Motor Control and Timing
How a BLDC Motor Knows When to Switch
Learn six-step commutation and Hall-sector signals.
You need to understand the internal functions in “How a BLDC Motor Knows When to Switch” before selecting, connecting or troubleshooting the real equipment.
See how a three-phase BLDC controller advances six inverter states, reads three Hall signals as rotor sectors, and validates the motor-to-controller phase map safely.
- Select or check first
- Define the required outcome, operating conditions and exact component or system boundary before choosing a part or method.
- Controlling assumption
- Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.
- Top risk
- A logic table does not make the power stage touch-safe. A three-phase inverter can source destructive fault current and can regenerate energy into its DC bus. Isolate energy before rewiring, provide current and voltage protection, and use a driver designed to prevent high-side and low-side overlap.
- Proof
- Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Start with the basics Optional foundation before “How a BLDC Motor Knows When to Switch”
Automation-and-control basics
An automatic system measures a condition, decides what it means and commands an actuator while checking whether the expected result actually happened.
- Sensor
- A sensor converts a physical condition into a signal. Its range, uncertainty, response time, environment and failure behaviour matter as much as its nominal output.
- Controller
- A controller reads inputs, applies logic or a control law and issues outputs. Software cannot correct missing measurements, inadequate hardware authority or unsafe architecture.
- Actuator
- An actuator turns a command into physical action, such as motor torque, valve position or switching. Its real limits depend on load, supply, drive and duty.
- Feedback
- Feedback compares measured behaviour with the desired state so error can be corrected. Open-loop command assumes the action occurred; closed-loop control checks it.
- Fault and safe state
- A fault is an abnormal condition that may invalidate control. The safe response must be defined from the hazard and may require independent hardware, not only software.
How to use this article: Trace one complete chain—physical condition, sensor, signal conditioning, controller, drive, actuator and feedback—then ask how each link can fail.
On this page
A brushless DC motor has no mechanical commutator to reverse winding current. The controller must switch a three-phase inverter in step with rotor position. In a common six-step method, each electrical revolution is divided into six 60-degree sectors. One phase is driven toward the positive bus, one toward the negative bus and the third is undriven during each sector.
Six inverter states move the stator field
Diagram description
A BLDC cutaway and commutation table show 120-degree conduction: two terminals conduct while the third floats in each 60-electrical-degree sector.
| Role | Electrical action | Purpose |
|---|---|---|
| High | Selected inverter leg connects the phase toward the DC positive bus, often with PWM | Provides current into the active winding path |
| Low | Another leg connects a phase toward the negative bus | Provides the return path |
| Floating | Neither switch is deliberately on for that terminal during the sector | Completes the 120-degree conduction pattern and can support back-EMF sensing |
Advancing the state rotates the stator magnetic field. Reversing the valid sequence reverses field rotation. Swapping arbitrary wires without updating the sensor map can instead create wrong torque, high current or a motor that jitters. Current limiting, gate-driver interlock and fault handling belong below the commutation table; the table alone is not a safe inverter.
Electrical and mechanical angle
θ_e = p θ_m
A rotor with p pole pairs completes p electrical cycles in one mechanical revolution. Six commutation sectors therefore repeat p times per shaft revolution. Use consistent angular units and the actual rotor pole count.
- θ_e
- electrical angle
- θ_m
- mechanical shaft angle
- p
- rotor pole-pair count
Three Hall signals identify a rotor sector
Diagram description
The plate shows one selected 120-degree Hall convention with codes 001, 101, 100, 110, 010 and 011; bit order, polarity and direction are not universal.
Hall devices respond to rotor magnetic field and usually present logic levels to the controller. A transition tells the controller that the rotor crossed a sector boundary. The controller reads all three bits, rejects invalid states and looks up the drive state that creates forward torque. The six codes shown in the figure are one selected bit order. Harness order, sensor polarity, physical placement and desired direction can produce a different valid sequence.
| Observation | Interpretation | Action |
|---|---|---|
| Six valid states repeat in one direction | Sensor transitions are plausibly spaced through an electrical cycle | Record the exact sequence at slow manual rotation |
| 000 or 111 appears persistently | Impossible for the selected 120-degree convention or evidence of wiring, supply or sensor fault | Disable drive and diagnose before commutating |
| State skips or chatters | Noise, loose connection, excessive transition speed or threshold problem | Filter only as justified; inspect signal integrity and timing |
| Motor draws current but produces weak or wrong torque | Hall-to-phase lookup may be misaligned | Return to current-limited mapping; do not guess under full power |
Commission the sensor-to-phase map as controlled evidence
Low-risk mapping workflow
- Identify phase wires, Hall supply, Hall ground and Hall outputs from authoritative motor documentation where available.
- With the power stage disabled, rotate the shaft slowly and log the three-bit sequence and direction.
- Confirm Hall voltage levels suit the controller inputs and that 000 or 111 are treated as faults for the selected convention.
- Configure the candidate lookup table from known motor geometry or a documented procedure.
- Use a current-limited low bus voltage for the first powered test; verify smooth starting direction and plausible current.
- Check both directions, transition timing, overcurrent shutdown and loss-of-Hall behavior before raising voltage or load.
Hall commutation remains available at standstill because the sensor observes magnetic position directly. That makes starting straightforward compared with simple back-EMF sensing. Its tradeoffs are extra sensor hardware, wiring and a discrete sector estimate rather than continuous high-resolution position. More advanced drives may interpolate, advance timing or use sinusoidal control, but the core obligation remains: energize windings according to verified rotor position and bounded current.
For the motor voltage-current relationship behind commutation and starting, review Why a Brushed DC Motor Starts Differently. The next control lesson removes Hall sensors and derives timing from the motor waveform.
Selection boundary and evidence to keep with the decision
Treat “How a BLDC Motor Knows When to Switch” as a selection problem, not only a definition. Record the required result, alternatives considered, the controlling assumption and the condition that would change the choice.
Keep evidence proportional to consequence: Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test. Revisit the decision when the load, environment, duty, interface, regulation or maintenance plan changes.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for How a BLDC Motor Knows When to Switch?
Define the required outcome, operating conditions and exact component or system boundary before choosing a part or method.
Which assumption can change the conclusion?
Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.
What can go wrong if I copy the example directly?
A logic table does not make the power stage touch-safe. A three-phase inverter can source destructive fault current and can regenerate energy into its DC bus. Isolate energy before rewiring, provide current and voltage protection, and use a driver designed to prevent high-side and low-side overlap.
How do I verify the result in the real system?
Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Sources and further reading
- Introduction to Six-Step Sensored Brushless DC Motor CommutationMicrochip Technology · 2026-10-13Manufacturer guidance reviewed for Hall-sector sensing, commutation lookup, three-phase inverter control and the role of dead-time insertion.
- Six-Step Commutation of Three-Phase BLDC MotorMicrochip Technology · 2026-10-13Primary manufacturer documentation reviewed for 60-degree sectors, 120-degree conduction and the relationship between electrical cycles, pole pairs and mechanical revolution.
- AVR443: Sensor-based Control of Three Phase Brushless DC MotorsMicrochip Technology · 2026-10-13Manufacturer application note reviewed for the six useful Hall combinations, two-driven-one-floating phase scheme and dependence of the lookup on phase and Hall mapping.