Is permanent magnet synchronous motor control too difficult?

The permanent magnet synchronous motor is developed from the electrically excited three-phase synchronous motor. It uses permanent magnets to replace the electric excitation system, thereby eliminating the excitation coil, collector ring and brush, and the stator is basically the same as the electrically excited three-phase synchronous motor, so it is called a permanent magnet synchronous motor (PMSM). The PMSM used for vector control requires that its permanent magnet excitation magnetic field waveform is sinusoidal, which is also a basic feature of PMSM.

FOC-Field Oriented Control (FOC) is usually called "vector control". It is a variable frequency drive control method that controls the three-phase AC motor by controlling the amplitude and frequency of the inverter output voltage. By measuring and controlling the stator current vector of the motor, the excitation current (Id) and torque current (Iq) of the motor are controlled respectively according to the principle of field oriented control, so that the three-phase AC motor is equivalent to DC motor control.

Through coordinate transformation, the three-phase stationary coordinate system is transformed into a two-phase rotating coordinate system, so that the three-phase AC coupled stator current is converted into mutually orthogonal, independently decoupled torque and excitation components, achieving the purpose of directly controlling torque similar to that of a separately excited motor. The principle block diagram of realizing FOC is as follows:

FOC is mainly achieved by controlling current and angle. The current comes from the decoupling after phase current acquisition. Usually, there are roughly the following types of current sampling using FOC control:

1 Three-resistance sampling; 2 Double-resistance sampling; 3 Single-resistance sampling; 4 Current sensor sampling; 5 Collecting MOS internal resistance Rdson current sampling.

Another element to realize the FOC solution with Hall is the angle estimator (angle observer when there is no Hall). Vector control requires continuous position signals and needs to subdivide the angle, as shown in the figure below:



The angular velocity of each 60° electrical angle hall sector can be calculated by the formula. In the next hall sector, the change in angle can be obtained by integrating the angular velocity over time, thereby realizing the estimation of position and speed.

Space vector modulation (SVPWM) is a relatively common and novel control method. It is a pulse width modulation wave generated by a specific switching mode composed of six power switching elements of a three-phase power inverter. Through the principle of volt-second balance, the stator flux is controlled to change in a sinusoidal manner, and the output current waveform can also be made close to the ideal sinusoidal waveform.

By processing the sampled current and the angle together to form a wave, the output waveform can be made close to the sine wave based on SVPWM by controlling the switching of the power device to finally realize the FOC solution. The overall control system solution is shown in the figure below