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Different methods for winding commutation of BLDC / EC motors

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Jürgen Wagenbach
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Question:

What is the difference between Block commutation versus Sinusoidal commutation versus Field Oriented Control (FOC)?

 

Technical background:

1.) Block commutation

Block commutation is a very simple way of motor winding commutation. The voltage applied to the motor windings is switched on / off in a defined sequence based on Hall sensor signal states, which results in generated torque and motor shaft motion. The drawback of this simple switching method is a remarkable torque ripple which can be critical, e.g. in case of (low speed) applications demanding for smooth operation, or position control at standstill, or precise torque output which should be independent of rotor position.

Most brushless motors (incl. maxon) have a sinusoidal (or close to sinusoidal) Back-EMF. In such a case block commutation causes a torque ripple of 14% due to the “hard switching” of the motor voltage at the motor windings. This torque ripple can be avoided by applying a sinusoidal motor current (as with “sinusoidal commutation” or FOC).

Main characteristics of Block commutation:

  • Rotor position measurement is based on Hall sensor signal states.
  • Motor voltage applied to the winding is switched every 60° (electrical), i.e. the motor current in each phase is turned on or off which results in a torque ripple of 14% (for motors with a sinusoidal Back-EMF).

 

2.) FOC vs. Sinusoidal Commutation

Field-oriented control (also known as FOC) and sinusoidal commutation are two different approaches to controlling three-phase BLDC motors. While sinusoidal commutation organizes the power supply to the motor in a sine wave, FOC uses a more complex control to regulate both the rotor's magnetic field and the motor's torque more efficiently. FOC generally offers better performance and efficiency, especially at variable speeds and loads, while sinusoidal commutation can be easier to implement.

Sinusoidal commutation:

  • The current in each phase is controlled independently with a dedicated control loop. The currents commanded to the control loops are sinusoidal with respect to the rotor position. They have identical amplitude but have a phase shift of 120° between them.
  • The achievable motor speed is limited by the bandwidth of the control loops. 
  • This method is relatively easy to implement and is often used in simple applications.

Field-oriented control (FOC):

  • FOC is a more advanced control method that treats the motor as two separate magnetic fields: the flux field (generated by the rotor) and the torque field (generated by the stator currents). FOC attempts to control these two fields independently of each other in order to achieve optimum performance and efficiency.
  • By using transformation matrices (e.g., Clarke and Park transformations), AC signal processing is converted to DC signal processing, resulting in more stable and efficient control.

FOC or Sinusoidal commutation?

  • FOC enables better torque control, higher efficiency over a wider speed range, and better dynamic response of the motor.
  • FOC is a more powerful method that is often used in demanding applications with dynamic servo motors and in electric vehicles, while sinusoidal commutation can be a good choice for simpler applications.
  • FOC requires more computing power than sinusoidal or block commutation. This means that the performance limit of the microcontroller is reached at a lower speed than with block commutation. For this reason, the maximum speeds vary depending on the commutation mode selected.

Caution: Risk of confusion and misunderstandings!

In everyday language and also in maxon manuals and software, the term “Sinusoidal commutation” is often used to refer to both FOC and sinusoidal commutation. Unfortunately, this is also the case in many manuals. Even in maxon software, only “Sinusoidal commutation” is mentioned during system setup (via the "Startup" wizard). In terms of terminology with a pure focus on commutation, this is correct, but the overall control concept is based on “FOC – Field Oriented Control”, although this is not explicitly mentioned.

 

Torque ripple: Practice versus theory?

  • Theoretically, the torque ripple should be 0% when using a motor with ironless winding and sinusoidal Back-EMF in combination with a FOC control. However, due to manufacturing tolerances of the motor components used (e.g. not 100% perfect sinusoidal Back-EMF) and sensors (e.g. tolerance of phase transition point), a slight torque ripple may still occur in practice.
  • For motors with iron-core windings (as often the case for third-party motors and also for maxon EC-i, EC-flat, EC-frameless, and IDX series), the cogging torque has to be considered too because it leads to additional torque ripple that can only be compensated for with special controller algorithms.
  • If no or just a neglectable low torque ripple within one motor shaft revolution is important for an application, a maxon motor with ironless winding in combination with a motor controller offering FOC and current controller clock rates of 25 kHz or higher should be selected. The EPOS4 and ESCON2 meet both requirements.

 

Commutation types of maxon controllers:

  • ESCON:
    Always "Block commutation".
    Regardless if an encoder is present in addition to the Hall sensors.
  • ESCON2:
    Always uses FOC.
    Even if only Hall sensors but no encoder are present.
  • EPOS4:
    “Block commutation” or “FOC - Field-Oriented Control” can be selected depending on the available sensors of the motor combination in use:
    • EPOS4 and Hall sensor only: 
      Only "Block commutation" is possible.
    • EPOS4 and Hall sensors plus encoder (incremental or SSI):
      "Block commutation" or "FOC" (mentioned as "Sinusoidal commutation") can be selected.
    • EPOS4 and SSI absolute encoder without Hall sensors:
      Only "FOC - Field-Oriented Control" is possible.
       

Cross references:

 

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