At an equipment expo, Xiao Q took a liking to a set of equipment. The manufacturer gave a detailed introduction to the equipment, specifically emphasizing the matching permanent magnet synchronous motor. After listening to the somewhat exaggerated presentation, Xiao Q remained silent for a long time. After all, the equipment's price was "high" due to its powerful power core, forcing the thrifty Xiao Q to weigh the pros and cons, making it a bit stressful. Today, Ms. Can will use Xiao Q's equipment purchase experience to briefly discuss permanent magnet synchronous motors.
I. Essential Requirements and Effects of Matching Permanent Magnet Motors
The installation of permanent magnet motors in equipment is not simply a matter of replacing the power core; it requires a corresponding upgrade from a conventional power supply to a controllable power supply, forming a permanent magnet motor system together with the motor itself. Compared to conventional motor systems, permanent magnet motor systems utilize power electronic converters for frequency and voltage transformation, allowing for smooth and wide-range adjustment of the motor output shaft speed and power. This results in a revolutionary improvement in equipment performance, enabling adaptive adjustment to economical operation based on load changes, leading to significant energy savings.
II. Permanent Magnet Motors and Ordinary Motors
1. Ordinary motor
Ordinary motors are divided into two main categories: asynchronous motors and synchronous motors. 70% to 80% of equipment uses asynchronous motors, while a small number of high-torque, low-speed equipment uses synchronous motors.
The origins of asynchronous and synchronous modes
The speed of an AC motor is closely related to the rotating magnetic field of the armature. The rotor speed of a synchronous motor is strictly synchronized with the rotating magnetic field of the armature, and the speed remains constant. However, the speed of an asynchronous motor is always lower than the speed of the rotating magnetic field. Under the action of the rotating magnetic field, the closed circuit of the rotor winding induces a current. In the magnetic field, the current-carrying winding of the rotor is subjected to the continuous action of the electromagnetic force of the magnetic field, which drives the rotor to rotate. Hence the terms asynchronous and induction motors.
The difference between synchronous motors and asynchronous motors
When a synchronous motor is running in steady state, there is a constant relationship between the rotor speed and the grid frequency: n = ns = 60f/p, where f is the grid frequency, p is the number of pole pairs of the motor, and ns is called the rotating magnetic field speed or synchronous speed. Asynchronous motors, on the other hand, have a slip problem, and their speed is close to the synchronous speed but slightly lower.
From the perspective of motor structure, the squirrel-cage rotor or wound rotor of an asynchronous motor is the same as the stator, with three-phase symmetrical windings. In contrast, the rotor of a synchronous motor is excited by salient-pole concentrated windings or non-salient-pole distributed windings, and the excitation power supply is DC.
2. Permanent magnet motor
Permanent magnet motors are divided into two categories: permanent magnet DC motors and permanent magnet AC motors.
Permanent magnet DC motors include permanent magnet commutator motors and permanent magnet brushless DC motors or permanent magnet electronic commutator motors. Permanent magnet AC motors, also known as permanent magnet synchronous motors, have the same structure as permanent magnet brushless DC motors, but their driving principles and control methods are quite different.
III. Permanent Magnet Synchronous Motors and Synchronous Motors
Permanent magnet synchronous motors and synchronous motors operate on the same principle, but the difference lies in the fact that permanent magnet synchronous motors are excited by permanent magnets and have no excitation windings, so there is no excitation loss; synchronous motors are electrically excited, and the excitation winding losses and heat generation are often the weak points of the ventilation and heat dissipation system.
Compared to synchronous and asynchronous motors, permanent magnet synchronous motors (PMSMs) do not have electrical excitation and corresponding losses. The permanent magnet rotor does not heat up, allowing for very high electrical load selection, resulting in smaller size and higher power density. With the rapid development of new motor control theories and rare-earth permanent magnet materials, the performance of PMSMs has been further improved, offering many unique advantages over ordinary motors. For example:
High efficiency and energy saving. Because the excitation magnetic field is provided by permanent magnets, the permanent magnet rotor does not require excitation, and the efficiency can reach over 90%. Compared with asynchronous motors, it has a wide high-efficiency operating speed range and significant energy savings. The advantages are even more obvious when operating at low speeds.
Low temperature rise. No electrical excitation means no heat generation, therefore, permanent magnet motors generally have very low temperature rise.
It has good starting performance. Self-starting permanent magnet synchronous motors generally also use asynchronous starting. During normal operation, the rotor winding of the permanent magnet synchronous motor is not active. The rotor winding of the permanent magnet motor can be designed to fully meet the requirements of high starting torque, such as increasing the starting torque multiple from 1.8 times to 2.5 times or more.
Impact on power grid operation. Asynchronous motors absorb a large amount of reactive current from the power grid, resulting in a significant amount of reactive current in the power grid transmission and transformation system. This leads to a decrease in the power grid's quality factor and increases the load on transmission and transformation equipment and power generation equipment. Simultaneously, reactive current consumes some electrical energy within the power grid and transmission and transformation system, causing low power grid operating efficiency. This, combined with the low efficiency of asynchronous motors and their excessive energy absorption from the grid, exacerbates energy loss and further burdens the power grid.
The unique advantages of permanent magnet motors, such as rotors without electrical excitation and high power factor, help improve the quality factor of the power grid or eliminate the need for compensators in the grid.
Permanent magnet motors are characterized by wide-range and high-efficiency operation and are widely used in the field of new energy vehicles.
Small size and light weight. The application of high-performance, ultra-strong permanent magnet materials has significantly reduced the size and weight of permanent magnet motors, and the power density is at least 1.5 times that of ordinary three-phase asynchronous motors.
