Introduction

The Permanent Magnet Synchronous Motor (PMSM) is one of the preferred choices for motor control applications. Due to its permanent magnet rotor, it has higher torque with a smaller frame size and no rotor current, all of which are advantages over AC Induction Motors (ACIMs). With their high power-to-size ratio, PMSMs can help make the design smaller without the loss of torque. These strengths enable the usage of PMSMs in a broad range of variable frequency drive (VFD) applications.

Field Oriented Control (FOC) is the conventional choice for controlling the PMSM-based VFD. The FOC takes phase currents and the rotor flux angle of the three-phase AC motor as inputs, and generates a commutation pattern for a three-phase voltage source inverter, such that the resulting stator flux is at a specified angle to the rotor flux. This provides optimal torque and speed performance. The FOC essentially achieves this by transforming the three-phase components in the stator reference frame to two decoupled components in the rotor flux reference frame: one for controlling the overall flux in the motor and the other for controlling the torque.

The transformation from the stator reference frame to the rotor reference frame requires precise knowledge of the rotor flux angle. The rotor flux angle can be obtained by using either the appropriate position sensors (in sensored FOC), or by using the position estimators (in sensorless FOC). In sensorless FOC, the challenge is to implement a robust speed estimator that can reject perturbations, such as temperature, switching noise, electromagnetic noise, and so on. The sliding mode observer provides an excellent choice when it comes to a rotor position estimator under the presence of unknown signals and uncertainties.

Several documents from Microchip explain the principles behind FOC. An example is AN2520 Sensorless Field Oriented Control (FOC) for a Permanent Magnet Synchronous Motor (PMSM) Using a PLL Estimator and Equation-based Flux Weakening (FW) (see References).

This document covers the following topics:

  • Permanent Magnet Synchronous Motor (PMSM)
  • Field Oriented Control (FOC) principle
  • Rotor position and speed estimation with a Sliding Mode Observer (SMO)
  • Design and implementation of Sensorless FOC Example Software