|Video training: How Permanent Magnet Synchronous Motors are built and what is Field Oriented Control|
Please watch this video before going further. The rest of the article is a summary of the video.
The PMSM's rotor (Fig. 2) is usually made of permanent magnets mounted on the surface (SPM) or inside the rotor (IPM) and a steel shaft that transit the torque to various devices.
The motor we are going to use thin this workshop was chosen to facilitate the development of the sensorless motor control. It has two pole pairs which are built in so called salient pole rotor type. This means that the stator flux path lengths varies in respect with the rotor position. This is a fancy way to say that the magnetic reactance is different between the motor flux and torque axes. This will helps us to build a magnetic salient tracking observer to estimate the rotor position and to eliminate the need for having a position sensor mounted on the motor shaft.
If we oversimplify the physical processes that happens inside the stator coils and yoke, and instead of a continuous AC current waveforms we conveniently choose only 6 strategic interest points then you can easily see how the resulting magnetic flux vector is shifting its direction in sync with the AC current systems.
Controlling the position and strength of the magnetic field produced in the stator is the first step towards the Field Oriented Control.
To align the stator flux vector with the rotor torque axis a the key aspect of Field Oriented Control technique is to know the position of the rotor at any moment (the angle between the rotor flux axis and the stator phase A magnetic axis). Without this critical piece of information the whole mechanism will fail to perform.
The Field Oriented Control gives us a the possibility to simplify the entire control structure of the synchronous machines using a series of mathematical transformations that allows us to think and act as in a linear domain rather that in a complex one where quantities varies in time and space. In other words, by using the FOC and the concepts derived from it we can transforms a 3-phase AC motor into a simple 1-phase DC motor where magnetic field and motor torque can be controlled separately and this mechanism is described in Fig. 5.
Starting from a 3-phase PMSM standard mathematical model that is described by 3 differential equations in a fixed reference system, we can perform a simple Clark transformation to reduce the number of equations and then have them translated using Park transformation from a time domain into a Laplace domain where the model can be described by 2 simple linear algebraic equations as shown in Fig. 6.
That's the beauty of Field Oriented Control.
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