Vector control strategy can be applied to induction and synchronous machine: in this section, the field-oriented control related to a PMSM is analysed. In the final chapter, the implementation of this technique and the obtained results will be described, for evaluating the performance of both surface-mounted and interior permanent magnet synchronous motors. The previously analysed dynamic equations of this kind of motor are useful for implementing the FOC algorithm: thanks to the possibility of mathematically decoupling the stator current along d and q axes, the motor torque can be directly controlled.
This strategy is based on the control principle of a separate excitation DC motor; in that case, the armature current directly control the torque, while the excitation current is responsible for the magnetizing flux generation. For that kind of DC motor, the two currents are independently accessible and the armature magnetomotive force is orthogonal with respect to rotor flux, through mechanical commutation system such as brushes and commutators.194 In case of AC motors (synchronous or induction), the spatial angle between rotating stator field and rotor flux changes depending on the load characteristics. Field-oriented control emulates the DC conditions in AC motor structure, by monitoring the rotor field position and orienting the stator field so that the angle between both fields is maintained at 90 degrees.195 Consequently, the maximum torque condition can be achieved, and the rotor speed can be controlled independently.
193 Parmar, Y., et al. (2016, December), op. cit., p. 365.
194 Wei, L. (2017), op. cit., p. 160.
195 Amin, F., et al. (2017, May), op. cit., p. 387.
FOC requires a position sensor, such as a Hall effect sensor, for constantly monitoring the rotor position, in order to use this value for decoupling the stator current in ππ and ππ. The Hall effect sensor is a device that measures the magnitude of a magnetic field:
the output voltage is directly proportional to the magnetic field density around the sensor.
This device is made with a thin piece of semiconductor material passing a continuous current through itself; the external magnetic flux exerts a force on this material, deflecting the charge carriers (electrons and holes) to either side of the slab. As a consequence, a measurable voltage is generated.
Alternatively, an encoder can be used for analysing directly the rotor angular position or for obtaining it by integration of the measured angular velocity β the actual rotor speed is measured also for realizing the speed control loop, instead of the torque-based one. Encoders are useful device for measuring angular and linear velocity, through the relationship between the encoderβs pulse frequency and its rotational speed. The incremental encoder is able to report immediate changes in position: it uses two output square waves in quadrature A-B and β depending on the phase difference between them β it can determine the direction of rotation. The frequency of the two pulses A or B is directly proportional to the encoderβs β and consequently the motorβs β velocity. As drawback, the incremental encoder doesnβt keep track of the absolute position when the mechanism starts: for a correct measurement, this type of encoder must be moved to a reference fixed point to align it with the magnetic field generated by the permanent magnets present in the rotor.
The absolute encoder is a device that doesnβt require any initialization because it maintains position information when power is removed from the system. During the assembly phase, the relationship between the encoder value and the corresponding physical position is established: the multiple rings of the device represent various binary weightings and the combination of them provide the binary data that represents an absolute position. By increasing the number of rings, the accuracy of the measurement can be improved.
Moreover, the speed can be obtained by counting the number of complete revolutions.
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Figure 3.15: Incremental encoder on the left, absolute encoder on the right.
As practical example, the speed-loop FOC structure of a surface-mounted PMSM is shown in figure 3.16. This scheme doesnβt include blocks that are essential for a real-time application of the system, such as the saturations, the proportional integral (PI) controller for managing the battery pack and inverter DC and the PI for regulating the direct current in the constant power region β flux weakening area. In chapter V, the effective block schemes are provided, together with the obtained output results for both IPM and SPM synchronous motors; in that case, all the previous elements are inserted for realizing a more accurate and more reliable vector control implementation. In figure 3.16 the represented elements are:
1. the SPMSM;
2. the inverter for driving the motor;
3. a sensor for measuring the actual rotor mechanical angular position ππ = ππ and a derivative block for obtaining the actual angular speed Οπ. The electrical angle ππ is obtained from ππ by multiplying it for the number of pole pairs ππ;
4. the Clarke-Park transformations for computing the direct-quadrature stator current from the three-phase current in abc reference frame. For this mathematical block, the measurement of the actual electric angle is needed β the value ππ is obtained from ππ. Thanks to this decoupling action, the feedback stator currents ππ and ππ are controlled as DC quantities and are compared with the reference values ππβ and ππβ through subtractors;
5. the PI regulator is used for obtaining the reference quadrature stator current ππβ from the rotor speed error. Alternatively, from the speed error the reference motor torque
πβ can be obtained and then this value is converted into ππβ. As described in previous chapter, a SPM synchronous machine, there is a proportional dependence between torque and quadrature current in the constant torque region, through the torque constant ππ;
6. the reference direct stator current ππβ is always equal to zero in this simple example, because the flux-weakening region is not considered for extending the range of rotor speed values;
7. the PI controllers are used for estimating the reference dq voltage values from the direct and quadrature current errors;
8. the Clarke-Park inverse transform blocks are then used for computing the three-phase reference voltage π£πππβ from the direct-quadrature values π£πβ and π£πβ;
9. finally, a PWM (or alternatively a SVPWM) technique is implemented based on the three-phase reference voltages, and the outputs of this block are sent to the inverter.
As simplification for the vector control used for the thesis project, the three-phase reference waveforms π£πβ, π£πβ and π£πβ are used for directly driving the synchronous motor, without inserting the inverter. In fact, within the LabVIEW development environment ideal configurations are considered for the PMSM and the applied control strategy.
Figure 3.16: Speed-based field-oriented control scheme for a PMSM.196
Thanks to its feedback structure, the field-oriented control algorithm guarantees optimal dynamic performance and fast response, such as rapid acceleration and deceleration.
196 Rusu, C., Birou, I., Radulescu, M., & Bara, A. (2014), Developing Embedded Control System Platform for Testing PMSM Drives, Presented at International Conference and Exposition on Electrical and Power Engineering (pp. 677-682), Iasi, Romania, p. 679.
96 Using the PI regulators, a certain amount of overshoot is associated to reference input variation; by modifying the parameters of the controllers, the output result can be improved.
Nowadays, in induction and synchronous AC machines drive this method has displaced the scalar volts-per-Hertz control, because of its accuracy and reliability; for the same reasons, in industrial applications FOC offers safer real-time performance. The continuous and fast evolution of microcontrollers β in terms of computational capability and reduction of power consumption β is boosting the adoption of vector control technique also for lower level motor drives.