dynamic response. Generally less popular than PWM for modern high-performance applications.
  • 3.Altering Commutation Timing (Field Weakening Control):
    • Principle: Above base speed (the maximum speed at rated voltage), advancing or retarding the commutation angle artificially weakens the motor's internal magnetic field. This allows the motor to accelerate further beyond its theoretical base speed without increasing the input voltage.
    • Application: Primarily used to extend the motor's speed range, especially in applications requiring high-speed operation (e.g., electric vehicle highway cruising, spindle motors).
    • Requirements: Needs complex control algorithms and precise position feedback (usually requiring a high-resolution encoder), implemented by advanced drivers. Some of our high-end BLDC drivers support field weakening control.
  • 4.Closed-Loop Speed Control (Using Feedback):
    • Principle: When operated open-loop (especially with PWM), changes in load cause speed variations. To achieve precise, stable speed control regardless of load changes, speed feedback must be introduced to form a closed loop.
    • Feedback Sensors:
      • (1) Hall Sensors: Built into the motor, provide commutation timing and can be used for moderate-accuracy speed estimation.
      • (2) Encoder: (Incremental or Absolute) Provides high-resolution position and speed information for the highest accuracy.
      • (3) Sensorless Algorithms: Estimate rotor position and speed by detecting the motor's back-EMF (BEMF), eliminating the need for extra sensors (lower cost, but performance may be limited at very low speeds or during startup).
    • Control Method: A speed controller (usually a PID controller) compares the actual speed with the set speed, generates an error signal, and adjusts the PWM duty cycle (or current command) to make the actual speed precisely track the setpoint. We offer driver options with built-in closed-loop control algorithms and support for various feedback schemes.
  • 5.Current Control (Indirect Speed Influence):
    • Principle: In a closed-loop system, the output of the speed loop typically becomes the setpoint for the current loop. Precisely controlling the phase currents (torque-producing current) is fundamental to achieving the desired torque and dynamic response. Since torque directly affects acceleration and the ability to counteract load disturbances, it indirectly impacts speed stability and response.
    • Implementation: Requires current sensing circuits (e.g., shunt resistors, Hall effect current sensors) and a fast-responding current loop controller (often also PID). This is core to high-performance drivers (like FOC drivers).
    • Control Method: A speed controller (usually a PID controller) compares the actual speed with the set speed, generates an error signal, and adjusts the PWM duty cycle (or current command) to make the actual speed precisely track the setpoint. We offer driver options with built-in closed-loop control algorithms and support for various feedback schemes.
  • 6.Advanced Control Algorithms (e.g., FOC - Field-Oriented Control):
    • Principle: FOC is an advanced vector control technique. It decomposes the three-phase currents into a component creating magnetic flux (Id) and a component producing torque (Iq), controlling them independently and precisely. This results in smoother motor operation, higher efficiency (especially at partial loads), faster and more linear torque response, and more precise speed control.
    • Advantages: Delivers optimal dynamic performance, efficiency, and speed control precision.
    • Requirements: Needs a powerful processor, accurate current and position sensing, and complex algorithms. We offer high-end BLDC driver solutions based on FOC algorithms for demanding applications.

• Step 3: Choosing the Right Speed Control Solution

  The choice of method depends on your specific application requirements:

  • Cost-sensitive, simple speed adjustment: Open-loop PWM control (with Hall sensors or sensorless startup).
  • Requires speed stability, resistance to load changes: Closed-loop PWM control (with Hall sensors, encoder, or sensorless algorithms).
  • Needs wide speed range (above base speed): Combine with field weakening control.
  • Pursuing ultimate performance, efficiency, and smoothness: FOC control (usually requires encoder feedback).
  • Ultra-high-speed applications: May require voltage regulation combined with field weakening.

• Step 4: Why Partner with a Professional Micro-Motor Manufacturer?

  Precise speed control relies not only on excellent control algorithms and drivers but also on the inherent quality of the motor itself:

  • Consistent electromagnetic characteristics (Ke): Ensures predictable speed vs. voltage/duty cycle relationships.
  • Low resistance (R): Reduces internal losses, improving efficiency and control precision.
  • High-quality magnets and windings: Guarantee stable torque output and back-EMF waveform.
  • Precision manufacturing: Affects balance, noise, lifespan, and indirectly influences control performance.
  • Optimal motor-driver matching: A professional factory can provide optimized motor + driver package solutions.

• YYMOTOR - Your Brushless DC Motor Solution Expert

  As a professional manufacturer with years of experience in the micro-motor field, we specialize in the design, development, and production of brushless DC motors (BLDC) and universal motors. We not only supply high-quality, highly reliable motor units but can also tailor solutions to your specific application needs:

  • Custom develop BLDC motors with specific speed ranges, torque characteristics, and feedback options.
  • Recommend or provide advanced driver solutions supporting various speed control strategies like PWM, closed-loop control, and FOC.
  • Offer comprehensive technical support to help you achieve optimal speed control performance.