BLDC Commutation and Hall-Sensor Sequencing

A motor turns because its magnetic field keeps pulling the rotor forward. To keep that pull going, current has to switch from one winding to the next at the right moment. That switching is called commutation. A brushed motor does it with brushes that rub against a spinning contact. A brushless DC motor, or BLDC, has no brushes, so a controller must track the rotor and switch the windings electronically.

Renesas lays out the options in its sensorless 120-degree conduction application note. Table 7-1 compares control methods, PWM methods, and ways to detect rotor position (R01AN6844EJ0121, Section 7.1, p. 35).

Six-step control is the simplest way to drive a BLDC motor. At any moment, one winding connects to the supply, one connects to ground, and the third is left unconnected. The controller moves through six of these combinations to complete one electrical turn. Each step covers 60 electrical degrees.

360° ÷ 6 steps = 60° per step

This method is also called 120-degree conduction, because each winding carries current for 120 degrees at a time.

The same 120-degree conduction application note confirms that current flows in two of the three phases at once. It also explains that the unconnected phase is what lets sensorless designs detect rotor position (R01AN6844EJ0121, Section 7.2, p. 38).

A Hall sensor outputs a digital high or low depending on which magnetic pole is passing it. A BLDC motor usually has three of them. Together they produce a 3-bit code, and each code marks one 60-degree slice of rotation. In the most common arrangement, the sensors produce six valid codes and never 000 or 111.

Renesas shows a real sequence in its Hall-sensor vector control application note. Turning clockwise, the Hall pattern steps through 1, 5, 4, 6, 2, and 3. Turning counterclockwise, it runs 1, 3, 2, 6, 4, and 5 (R01AN6466EJ0131, Section 7.6.2, p. 42).

It uses a small lookup table called a commutation table. Each Hall code points to one drive step. A typical forward sequence looks like this.

Step 1: U high, V low
Step 2: U high, W low
Step 3: V high, W low
Step 4: V high, U low
Step 5: W high, U low
Step 6: W high, V low

Which code matches which step depends on how the sensors are mounted in that motor. Always check the motor datasheet before building the table.

Renesas firmware uses this same idea. The FSP Motor 120-Degree Driver Interface names each step by the two switches it uses, such as “Up(PWM) to Vn(on)” (FSP v6.6.0, motor_120_driver_phase_pattern_t, Enumeration Type Documentation).

Flip the polarity for each step. If forward calls for U high and V low, reverse calls for V high and U low. The magnetic field now pulls the rotor backward, and the Hall codes arrive in reverse order. Slow the motor down first. Reversing at full speed can cause large current spikes.

In the Renesas Flexible Software Package, or FSP, the 120-degree Hall control module stores direction as a single setting. A value of 0 means clockwise and 1 means counterclockwise (FSP v6.6.0, rm_motor_120_control_hall, Data Structure Documentation).

A code of 000 or 111 usually means something is broken. A wire may be loose, or noise may have corrupted the signal. Switching windings based on a bad code can brake the motor or cause overcurrent. The safe response is to stop driving the motor and report a fault. A timeout check helps too. If no new Hall edge arrives when the motor should be turning, the rotor may be stuck.

The FSP 120-degree Hall control module includes both checks. It has a pattern error flag and a timeout error flag, with a default timeout of 200 ms (FSP v6.6.0, rm_motor_120_control_hall, Configuration and Function Documentation).

Each Hall edge means the rotor moved 60 electrical degrees. Time the gap between edges and you can calculate speed. Here is an example for a motor with four pole pairs and 500 µs between edges.

One electrical turn = 6 × 500 µs = 3 ms
Electrical frequency = 1 ÷ 3 ms = 333 Hz
Shaft speed = 333 ÷ 4 × 60 = about 5,000 rpm

Averaging the last six gaps smooths out small sensor errors.

The Hall-sensor vector control application note uses this method. It times each 60-degree interval and adds up the last six to estimate speed (R01AN6466EJ0131, Section 7.6.1, pp. 41–42).

Mechanical degrees describe how far the shaft turns. Electrical degrees describe how far the magnetic cycle has moved. A motor with more than one pair of magnetic poles completes several electrical cycles per shaft turn. A motor with two pole pairs sees 12 Hall steps per shaft turn instead of six.

Electrical frequency = rpm ÷ 60 × pole pairs
At 3,000 rpm with 2 pole pairs: 3,000 ÷ 60 × 2 = 100 Hz

The glossary in the 120-degree conduction application note gives the rule. To get mechanical angle, divide electrical angle by the number of pole pairs (R01AN6844EJ0121, Section 2, Table 2-1, p. 6).

Six-step control only switches six times per electrical turn, so torque dips slightly at each step. That causes some ripple and noise. Placement errors make it worse. If a sensor sits a little off its ideal spot, the controller switches early or late. The motor then runs rougher and less efficiently.

Renesas corrects for this in its Hall-sensor vector control application note. The software adds a motor-specific offset to make up for Hall signal errors caused by sensor mounting (R01AN6466EJ0131, Section 7.6.2, p. 44).

Hall sensors report rotor position even when the motor is standing still. The controller reads the code and applies the right step from the very first moment. Sensorless motors can’t do this. They have to nudge the rotor into motion before they know where it is. One limit remains with Hall sensors. At low speed, edges are far apart, so speed readings update slowly.

The FSP 120-degree Hall control module reflects this. By default it waits for 12 Hall interrupts before it starts calculating speed (FSP v6.6.0, rm_motor_120_control_hall, Configuration).

A timer can capture the exact count at each Hall edge in hardware. Speed readings stay accurate even when the CPU is busy. The same timer can generate the motor’s PWM signals with a built-in safety gap called dead time. For protection, a hardware shutdown path can turn off the outputs instantly when a fault occurs, without waiting for software.

The FSP general PWM timer, or GPT, driver for RA MCUs supports input capture, pulse period measurement, and automatic dead time. It also works with the port output enable for GPT, or POEG, which disables GPT outputs when an error is detected (FSP v6.6.0, r_gpt, Overview and Features).

Six-step control with Hall sensors is simple and reliable. It works well for fans, pumps, and tools where a little noise is fine. Sensorless control removes the sensors and their wiring, which lowers cost. Field-oriented control, or FOC, drives smooth sine-wave currents for quieter, more efficient operation. Today’s motor control MCUs make FOC affordable even in low-cost products.

A Renesas blog on motor control for power tools describes this shift. It covers replacing Hall-sensor BLDC control with sensorless FOC on an RA4T1 MCU to reduce complexity, lower cost, and improve reliability (Renesas blog, Feb. 5, 2025).