Why does a BLDC motor need three Hall sensors?

Three Hall sensors at 120° electrical spacing give the controller six unique state combinations per electrical revolution — exactly the resolution needed to drive a six-step commutation sequence on a three-phase stator. With only one Hall, the controller can detect rotor position once per revolution (not enough to commutate). With two Halls, four states (still under-resolved for six steps). Three Halls give six states (HA/HB/HC = 100, 110, 010, 011, 001, 101), each corresponding to a 60° electrical sector and a specific phase-pair to energise (e.g., A+/B- in sector 1, A+/C- in sector 2). The mapping is shown in the commutation timing diagram on this page.

What's the difference between six-step trapezoidal and FOC drive?

Six-step trapezoidal drive energises two of the three phases at a time (one high-side and one low-side MOSFET conducting), switching the active pair every 60° electrical based on Hall sensor feedback. Simple, robust, low controller-side compute load — but the torque ripple at each commutation step shows up as 6× electrical-frequency ripple in the output torque and audible noise. FOC (Field-Oriented Control) energises all three phases simultaneously with sinusoidally-shifted voltages calculated by SVPWM modulation from a dq-axis current setpoint, with continuous closed-loop on rotor angle from an encoder. Torque ripple is much lower (better for robotics, precision positioning, low-noise applications) but compute load is higher and the controller needs an encoder. Most Shenghe BLDC motors run trapezoidal on BLD22010/BLD6010 controllers; BLDC servo motors run FOC on the BLDB6010 servo controller — see the FOC block diagram below for the control-loop structure.

How does sensorless BLDC motor control work?

Sensorless commutation replaces Hall-sensor position feedback with back-EMF detection on the un-energised phase. At any moment during six-step commutation, two phases are energised and one is floating; the floating phase has back-EMF induced by the rotating rotor that crosses through zero at the centre of each 60° commutation sector. A voltage-divider network samples the three phase voltages, firmware synthesises a virtual neutral, a zero-crossing detector flags the moment, a 30° delay timer aligns the actual commutation instant, and the commutation logic switches the inverter to the next phase pair. The sensorless back-EMF diagram below illustrates the signal flow. Sensorless wins on cost (no Hall harness or encoder) and reliability (no extra sensor to fail) but has a known weakness at low speed and at startup, where the back-EMF amplitude is too small to detect reliably. Most sensorless BLDC controllers use a forced-commutation startup sequence then hand off to back-EMF mode above ~5–10% rated speed.

What MOSFETs are used in a typical BLDC motor inverter?

MOSFET selection depends on bus voltage and continuous current. Typical choices: for 24V/36V/48V systems at 10A continuous, N-channel power MOSFETs rated 60–100V VDSS and 30–50A ID continuous with RDSon under 10mΩ (e.g., IRFB4110, AOT460 class) work well. For 72V/110V systems at 20–30A continuous, 150–200V VDSS MOSFETs with RDSon under 15mΩ. The three half-bridges use six identical parts. Gate drive comes from a dedicated half-bridge gate driver IC (e.g., IR2104, IRS21531D class) or a three-phase BLDC controller chip that includes the gate drivers. Dead-time between high-side and low-side conduction is critical to prevent shoot-through — typically 500ns to 2µs depending on MOSFET turn-off time and the gate driver's built-in dead-time generator. See the MOSFET inverter diagram below for the topology and the driver-circuit blog at /blog/bldc-motor-driver-circuit/ for the gate-drive detail.

Where can I get ready-made BLDC motor controllers instead of building from these diagrams?

Most engineering teams find that building a BLDC motor controller from the inverter and commutation diagrams below takes 4–6 months of board design, firmware development, EMC compliance and field validation — far longer than buying a ready-made controller and integrating it. Shenghe manufactures three controller families in-house: BLD22010 for 24V/36V/48V systems (10A continuous), BLD6010 for 72V/110V/220V systems (30A continuous), and BLDB6010 servo controller for FOC + encoder feedback applications. All three are available as matched motor + controller kits where the motor and drive are pre-paired and dyno-tested at end-of-line — see /products/motor-controller/ for the kit catalog. For DIY / educational / prototype builds, the diagrams below cover the architecture; the driver-circuit blog at /blog/bldc-motor-driver-circuit/ covers the schematic-level detail.

Engineering Reference · 5 Inline SVG Diagrams

BLDC Motor Diagram — Block, Winding, Commutation, Inverter, FOC & Sensorless.

Q: What does a BLDC motor diagram show?

A BLDC motor diagram typically covers five layers: (1) the block diagram showing DC power source → controller → motor → Hall / encoder feedback → back to controller; (2) the motor cross-section with three-phase stator windings (phase A, B, C at 120° spacing) and the permanent-magnet rotor with N/S poles plus three Hall sensors at 120° electrical spacing; (3) the commutation timing strip showing how Hall signals (HA, HB, HC) drive the six-step phase-pair activation sequence over one electrical revolution; (4) the three-phase MOSFET inverter schematic with six switches arranged as three half-bridges driving phase outputs U, V, W; (5) for servo-grade or sensorless operation, the FOC control loop or the sensorless back-EMF detection block diagram. This page draws all five as inline SVGs with engineer-friendly annotations.

Five engineering diagrams covering how a brushless DC motor works at the level engineers source, size or build controllers at. Each diagram is drawn as inline SVG with annotated labels — no vector-graphics editor required to view, no rasterised PDF to download. Use this article as reference when sizing a BLDC controller against a motor, when debugging a winding or Hall harness, or when teaching a junior engineer how trapezoidal six-step commutation differs from FOC drive.

Author: Shenghe Motor Engineering Team · Published 2026-05-11

Contents

Three-phase MOSFET inverter diagram
Three-phase winding + Hall sensor placement
Hall commutation timing strip
FOC control loop block diagram
Sensorless back-EMF detection block diagram
Frequently asked questions

1. Three-Phase MOSFET Inverter Diagram

The three-phase MOSFET inverter is the power stage that converts the DC bus voltage into the three-phase pulsed waveform applied to the motor windings. Six MOSFETs arranged as three half-bridges (one high-side + one low-side per phase) — the controller switches them on and off in sequence to energise the phase pairs that produce torque on the rotor. The diagram below is the canonical topology for both six-step trapezoidal drive and FOC drive — the same MOSFETs, the same wiring; only the firmware that switches them changes.

Three-Phase MOSFET Inverter Topology

Six MOSFETs arranged as three half-bridges. Q1+Q4 form the phase-U bridge, Q3+Q6 phase V, Q5+Q2 phase W. The high-side MOSFET sources current from DC+ when on; the low-side sinks current to DC− when on. Only one side of each bridge conducts at a time — turning both on simultaneously causes shoot-through and destroys the MOSFETs. Dead-time between switching prevents this; typical dead-time 500ns–2µs.

Reading the schematic: at any instant during six-step commutation, two MOSFETs are conducting (one high-side on one phase, one low-side on a different phase), creating a current path from DC+ through the motor winding back to DC−. The third phase is floating (both MOSFETs off). The six commutation steps cycle through the six possible (high-side, low-side) pair combinations, advancing the magnetic field around the stator and rotating the rotor. Engineers sourcing MOSFETs for a build should target VDSS ≥ 1.5× bus voltage to absorb back-EMF spikes during phase-current decay through the body diodes. For 48V systems, 100V VDSS MOSFETs are typical; for 72V systems, 150V; for 110V systems, 200V.

For the gate-drive and dead-time detail, see the companion article BLDC Motor Driver Circuit — Three-Phase MOSFET Inverter Walk-Through →.

2. Three-Phase Winding + Hall Sensor Placement Diagram

A cross-section view of the BLDC motor showing where the three-phase stator windings sit and where the three Hall sensors mount relative to the rotor magnet. The 120° electrical spacing between phases and between Hall sensors is what makes the six-step commutation algorithm work — three sensors give six unique state combinations per electrical revolution, exactly enough to drive the six commutation steps.

BLDC Motor Cross-Section — Stator Windings + Rotor + Hall Sensors

Three stator coils (A, B, C) spaced 120° electrical from each other. Three Hall sensors (HA, HB, HC) mounted on the stator inner bore, also at 120° electrical spacing, offset 30° from the coils. Permanent-magnet rotor at centre with N pole (red top) and S pole (blue bottom). As the rotor rotates, the magnetic field passing each Hall sensor flips the sensor's digital output, and the unique combination of HA/HB/HC states tells the controller which 60° electrical sector the rotor is in — see commutation timing diagram below.

In real Shenghe BLDC motors, the three Hall sensors come pre-mounted on a small PCB inside the rear motor housing, with a 5-wire harness (+5V, GND, HA, HB, HC) running out to the matched controller. The Hall sensors are typically Allegro A3144 or equivalent latching Hall-effect devices with open-collector outputs. Pull-up resistors (typically 10kΩ) sit on the controller side. Reversed Hall pairs (e.g., swapping HA and HB on the harness) cause the motor to twitch instead of rotate — a common field-debug symptom. Matched motor + controller kits eliminate this risk because both connectors are pre-labelled and the firmware is parameterised against the actual motor's Hall sequence.

3. Hall Commutation Timing Diagram

The six-step commutation strip showing how the three Hall sensors (HA, HB, HC) advance through six unique state combinations over one electrical revolution, and which phase pair the controller energises in each step. Each step covers 60° electrical; six steps make one full revolution of the magnetic field around the stator. Read left-to-right.

Six-Step Commutation Timing — One Electrical Revolution

Each of the six rotor positions (60° electrical sectors) generates a unique HA / HB / HC code. The controller looks up the code in a 6-row table and switches on the two MOSFETs that energise the correct phase pair (e.g., HA=1, HB=0, HC=1 → Q1+Q6 → phase A+, phase B−). One full electrical revolution = 6 steps. Mechanical revolutions per electrical revolution = number of rotor pole pairs (typically 2, 3 or 4 — so a 4-pole-pair motor needs 24 commutation steps per shaft turn).

Engineers debugging a new BLDC motor or a winding modification check the Hall sequence on a scope first thing — the motor's nameplate or datasheet should specify the exact HA/HB/HC pattern. If the actual pattern is different (rotated or inverted), either the Hall harness is swapped (two-pin re-wire fixes it) or the rotor magnetisation is inverted (rare, but happens with custom builds). Shenghe BLDC motors ship with the Hall sequence and the corresponding commutation table documented on the datasheet; matched motor + controller kits skip the documentation step because firmware is pre-loaded.

4. FOC Control Loop Block Diagram

Field-Oriented Control (FOC) replaces the six-step trapezoidal commutation with a continuous sinusoidal drive computed from a rotating-frame (dq-axis) current setpoint. The output is much smoother torque (lower 6× commutation ripple, lower audible noise), better dynamic response, and the ability to do high-precision position / torque control on a BLDC servo motor with an encoder. The diagram below is the canonical FOC block diagram — same structure on every BLDC servo controller from textbook references to production firmware.

FOC Control Loop — Speed PI + Current PI + Park/Clarke Transforms + SVPWM

Forward path (top, blue): speed setpoint → speed PI → Iq* reference → current PI → Vd/Vq → inverse Park to αβ frame → SVPWM duty cycles → 3-phase inverter → motor. Feedback path (bottom, green): phase currents iU iV measured at shunts → Clarke transform to αβ frame → Park transform to dq frame using rotor angle from encoder → Iq feedback into current loop, speed derived from encoder feeds back into speed loop. SVPWM (Space Vector Pulse Width Modulation) drives all three half-bridges with sinusoidally-shifted duty cycles, producing smooth continuous rotation instead of stepped commutation.

Shenghe's BLDB6010 servo controller runs this FOC loop on an STM32-class microcontroller at typically 16–20 kHz PWM frequency. Speed loop runs at 1–2 kHz (slower outer loop), current loop at the PWM rate (fastest inner loop). The Park/Clarke transforms require the rotor angle from a quadrature encoder, typically 1000–2500 line counts, mounted on the rear motor shaft. For sensorless FOC (no encoder), the rotor angle is estimated by a position observer running off back-EMF — works at higher speeds but degrades near zero speed.

5. Sensorless Back-EMF Detection Block Diagram

Sensorless BLDC commutation replaces the three Hall sensors with a back-EMF detection circuit. At any moment during six-step commutation, one of the three phases is un-energised (floating); the rotating rotor magnet induces a back-EMF voltage on that floating phase, which crosses through zero at the midpoint of the 60° commutation sector. Detecting that zero-crossing and delaying 30° before commutating gives the controller the same rotor-position information as Hall sensors — without the cost, harness or sensor reliability issue. The diagram below shows the signal flow.

Sensorless BLDC Back-EMF Detection & Commutation

Phase voltages VU / VV / VW are sampled through a voltage divider, fed into the controller's ADC. A virtual neutral is synthesised in firmware (typically (VU + VV + VW) / 3 since the actual motor neutral is usually inaccessible). The back-EMF zero-crossing detector watches the floating phase per six-step sector, flagging when its voltage crosses the virtual neutral. A 30° delay timer then aligns the actual commutation instant to the centre of the next 60° sector. Commutation logic switches the inverter to the next phase pair. Startup uses forced open-loop commutation (no back-EMF feedback yet because the rotor is too slow) and hands off to sensorless mode above ~5–10% rated speed.

Sensorless wins on: no Hall harness, no encoder, no extra sensor to fail. Loses on: not reliable below ~5–10% rated speed where back-EMF amplitude is too small to detect, requires a forced-startup sequence that can twitch the rotor in the wrong direction briefly, weaker low-speed precision. Best fit: continuous-duty fans, pumps, blowers, e-mobility traction motors where the motor lives above the minimum-speed threshold most of the time. Not a good fit for AGV traction at standstill, robotic actuators, automatic doors — those should use Hall sensors or an encoder.

Shenghe BLD22010 and BLD6010 controllers support both Hall-sensor and sensorless modes — configured by a firmware parameter at end-of-line. Most kit shipments come in Hall-sensor mode (matches the 5-wire harness on the motor side); sensorless mode is requested for cost-sensitive builds where the customer wants to skip the Hall harness on the production motor.

6. Frequently Asked Questions

What does a BLDC motor diagram show?

Five layers: block overview (DC → controller → motor → feedback); cross-section showing 3-phase windings + Hall placement; commutation timing strip with Hall states and active phase pairs; 3-phase MOSFET inverter schematic; FOC control loop or sensorless back-EMF detection block. This page draws all five as inline SVGs.

Why three Hall sensors at 120°?

Three Halls give six unique state combinations per electrical revolution — exactly what's needed to drive six-step commutation. Two Halls → only four states (under-resolved). The 120° electrical spacing matches the 120° spacing between the three stator phases.

Six-step trapezoidal vs FOC drive?

Six-step energises 2 of 3 phases at a time (one high-side, one low-side), switching every 60° electrical. Simple, low compute load, but 6× commutation ripple in torque. FOC energises all 3 phases with SVPWM-modulated sinusoidal voltages from a dq-axis current setpoint, with continuous encoder feedback. Smoother torque, lower noise, higher compute load. BLDC servo motors run FOC; standard BLDC motors run six-step.

How does sensorless control work?

Back-EMF from the un-energised phase crosses through zero at the midpoint of each 60° commutation sector. Voltage divider + ADC + virtual neutral synthesis + zero-crossing detector + 30° delay timer + commutation logic → drives the inverter without Halls or encoder. Loses reliability below ~5–10% rated speed; needs forced-startup ramp.

MOSFET selection for BLDC inverter?

VDSS ≥ 1.5× bus voltage (100V VDSS for 48V system, 150V for 72V, 200V for 110V). ID continuous ≥ 3× motor continuous current. RDSon < 10mΩ for low conduction loss. Dead-time 500ns–2µs between high-side and low-side switching to prevent shoot-through. See BLDC Motor Driver Circuit → for gate-drive detail.

Where to get a ready-made BLDC controller?

Building from these diagrams takes 4–6 months. Shenghe ships 3 controller families ready-made: BLD22010 (DC 18–60V, 10A — for 24V/36V/48V systems), BLD6010 (DC 80–220V, 30A — for 72V/110V/220V), BLDB6010 (servo with FOC + encoder). Available as matched motor + controller kits, pre-paired and dyno-tested. See Motor Controller Hub →.

Skip The 6-Month Controller Build — Order A Matched BLDC Kit 3 controller families covering 24V to 220V DC bus systems, pre-paired with Shenghe BLDC motors on the dyno bench at end-of-line. Firmware loaded against motor electrical spec, Hall harness pre-wired, no field-tuning. Sample 7–10 days, production 2–3 weeks from Cixi factory. ISO 9001 / CE / RoHS.

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