How does sensorless BLDC motor control work?

Sensorless control detects rotor position from the back-EMF (back electromotive force) the motor windings generate as the rotor spins. The MCU monitors voltage on the unenergized phase during each commutation step — that voltage's zero-crossing tells the controller when to commutate next. No Hall sensors required.

When does sensorless BLDC fail?

At low speed and at startup. Back-EMF is proportional to rotor speed, so below ~5–10% of rated RPM the back-EMF signal is too small to read reliably. Sensorless motors typically use an open-loop 'align-and-go' startup sequence (force a known position, then accelerate blindly until back-EMF is readable). This makes sensorless a poor choice for AGV traction, low-speed conveyors and any application that needs precise low-speed torque.

When does sensorless BLDC win?

When operating at near-constant high speed and Hall-sensor wiring is hard to maintain. Top wins: fans (computer cooling, HVAC blowers), pumps (water, fluid), blowers (industrial, vacuum), sealed hub motors (e-bike, e-scooter — Hall wires can't be routed through axle), refrigerator compressors, washing-machine drum drives. The fewer-wires advantage often outweighs the low-speed limitation in these applications.

Do Shenghe controllers support sensorless mode?

Yes. The BLD22010, BLDB6010 and BLD6010 BLDC controllers all support both Hall-sensor and sensorless commutation, switchable in firmware. For sensorless mode, the motor needs balanced symmetric windings and you'll see longer startup latency (typically 1–2 seconds align-and-go vs instant Hall startup).

Can I switch a Hall-sensor motor to sensorless?

Sometimes. If the motor's windings are symmetric and the controller supports both modes, you can disconnect Hall sensors and run sensorless. But factory-configured sensorless motors often have winding asymmetries (skip the Hall mounting cost) that hurt sensorless performance. For best results, specify sensorless up front and the manufacturer matches the winding accordingly.

Control Theory

Sensorless BLDC Motor Control — Back-EMF Detection Explained.

Sensorless BLDC motor control replaces the three Hall sensors with back-EMF detection — the controller reads the voltage on the unenergized phase to know where the rotor is. Fewer wires, better reliability in dusty / wet environments, but harder low-speed startup. This guide covers the theory, the trade-offs, and where sensorless wins (fans, pumps, blowers, sealed hub motors) vs where it fails (AGV traction, low-speed conveyors).

Reviewed by Shenghe Motor Engineering Team · Updated 2026-04-30

1. What Sensorless BLDC Means

A standard BLDC motor uses three Hall sensors mounted in the stator to detect rotor position. The controller reads a 3-bit Hall code (six possible sectors per electrical revolution) and energizes the next two phases accordingly. Sensorless BLDC removes those three Hall sensors and infers rotor position from a different signal: back-EMF, the voltage that the spinning rotor's permanent magnets induce in the unenergized phase.

2. How Back-EMF Detection Works

At any instant, only two of the three motor phases carry drive current. The third phase is electrically floating — and the rotating rotor magnets induce a voltage on that floating phase. The shape of that induced voltage tracks rotor position:

The MCU samples the floating-phase voltage continuously.
When the back-EMF voltage crosses zero (relative to the bus midpoint), it signals the rotor has reached the next commutation point.
The MCU energizes the next phase pair — same 6-step commutation as a Hall-driven motor, just with different sensing.
For higher accuracy and low-torque-ripple operation, sensorless drives also implement FOC (Field-Oriented Control), which estimates rotor flux angle from back-EMF in real time using a state observer.

The key signal is the back-EMF zero-crossing. Position accuracy is a few electrical degrees — comparable to Hall-driven, sometimes better at high speed.

3. Why Sensorless Fails at Low Speed

Back-EMF is proportional to rotor speed: zero RPM = zero back-EMF. Below ~5–10% of rated speed, the signal is too small to read reliably above electrical noise. Sensorless motors handle startup with a special sequence:

Align: Apply DC current to two phases. The rotor pulls into a known position.
Open-loop ramp: The MCU commutates blindly at a programmed accelerating frequency. The rotor follows because the load is small at low speed.
Closeloop handover: Once back-EMF is detectable (typically above 10% of rated RPM), the MCU switches from blind ramp to back-EMF-driven commutation.

This align-and-go sequence takes 1–2 seconds typically and assumes the load is light at startup. If the load is heavy at startup (AGV pulling a 1000kg payload, conveyor with full belt load, low-speed positioning task), sensorless fails — the motor stalls during the open-loop ramp because it can't develop enough torque without precise commutation timing. That's why sensorless is rarely used in AGV traction.

4. When Sensorless BLDC Wins

Fans (computer cooling, HVAC blowers, server-room ventilation): runs at near-constant high speed, light load, minimum wiring matters because fan housings are sealed.
Pumps (water, fluid, fuel, oil): same pattern as fans — high-speed steady-state, sealed motor housing, no Hall wires needing waterproofing.
Blowers (industrial, vacuum, dust extraction): high-speed, high-flow, dirty environment.
Sealed hub motors (e-bike, e-scooter): Hall wires are hard to route through the axle bearing seal — sensorless eliminates that pain. Acceptable because hub motors usually start under low load (rider pedaling first, motor assists).
Refrigerator compressors and washing-machine drum drives: sealed enclosures, near-constant speed, fewer-wires advantage outweighs startup latency.
Tool batteries (drills, saws, screwdrivers): small motors, no Hall wires through trigger/switch, light low-speed load.

5. When Sensorless BLDC Fails

AGV traction: heavy startup load, needs precise low-speed torque control. Hall sensors are mandatory.
Low-speed conveyors (especially with variable load): Hall sensors required for closed-loop speed regulation at low RPM.
Servo applications: position control needs accurate low-speed positioning and zero-speed holding torque. Sensorless can't do either.
Indexing tables and robotics joints: same constraint as servo — needs high-resolution position feedback at all speeds, especially low and zero.
Heavy-load startup: any application where the motor must develop full torque from rest. The open-loop ramp can't deliver it.

6. Sensorless on Shenghe Controllers

Shenghe's BLD-series BLDC controllers (BLD22010, BLDB6010, BLD6010) all support both Hall-sensor and sensorless commutation, switchable in firmware via the configuration registers. For sensorless mode:

The motor needs balanced symmetric windings for clean back-EMF zero-crossings.
Startup sequence is align-and-go with configurable ramp rate (default 1.5 seconds).
Closed-loop speed control kicks in at ~10% rated RPM after handover.
FOC firmware available for low-torque-ripple operation in fans / pumps / blowers.

For applications that benefit from sensorless (sealed motors, dusty environments, fewer-wire builds), specify sensorless mode at order time and the motor winding will be tuned for it.

7. Decision Summary

Requirement	Sensorless	Hall Sensors
Heavy startup load	✗ Stalls during ramp	✓ Full torque from rest
Low-speed precision	✗ Below 10% RPM blind	✓ Accurate at all speeds
Sealed motor / dusty environment	✓ Fewer wires through seal	✗ Hall cable to manage
Reliability	✓ No Hall wear-out	OK (Hall sensors very reliable)
Cost	✓ Lower (no Hall ICs)	OK
Servo / position control	✗ Not feasible	✓ With encoder addition

8. Further Reading

BLDC Motor Controller — Shenghe BLD-series controllers (Hall + sensorless modes)
What Is a BLDC Motor? — fundamentals
Hall Sensor in BLDC Motor — companion piece
BLDC Servo Motor — when you need sub-0.1° position (encoder + Hall, never sensorless)