A Hall sensor is the small semiconductor element that tells a BLDC controller where the rotor is. Three of them, spaced 120 electrical degrees apart inside the stator, provide the position feedback needed to energize the correct phase pair in the right sequence. This article explains how Hall sensors work, how they are wired, why most industrial BLDC drives still prefer them, and how to diagnose a faulty sensor.
| Hall A | Hall B | Hall C | Active Phases |
|---|---|---|---|
| 1 | 0 | 1 | A+ B- |
| 1 | 0 | 0 | A+ C- |
| 1 | 1 | 0 | B+ C- |
| 0 | 1 | 0 | B+ A- |
| 0 | 1 | 1 | C+ A- |
| 0 | 0 | 1 | C+ B- |
As the rotor turns, its permanent magnet poles rotate past each Hall sensor mounted in the stator. The magnetic field at the sensor surface changes from north to south.
Current flowing through the sensor semiconductor is deflected by the magnetic field, producing a small voltage across the output terminals. An internal amplifier and Schmitt trigger convert this into a clean digital signal.
The three Hall outputs form a 3-bit binary code that changes through six unique states per electrical cycle. Each state corresponds to one of the six commutation steps.
The controller looks up the Hall state in its commutation table and switches the matching pair of MOSFETs to energize two of the three motor phases, producing torque in the correct direction.
Most industrial BLDC motors expose a separate 5-wire or 6-wire Hall cable that runs to the controller next to the three thick phase leads. The Hall cable carries a regulated +5V supply from the controller, a ground return and the three Hall outputs. Some motors add a temperature sensor line, making the connector six wires instead of five. Never connect the Hall supply to the main bus voltage — the sensors are rated for 5V logic level only.
Hall sensor drives read rotor position directly and work from standstill. Sensorless drives use back-EMF on the unenergized phase to estimate rotor position, which only works once the rotor has enough speed to generate a readable signal. For conveyors, AGVs, robotic joints and any application where torque must be available immediately at zero speed, Hall sensor control is the default. Sensorless is common in fans, pumps and appliances that ramp through the start region briefly and then run at fairly constant speed.
| Parameter | Typical Value |
|---|---|
| Supply voltage | 4.5 - 5.5 V DC |
| Output type | Open-collector digital |
| Switching point | +/- 10 mT (typical) |
| Operating temperature | -40 to +150 °C |
| Response time | < 10 microseconds |
| Common parts | A3144, SS41, US1881 |
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It detects the rotor magnet position and outputs a digital signal so the controller knows which stator phases to energize next.
Three, spaced 120 electrical degrees apart. Their combined output forms a 3-bit code with six valid states per electrical cycle.
Only with a sensorless controller designed for it. The motor and controller type must match or the drive will not commutate correctly.
