How To Control The Speed Of A BLDC Motor

Quick Summary

• Speed is set by PWM duty cycle on the inverter transistors inside the BLDC controller.
• Open-loop mode holds duty cycle constant; closed-loop PID holds speed constant under load.
• Feedback comes from Hall sensors, encoders, or back-EMF sensing in sensorless drives.
• 6-step trapezoidal control is simple and low cost; FOC provides smoother low-speed torque.
• Command input is usually analog 0-5V, potentiometer, PWM signal, RS485 Modbus or CAN bus.

Why Not Just Change The DC Voltage?

• Varying the bus voltage is inefficient and requires bulky linear regulators or variable power supplies.
• PWM at 15-20 kHz delivers the same average voltage effect with almost no loss in the switching devices.
• The motor windings act as an inductor and integrate the PWM pulses into a smooth current.
• PWM also lets a single fixed-voltage controller handle the full speed range from zero to rated speed.
• Modern BLDC controllers always use PWM, not linear voltage control.

Open-Loop PWM Speed Control

In open-loop mode, the controller maps the command input directly to a PWM duty cycle and applies it to the inverter. There is no feedback correction. Speed will sag when load increases because the back-EMF rises more slowly and the available torque margin shrinks. Open-loop control is simple and adequate for fans, pumps and other loads where a small speed variation is acceptable. It is not suitable for conveyors or AGV drives that need a stable speed under changing weight.

• Command input mapped linearly to PWM duty cycle
• No speed feedback, no PID loop
• Speed drops under increasing load
• Lowest cost, simplest firmware
• Typical use: fans, blowers, centrifugal pumps

Closed-Loop PID Speed Control

Closed-loop control adds a PID routine that reads actual speed from Hall sensors or an encoder and compares it to the target. The proportional term reacts to the current error, the integral term eliminates steady-state offset, and the derivative term damps overshoot. The PID output modulates the PWM duty cycle to hold the commanded speed even as load or voltage changes. This is the default control mode for industrial BLDC drives.

• Feedback source: Hall sensors, encoder or back-EMF
• PID tunes proportional, integral and derivative gains
• Holds target speed under load and voltage variation
• Required for conveyors, AGVs, winders and most automation loads
• Most Shenghe controllers default to closed-loop mode

6-Step Trapezoidal vs FOC Speed Control

The commutation method affects how smoothly speed is controlled, especially at low RPM. 6-step trapezoidal switches the active phase pair in discrete steps, producing a small torque ripple at each transition. FOC (Field Oriented Control) uses Park and Clarke transforms to produce smooth sinusoidal currents, giving the motor almost constant torque across the electrical cycle. At higher speeds the difference is minor, but at 10-20% of rated speed the FOC drive is noticeably smoother and quieter.

• 6-step: simple lookup table, acceptable for most industrial loads
• FOC: smoother low-speed torque, less audible noise
• FOC needs a more powerful processor (DSP or ARM Cortex-M)
• FOC is preferred for robotics, medical and servo-style positioning
• Both methods use the same PWM and closed-loop PID structure

Practical Tuning Tips

• Set acceleration and deceleration ramps: Abrupt speed jumps cause overcurrent trips. Most controllers allow setting a ramp time in the parameter list.
• Match PID gains to inertia: A high-inertia load needs lower proportional gain and higher integral gain to avoid overshoot.
• Check Hall wiring first: Swapped Hall wires cause rough running and failed speed regulation. Verify phase and Hall alignment before tuning PID.
• Use encoder feedback for precision: Hall-based speed feedback has resolution limited to six pulses per electrical cycle. For tight regulation, add an incremental encoder.
• Protect against overcurrent during start: Set current limit to roughly 2x rated for short acceleration periods, not continuous.
• Verify the command scale: A 0-5V command should map to zero-to-rated speed, not over-range. Many trip faults come from a miscalibrated analog scale.

Matching Controller To Shenghe BLDC Motors

Shenghe BLDC motors are typically paired with BLD-series controllers that handle Hall-feedback closed-loop control, PWM speed regulation and industry-standard protection. For FOC or servo-style applications, the BLDB6010 provides position, speed and torque mode switching with PID tuning through the host interface.

• BLD6010: DC 80-220V, up to 10A, RS232/RS485
• BLD22010: DC 18-60V, up to 10A, same control logic
• BLDB6010: AC/DC 24-80V, FOC with 3-mode operation
• All models include overcurrent, overvoltage and overtemperature protection

Related Pages

• BLDC Motor Controller
• 48V BLDC Motor Controller
• Brushless DC Motor
• What Is a BLDC Motor Controller?
• How Does a Brushless DC Motor Work?
• BLDC Motor vs DC Motor