Hall Sensor in BLDC Motor: How It Works, Commutation Table & Troubleshooting

The Hall sensor in a BLDC motor is the component that makes brushless operation possible. Without it, the controller has no way to know where the rotor is — and cannot energize the correct stator winding at the correct time. Hall sensor failures are the number one cause of BLDC motor field problems that we see in our technical support, yet they are almost always diagnosable and fixable with basic tools. This guide explains how Hall sensors work, how to read the commutation table, and how to diagnose the five most common faults.

What Is a Hall Effect Sensor?

A Hall effect sensor is a semiconductor device that produces a voltage proportional to the magnetic field passing through it. Named after physicist Edwin Hall who discovered the effect in 1879, these tiny chips (typically 3mm x 4mm) are the most common rotor position sensors in BLDC motors because they are inexpensive (under $0.10 each), reliable (no mechanical wear), and operate across a wide temperature range (-40 to +150 degrees C).

In a BLDC motor, the Hall sensor does not measure the strength of the magnetic field — it acts as a digital switch. When a rotor magnet's north pole passes over the sensor, the output goes high (5V). When a south pole passes, the output goes low (0V). This binary output is all the controller needs to determine which commutation step to execute next.

How Hall Sensors Work in a BLDC Motor

A 3-phase BLDC motor has three Hall sensors (labeled Hall A, Hall B, Hall C) mounted on a small PCB inside the motor housing, positioned at the stator end cap. They are spaced 120 electrical degrees apart — the exact mechanical spacing depends on the motor's pole count:

As the rotor spins, the permanent magnets sweep past the Hall sensors. Each sensor outputs a square wave — high when a north pole is overhead, low when a south pole is overhead. The three square waves are phase-shifted by 120 electrical degrees from each other. At any instant, the combination of three high/low states tells the controller exactly which 60-degree electrical sector the rotor is in.

Hall Sensor Commutation Table

The BLDC motor commutation table maps each combination of Hall sensor states to the correct phase energization. This is the core logic that every BLDC controller implements — either in hardware (gate driver IC) or firmware (microcontroller lookup table):

Important notes on the commutation table:

• Hall codes 0 (000) and 7 (111) are invalid — if the controller reads either, it indicates a wiring error or sensor failure.
• The table above shows the most common commutation sequence. Some motor-controller combinations use a different sequence — always verify with the motor's datasheet.
• Reversing motor direction requires either swapping two phase wires AND the corresponding two Hall wires, or changing the commutation table in the controller firmware.

Hall Sensor Wiring: Pin by Pin

A BLDC motor's Hall sensor connector has 5 wires:

Wire color warning: There is no universal color standard for Hall sensor wires. Chinese, Japanese, and European motor manufacturers may use completely different color codes. Our motors include a pinout label on the connector and a wiring diagram in the documentation. Always verify before connecting.

Hall Sensor vs Sensorless BLDC Control

Some BLDC controllers offer "sensorless" operation — detecting rotor position from the back-EMF voltage on the un-energized phase instead of using Hall sensors. Here is when each approach makes sense:

Our recommendation: For industrial and commercial applications — AGV electric tugs, pumps, conveyors, automatic doors and gates — always use Hall sensor control. The $0.30 cost of 3 Hall sensors buys you reliable startup under load and smooth low-speed operation. Sensorless is appropriate only for applications where the motor always starts unloaded and runs at medium-to-high speed (fans, drone propellers).

5 Common Hall Sensor Faults and How to Fix Them

Fault 1: Motor Vibrates But Does Not Rotate

Symptom: The motor hums, vibrates, or jerks when powered but does not spin. It may get hot quickly.

Cause: Hall sensor wiring is incorrect — the A/B/C signals are connected to the wrong controller inputs, so the controller energizes the wrong phase at the wrong time.

Fix: Verify the Hall A/B/C wire mapping between motor connector and controller connector. Try swapping Hall A and Hall B signals. If still incorrect, try all 6 possible permutations (ABC, ACB, BAC, BCA, CAB, CBA). Only one permutation produces smooth rotation in the desired direction.

Fault 2: Motor Runs in Wrong Direction

Symptom: The motor spins, but in the opposite direction from what you need.

Fix: Swap any two of the three phase power wires (e.g., swap U and V). You must ALSO swap the corresponding two Hall sensor signal wires (swap Hall A and Hall B). Swapping only the phase wires without the Hall wires will cause erratic operation or Fault 1 above.

Fault 3: One Hall Sensor Dead — Erratic Speed or Cogging

Symptom: The motor runs but with periodic cogging, jerking, or speed fluctuation every 60 electrical degrees (you can feel it when rotating the shaft slowly by hand with the controller off).

Diagnosis: Disconnect the motor from the controller. Apply 5V to the Hall VCC pin and ground to GND. Connect a multimeter to each Hall output (A, B, C) one at a time. Slowly rotate the shaft by hand. A healthy Hall sensor output will alternate between approximately 0V and 5V. A faulty sensor will stay stuck at one voltage or show intermediate values (1-3V).

Fix: Replace the faulty Hall sensor IC. These are typically Honeywell SS41, Allegro A3144, or equivalent latching Hall switches, available for under $0.10 each. If the sensor PCB is potted or inaccessible, the motor must be returned for repair. Our motors use socketed Hall sensor PCBs that can be replaced without disassembling the stator — contact us for replacement boards.

Fault 4: Hall Sensor Burned Out — Overvoltage

Symptom: All three Hall sensors are dead (no output on any channel). The motor worked fine previously.

Cause: The controller supplied 12V or 24V to the Hall sensor VCC pin instead of 5V. This is the most common cause of Hall sensor destruction and happens when connecting a motor to an unfamiliar controller without checking the Hall sensor voltage specification.

Prevention: Always measure the voltage on the controller's Hall VCC output pin before connecting the motor. If it reads above 5.5V, do NOT connect the motor. Some controllers have a jumper or DIP switch to select 5V or 12V Hall sensor voltage — set it to 5V for all our motors.

Fix: Replace all three Hall sensors on the sensor PCB. The sensors themselves are cheap, but disassembly labor is significant.

Fault 5: Intermittent Hall Signal — Loose Connection or Noise

Symptom: The motor runs normally most of the time but occasionally stutters, hesitates, or makes a brief grinding noise. The problem is intermittent and may be related to vibration or temperature.

Cause: Loose connector pins, corroded contacts, or electromagnetic interference (EMI) coupling into the Hall sensor cable from nearby power electronics (VFDs, welding equipment, high-current switching).

Fix:

• Re-crimp or replace the Hall sensor connector pins. Check for corrosion (green or white deposits on copper contacts).
• Use a shielded cable for the Hall sensor connection if running near power cables or noisy equipment. Connect the shield to GND at the controller end only.
• Add 100nF ceramic capacitors between each Hall output and GND at the controller connector as close as possible to the controller pins — this filters high-frequency noise spikes.
• Separate Hall sensor wiring from motor power wires by at least 50mm and cross at 90 degrees where they must intersect.

Hall Sensor Specifications for Common Motor Sizes

The following table shows the Hall sensor specifications across our standard BLDC motor range:

All our BLDC motors use latching Hall sensors (not unipolar). Latching sensors have a built-in hysteresis band that provides clean switching even when the rotor is moving slowly — critical for smooth low-speed operation in machinery movers and AGV electric tugs.

Testing Hall Sensors: Step-by-Step Procedure

This procedure works for any 3-phase BLDC motor with Hall sensors. You need a multimeter or oscilloscope, a 5V DC power supply (or 4x AA batteries = 6V, acceptable for testing), and the ability to rotate the motor shaft by hand.

1. Disconnect the motor from the controller completely — both power and Hall sensor connectors.
2. Identify the Hall sensor pins. Refer to the motor's datasheet or pinout label. Find VCC (5V), GND, Hall A, Hall B, Hall C.
3. Apply 5V DC to VCC and GND. Confirm correct polarity — reversing VCC and GND may damage the sensors.
4. Set multimeter to DC volts. Probe Hall A output against GND.
5. Slowly rotate the shaft by hand in one direction. The voltage should alternate between approximately 0V and 5V as each magnet pole passes the sensor.
6. Count transitions: One complete revolution should produce (number of pole pairs) transitions from high to low. For an 8-pole motor (4 pole pairs), you should see 4 high-to-low transitions per revolution on each Hall output.
7. Repeat for Hall B and Hall C. All three should produce the same number of transitions, phase-shifted from each other.
8. If using an oscilloscope: Monitor all 3 channels simultaneously. You should see three square waves, each shifted by 120 electrical degrees. The duty cycle should be approximately 50%. Any waveform that is noisy, has slow rise/fall times (over 1 microsecond), or has incorrect duty cycle indicates a problem.

Frequently Asked Questions