Permanent magnets are more and more widely used in motors, and the degree of popularity is showing an increasingly strong trend. Why is there such a sensational effect? In addition to the drive of the energy-saving and environment-friendly economic development model, the deep-seated boosting factor is the inherent characteristics of permanent magnet motors.
The latest achievements in high performance permanent magnetic materials and power electronics technology have greatly promoted the application fields of permanent magnet motors, such as robots, aerospace, electric tools, generators, new energy, various medical equipment and electric or hybrid vehicles, etc. Permanent magnet motors are everywhere, and without exception declare the fact that permanent magnet motors have many advantages beyond traditional motors such as brush commutation DC motors, synchronous motors and induction (asynchronous) motors.
(1) The copper loss of the rotor is zero, and the natural efficiency is higher;
(2) High drive torque and output power per unit volume, making compact design possible;
(3) Slip rings, commutators, carbon brushes, etc. are eliminated, and the structure and maintenance of the motor are simplified;
(4) The air gap flux density is higher than that of traditional motors, so the dynamic performance is better;
(5) It can operate with high power factor;
(6) A simple six-phase switching voltage source can achieve precise torque, speed and position control.
Permanent magnet synchronous motor is mainly composed of rotor, end cover, stator and other components. The stator structure of permanent magnet synchronous motor is very similar to that of ordinary induction motor. The biggest difference between rotor structure and asynchronous motor is that high-quality permanent magnet poles are placed on the rotor. According to the position of permanent magnet placed on the rotor, permanent magnet synchronous motor is usually divided into surface rotor structure and built-in rotor structure.
The placement of permanent magnets has a great impact on the performance of motors. Surface rotor structure-the permanent magnet is located on the outer surface of the rotor core. This kind of rotor has a simple structure, but the asynchronous torque generated is very small. It is only suitable for occasions with low starting requirements and is rarely used. Built-in rotor structure-the permanent magnet is located in the iron core between the squirrel cage guide bar and the rotor shaft, and has good starting performance. Most permanent magnet synchronous motors adopt this structure.
The start-up and operation of permanent magnet synchronous motor (PMSM) is formed by the interaction of the magnetic field generated by stator winding rotor squirrel cage winding and permanent magnet. When the motor is at rest, a three-phase symmetrical current is passed through the given stator winding to generate a stator rotating magnetic field. The stator rotating magnetic field generates a current in the cage winding relative to the rotor rotation, forming a rotor rotating magnetic field. The asynchronous torque generated by the interaction between the stator rotating magnetic field and the rotor rotating magnetic field makes the rotor accelerate from rest. In this process, the rotor permanent magnetic field is different from the stator rotating magnetic field, which will produce alternating torque.
When the rotor accelerates to a speed close to the synchronous speed, the rotor permanent magnet magnetic field and the stator rotating magnetic field are nearly equal, and the stator rotating magnetic field speed is slightly higher than the rotor permanent magnet magnetic field. Their interaction generates torque and pulls the rotor into the synchronous operation state. In the synchronous operation state, no current will be generated in the rotor winding. At this time, only the permanent magnet on the rotor generates a magnetic field, which interacts with the rotating magnetic field of the stator to generate driving torque. It can be seen that permanent magnet synchronous motor starts by the asynchronous torque of the rotor winding. After the start-up is completed, the rotor winding no longer functions, and the magnetic field generated by the permanent magnet and the stator winding interacts to generate the driving torque.
