A brushless servo motor produces torque by pushing magnetic fields against each other, and to keep pushing as the rotor turns, the drive must constantly switch current between the motor's phases in step with the rotor's position. That coordinated switching is called commutation. In a brushed motor a mechanical part called the commutator does it automatically; in a brushless motor the drive must do it electronically, and it can only do so if it knows where the rotor is. This guide explains how electronic commutation works, the roles of Hall sensors and encoder-based field-oriented control, and why a wrong commutation offset leaves a motor weak or sends it into runaway.
Motor Commutation in one line: Commutation is the process of energizing a motor's phases in the right sequence and proportion relative to the rotor's position so that the magnetic fields stay aligned to produce smooth, continuous torque. In a brushless servo motor the drive performs commutation electronically, which requires it to know the rotor's angle from Hall sensors or an encoder. If the drive's idea of rotor position is wrong, commutation misaligns the fields, producing weak, rough, or even reversed torque.
Torque in a motor comes from the interaction of two magnetic fields, one on the rotor and one created by current in the stator windings, and the force is strongest when those fields are held at the right angle to each other. As the rotor spins, that ideal angle would be lost if the stator field stayed fixed, so the stator field must be continually rotated to stay correctly positioned ahead of the rotor. Commutation is the act of steering the stator field by switching current between the motor's phases, keeping the two fields in the productive relationship no matter where the rotor is.
In a brushed DC motor this is done mechanically. A commutator and brushes physically switch the current in the windings as the shaft turns, so the alignment is maintained automatically by the motor's own construction. The brushes wear and spark, which is a maintenance and reliability drawback, but the commutation logic is built into the hardware and needs no external intelligence. This is the original meaning of the word, and it is where the concept comes from.
A brushless servo motor removes the brushes and the wearing commutator, which improves reliability and performance, but it moves the responsibility for commutation to the drive. The drive must now decide, moment by moment, how to energize each phase so the stator field leads the rotor correctly, and it can only make that decision if it knows the rotor's angular position. That is why every brushless servo has some form of rotor position feedback: without knowing where the rotor is, the drive cannot commutate, and the motor cannot produce useful torque.
The simplest rotor position feedback for commutation is a set of Hall-effect sensors built into the motor, usually three of them, that report which broad sector the rotor is currently in. With this coarse position the drive can perform six-step or trapezoidal commutation, switching the phases in a fixed pattern as the rotor passes from one sector to the next. This is inexpensive and adequate for many applications, but because the position is only known in coarse sectors, the torque has a slight ripple as the drive switches abruptly between steps, which is felt most at low speed.
For smooth, high-performance servo control the drive uses much finer position information, usually from the same high-resolution encoder or resolver that closes the position loop, and applies field-oriented control. In field-oriented control the drive knows the rotor angle precisely and continuously, and it shapes the phase currents sinusoidally so the stator field is placed at exactly the optimal angle to the rotor at every instant. This produces smooth, ripple-free torque and gets the most torque per amp from the motor, which is why it is the standard for demanding motion applications.
Field-oriented control effectively separates the motor's current into a part that produces torque and a part that would only weaken or strengthen the field, and it keeps the torque-producing component aligned for maximum effect while holding the other near zero under normal operation. Achieving this depends entirely on the drive knowing the true electrical angle of the rotor, because the whole scheme is about placing current relative to that angle. The precision of the feedback and the accuracy of the drive's knowledge of the rotor angle are therefore what make the difference between rough and glassy-smooth torque.
For commutation to work, the drive must know not just the rotor's position but how that position relates to the motor's magnetic poles, and this relationship is captured in the commutation offset, sometimes called the phasing or the electrical angle offset. It is the alignment between the feedback device's zero and the motor's magnetic zero. This offset must be established when the motor and feedback are paired, either measured during a commissioning routine or stored in the motor's feedback device, and it must be correct for the drive to place the stator field where it belongs.
If the commutation offset is wrong, the consequences range from poor to dangerous. A modest error simply misaligns the fields so the motor makes less torque than it should and runs rough and hot, wasting current because much of it no longer produces useful torque. A large error can be far worse: if the drive's idea of the rotor angle is off by enough, the feedback that should stabilise the motor instead pushes it the wrong way, so commanding it to hold still or move gently causes it to accelerate uncontrollably. This runaway is a well-known symptom of bad phasing, and it is why commissioning a new motor and feedback combination always includes verifying commutation before trusting the axis.
In the field, commutation runs deep inside each drive, but its failures surface as exactly the kind of anomalies a monitoring system is built to catch. A motor that suddenly draws far more current for the same work, runs unusually hot, or trips on overcurrent may be suffering degraded commutation from a failing feedback device or a shifted offset, and a runaway on power-up points straight at a phasing problem. A cloud SCADA platform such as Merobix can trend motor current, temperature, and fault codes reported by drives across many machines and sites, so a drive that starts drawing excessive current or repeatedly faulting stands out in the trend. For remote equipment, catching that pattern centrally lets a team dispatch help for a commutation or feedback problem before it turns into a burned-out motor or an unexpected stoppage.
Hall sensors give the drive coarse rotor position, enough for six-step or trapezoidal commutation that switches phases in blocks and produces slight torque ripple, especially at low speed. A high-resolution encoder or resolver gives precise, continuous rotor angle, enabling field-oriented sinusoidal commutation that shapes the currents smoothly for ripple-free torque and the best torque per amp. Many servos use Halls for initial startup information and the encoder for fine running commutation.
The commutation offset tells the drive how the feedback's zero relates to the motor's magnetic poles, so the drive can place the stator field at the right angle to the rotor. If that offset is badly wrong, the drive misjudges the rotor angle and applies current that pushes the rotor the wrong way, so instead of the feedback stabilising the motor it drives it faster. That positive feedback appears as uncontrolled acceleration, which is why commutation is always verified when a motor and feedback are first paired.
No. A brushed DC motor commutates mechanically through its commutator and brushes, which switch the winding current automatically as the shaft turns, so no external position feedback or drive intelligence is needed for commutation. The trade is that the brushes and commutator wear and spark, reducing reliability and life. Brushless motors remove those wearing parts but require the drive to perform commutation electronically using rotor position feedback.
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