A Simple Introduction to PMSM
1. Introduction to PMSM
PMSM stands for Permanent Magnet Synchronous Motor.
Motion control is a key requirement for many household appliances today, such as dryers, washing machines, refrigerators, air conditioners, and various kitchen appliances. To keep these devices working at their best, new and improved motor control technology is key. The new technology offers additional benefits, such as smoother operation and significantly reduced noise. This has led to the gradual elimination of inefficient AC induction motors in favor of more efficient alternatives, such as permanent magnet synchronous motors (PMSM) and brushless DC (BLDC).
Permanent magnet synchronous motors have gradually been widely used in industrial production and daily life due to a series of advantages such as high efficiency, large specific power, simple structure, and significant energy saving effect. Especially in recent years, with the successful development of high heat resistance and high magnetic performance NdFeB permanent magnets and the further development of power electronic components, the research and development of permanent magnet synchronous motors has entered a new period at home and abroad. In theoretical research and All application fields will undergo a qualitative leap, and are currently developing in the direction of ultra-high speed, high torque, high power, miniaturization, and high functionality.
Figure 1
2. PMSM Structure Analysis
The cross-sectional view of a high-power industrial PMSM is shown below. The stator and rotor are the most critical parts of the entire motor design, which determine the performance of the entire motor. Let's analyze them one by one.
Figure 2 PMSM Cross-section
The stator structure of permanent magnet synchronous motors is mostly 4-pole. The stator core of the motor in the figure below has 24 slots and is installed in the base. The motor winding is arranged in 3-phase 4-pole, using a single-layer chain winding, and a 4-pole rotating magnetic field is generated when power is applied. As shown in the figure below.
Figure 3 PMSM stator winding
The difference between permanent magnet synchronous motors and ordinary asynchronous motors is the rotor structure. Permanent magnet poles are installed on the rotor. There are many ways to arrange the permanent magnets in the rotor. The following introduces several main forms.
The first form: The permanent magnet poles are installed on the circumferential surface of the rotor core, which is called a surface protruding permanent magnet rotor.
According to the principle of minimum magnetic resistance, that is, the magnetic flux is always closed along the path with the minimum magnetic resistance, and the magnetic attraction is used to pull the rotor to rotate, so the permanent magnet rotor will rotate synchronously with the rotating magnetic field generated by the stator.
The second form: the permanent magnet poles are embedded in the surface of the rotor core, which is called a surface embedded permanent magnet rotor.
The third form: It is more commonly used in larger motors to embed permanent magnets inside the rotor, which is called an embedded permanent magnet rotor. The permanent magnets are embedded inside the rotor core, and there are slots for installing permanent magnets in the core. The main arrangement of permanent magnets is shown in the figure below. In each form, there is a combination of multiple layers of permanent magnets.
Figure 4 Embedded permanent magnet rotor
The following is an introduction to the radially arranged rotor as an example. To prevent the permanent magnet flux from short-circuiting, magnetic isolation slots are also opened in the rotor core, and magnetic isolation materials can also be filled in the slots. The permanent magnet is inserted into the installation slot of the rotor core, as shown in the left figure below. The polarity of the magnetic pole and the direction of the magnetic flux are shown in the right figure below. It can be seen that the magnetic isolation slots reduce the leakage of magnetic flux. This is a 4-pole rotor.
Figure 5 PMSM rotor
After installing the permanent magnet rotor core, insert the shaft and install the cooling fans on both sides of the rotor core. Then insert the rotor into the stator, install the end cover, and assemble the whole machine.
3. Working Principle and Control Method of PMSM
First, let's look at how to generate a rotating magnetic field. To simplify the analysis, we take a 3-phase 2-pole PMSM as an example. As shown in the figure below, the magnetic field in the motor rotates once in one cycle. The figure below also shows the dynamic changes of the magnetic field in the first half of the cycle.
Figure 6 Generation of rotating magnetic field
After knowing the working principle of PMSM, you need to know how to generate 3-phase AC power. Currently, the common method for low-power PMSM is to use MCU to generate 6 ways SPWM to control the MOS driver and drive 6 MOS tubes to generate 3-phase AC power, as shown in the figure below.
Figure 7 Hardware Diagram
Unlike the square wave control of BLDC, PMSM uses sine wave control, so the requirements for MCU are higher. Here we introduce a 3-phase SPWM generation method.
The following figure shows the principle of 3-phase SPWM generation, which is based on the triangle wave comparison method. For example, when the voltage of phase U is higher than the voltage of the triangle wave, the PWM output is high level, otherwise it outputs low level. When the frequency of the triangle wave is much higher than the input voltage frequency, the duty cycle of PWM changes linearly with the input voltage, and the period of PWM is equal to the period of the triangle wave.
Figure 8 3-phase SPWM generation principle
In actual applications, in order to reduce the workload of the MCU, the PWM waveform is not obtained by real-time triangle wave comparison, but the PWM duty cycle is first taken from the table and stored in the memory according to the sine law, and then obtained by table lookup output. As shown in the figure below, the entire system is a DDS frequency synthesis system, except that the traditional DAC is replaced by a PWM generation module.
In this system, the size of the waveform data table is 1024 points, and the PWM carrier frequency is 10KHz. The waveform data table is 1024 points, firstly for the convenience of calculation, because there is a phase interception operation in the process of looking up the table after phase accumulation (our phase accumulator is 16 bits, and the waveform data table is 1024 points--10 bits). In order to speed up this process, the use of a table with a size of 2n is conducive to speeding up the process and saving the MCU's computing time as much as possible. Secondly, a larger data table is also conducive to ensuring the accuracy of the waveform at low frequencies.
Figure 9 3-phase SPWM generation principle