Latching Hall Effect Sensors in BLDC Motors: Commutation & Encoders

Understanding Commutation in Brushless Motors

Unlike brushed DC motors which use mechanical copper brushes and split-ring commutators to reverse the current direction, brushless DC (BLDC) motors perform commutation electronically. The stator windings must be energized sequentially based on the physical position of the permanent magnets on the rotor.

To know when to energize which phase winding, the Electronic Speed Controller (ESC) needs continuous feedback indicating where the rotor magnets are positioned. This feedback is supplied by three latching Hall effect sensors embedded directly inside the motor.

Why Latching Sensors are Mandatory

A standard latching Hall sensor (such as the US1881) exhibits a memory-like switching operation. When exposed to a South magnetic pole, the sensor turns ON and outputs a LOW electrical state. It remains in this state even if the South pole is removed. The sensor will only turn OFF (output HIGH) when exposed to an opposite North magnetic pole.

This latching action is critical for BLDC commutation because the rotor is surrounded by alternating North and South permanent magnet segments. As the rotor rotates:

1. A South pole passes, latching the output LOW.
2. The gap between poles passes, but the sensor remains LOW.
3. A North pole passes, unlatching the output to HIGH.

This yields a highly reliable, noise-tolerant square wave output with a 50% duty cycle, matching the motor speed and rotor alignment.

6-Step Commutation & Spacing

Most sensored BLDC motors place three Hall sensors at either 120 electrical degrees or 60 electrical degrees apart. In a 120-degree setup, the outputs of the three sensors (H1, H2, H3) never display all zeros (0-0-0) or all ones (1-1-1). This provides 6 distinct states per electrical revolution:

By mapping these 6 digital sensor states directly to switching states in a 3-phase MOSFET bridge (H-Bridge array), the ESC maintains continuous, efficient rotation.

Arduino Speed Tracking from BLDC Hall Signals

In addition to commutation, you can use the pulses from one or all three Hall sensors to calculate motor speed. Connect the Hall signal output to an interrupt-enabled pin on a microcontroller.

// Arduino Uno BLDC Hall Speed Sensor Reader
const int HALL_A_PIN = 2; // Interrupt Pin 0
volatile unsigned long pulseCount = 0;
unsigned long lastTime = 0;
const float polePairs = 4.0; // Number of rotor pole-pairs

void IRAM_ATTR countPulse() {
  pulseCount++;
}

void setup() {
  Serial.begin(115200);
  pinMode(HALL_A_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(HALL_A_PIN), countPulse, RISING);
}

void loop() {
  unsigned long currentTime = millis();
  if (currentTime - lastTime >= 1000) { // Update once per second
    noInterrupts();
    unsigned long pulses = pulseCount;
    pulseCount = 0;
    interrupts();

    // RPM = (pulses / polePairs) * 60 seconds
    float rpm = (float(pulses) / polePairs) * 60.0;
    
    Serial.print("Rotor Speed: ");
    Serial.print(rpm);
    Serial.println(" RPM");
    
    lastTime = currentTime;
  }
}

Latching Hall Sensors & BLDC Commutators Sourcing in India

Latching Hall sensors like the US1881, Honeywell SS41F, and Allegro A1220 are readily available at retail and wholesale electronic markets in India for motor encoders and custom ESC building:

Frequently Asked Questions

Conclusion

Latching Hall effect sensors form the backbone of modern sensored motor control, linking physical rotation with digital electronic commutators. Understanding their binary memory properties, the 6-step commutation matrix, and standard microprocessor timing techniques allows you to design highly efficient motor drivers and positioning loops.

Ready to go deeper? Interface your speed encoders with the PID feedback loop guide, explore phase switching using the H-Bridge motor driver breakdown, or read sensor waveforms in the General Hall Effect overview.