Field-Oriented Control

Field-oriented control (FOC) may also be known as vector control. It applies to three-phase brushless motors that operate in sinusoidal mode. FOC is a variable frequency drive (VFD) control method and is used to precisely control motor speed and torque by varying the input frequency and voltage. At Fisher & Paykel Technologies we use this control method for our direct drive motors, HVAC motors and high-speed motors.

The basic idea of the vector control algorithm is that it decomposes the stator current into a magnetic field-generating part and a torque-generating part. Both components can be controlled separately after decomposition, which simplifies the control of a permanent magnet synchronous motor (PMSM).

In sinusoidal commutation, the three phases of the motor are loaded with a sinusoidal current that are 120 degrees apart. This produces a magnetic field that rotates between the stator poles and rotor magnets. When the magnetic poles are synchronizing and aligning with the energized stator poles, rotating motion is created.

As shown below, we can see that the maximum torque is produced when the rotor’s magnet is at 90 degrees to the stator field. If the rotor magnet is fully aligned with the stator field, the magnet experiences a very strong outwards pull and creates no torque or rotation whatsoever. All other alignments will produce some amount of non-optimal torque.

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To make the motor run optimally, we need to know the real-time angular position of the rotor. This position determines where voltage will be applied next to ensure that the magnetic field on the stator is at 90 degrees. If we go back to our example and measure the rotor to be at 90 degrees, we need to create a magnetic field in the stator at 180 degrees. The sinusoidal wave diagram shows that to achieve this, we have to apply full positive voltage on V1, and 50% negative voltage on V2 and V3.

The sinusoidal commutation method, in which all three phases are permanently energized with a sinusoidal current, replaces the position sensor with a sophisticated algorithm that estimates the rotor position while maintaining precise control of the motor speed and torque delivery.

We have optimized the use of FOC for our motor controllers to monitor motor run parameters and adjust how your product behaves in real time. For washing machines, this means sophisticated load-sensing, with the controller enabling detection of load size, position, and content. The motor controller uses this information to manage the real-time behavior of the machine for the best wash performance.

Sensorless FOC offers many benefits which include:

  • High efficiency,
  • The elimination of the position sensor,
  • Improved response,
  • Reduced torque ripple,
  • Low audible noise,
  • Extended speed range operation, and
  • Reliability.