1 Field Oriented Control (FOC)
In case of the PMSM, the rotor field speed must be equal to the stator (armature) field speed (i.e., synchronous). The loss of synchronization between the rotor and stator fields causes the motor to halt.
FOC represents the method by which one of the fluxes (rotor, stator, or air gap) is considered as a basis for creating a reference frame for one of the other fluxes with the purpose of decoupling the torque and flux-producing components of the stator current. The decoupling assures the ease of control for complex three-phase motors in the same manner as DC motors with separate excitation. This means the armature current is responsible for the torque generation, and the excitation current (for a PMSM motor, permanent magnet) is responsible for the flux generation. In this application note, the rotor flux is considered as a reference frame for the stator and air gap flux.
Several application notes from Microchip describe the principles behind FOC. Two such examples are (see References): "AN1078 - Sensorless Field Oriented Control of PMSM Motors" and "AN908 - Using the dsPIC30F for Vector Control of an ACIM".
It is beyond the scope of this application note to explain the FOC details; however, the particulars of the new implementation will be covered with respect to the previously indicated application notes.
The control scheme for FOC is illustrated in Figure 1-1. This scheme was implemented and tested using the dsPICDEM™ MCLV-2 Development Board (DM330021-2), which can drive a PMSM motor using different control techniques without requiring any additional hardware.
The control scheme is similar to the one described in application note "AN1292 - Sensorless Field Oriented Control (FOC) for a Permanent Magnet Synchronous Motor (PMSM) Using a PLL Estimator and Field Weakening (FW)" (see References), except for the flux weakening.
The particularity of the FOC in the PMSM is that the stator’s d-axis current reference Idref (corresponding to the armature reaction flux on d-axis) is set to zero. The rotor’s magnets produce the rotor flux linkage, ΨPM, unlike ACIM, which needs a constant reference value, Idref, for the magnetizing current, thereby producing the rotor flux linkage.
The air gap flux is equal to the sum of the rotor’s flux linkage, which is generated by the permanent magnets plus the armature reaction flux linkage generated by the stator current. For the constant torque mode in FOC, the d-axis air gap flux is solely equal to ΨPM, and the d-axis armature reaction flux is zero.
On the contrary, in constant power operation, the flux generating component of the stator current, Id, is used for air gap flux weakening to achieve higher speed.
In Sensorless control, where no position or speed sensors are needed, the challenge is to implement a robust speed estimator that is able to reject perturbations, such as temperature, electromagnetic noise and so on. Sensorless control is usually required when applications are cost sensitive, where moving parts are not allowed, such as position sensors or when the motor is operated in an electrically hostile environment. However, requests for precision control, especially at low speeds, should not be considered a critical matter for the given application.
The position and speed estimation is based on the mathematical model of the motor. Therefore, the closer the model is to the real hardware, the better the estimator will perform. The PMSM mathematical modeling depends on its topology, differentiating mainly two rotor types: surface-mounted and interior permanent magnet. Each type has its own advantages and disadvantages with respect to the application needs. The proposed control scheme has been developed around a surface-mounted permanent magnet synchronous motor, see Figure 1-2, which has the advantage of low torque ripple and lower price in comparison with other types of PMSMs. The air gap flux for the motor type considered is smooth so that the stator’s inductance value, Ld = Lq (non-salient PMSM), and the Back Electromotive Force (BEMF) is sinusoidal.
The fact that the air gap is large (it includes the surface mounted magnets, being placed between the stator teeth and the rotor core), implies a smaller inductance for this kind of PMSM with respect to the other types of motors with the same dimension and nominal power values. These motor characteristics enable some simplification of the mathematical model used in the speed and position estimator, while at the same time enabling the efficient use of FOC.
When using a surface PMSM, the FOC maximum torque per ampere is obtained by keeping the motor’s rotor flux linkage situated at 90 degrees behind the armature generated flux linkage, see Figure 1-3.
Considering the FOC constant power mode, the flux weakening for the motor considered cannot be done effectively because of the large air gap space, which implies weak armature reaction flux disturbing the rotor’s permanent magnets flux linkage. Due to this, the maximum speed achieved should not exceed more than double the base speed for the motor considered for testing. Figure 1-4 illustrates the phasors orientation in constant power, Flux Weakening (FW) mode.
