What is Permanent Magnet Synchronous Electric Motor?

Figure 1: Permanent magnet synchronous motor.

Permanent magnet synchronous motor (PMSM) is a synchronous motor that uses permanent magnets to establish an excitation magnetic field. It is mainly composed of stators, rotors and end covers, with the advantages of simple structure and energy saving. It is widely used in industry, agriculture and other industrial fields.

1. Internal Structure of Permanent Magnet Synchronous Motor

Figure 2: Structure of permanent magnet synchronous motor.

Permanent magnet synchronous motor generally consists of: stator, rotor, end cover and other components.
The stator winding surrounds the stator iron core. By controlling the frequency of the input current of the stator winding, the rotation frequency of the magnetic field can be controlled, thereby controlling the rotational speed.
There are permanent magnets on the rotor. According to the different positions of the permanent magnets, permanent magnet rotors are divided into: surface type and interior type.

Figure 3: Rotor of a surface permanent magnet synchronous motor.

Figure 4: All kinds of rotors of an interior permanent magnet synchronous motor.

The way the permanent magnets are placed has a big impact on motor performance. Surface rotor structure—the permanent magnets are located on the outer surface of the rotor core. This type of rotor has a simple structure, but generates a small asynchronous torque. It is only suitable for occasions with low starting requirements and is rarely used.
Buried rotor structure—the permanent magnet is located in the iron core between the squirrel cage bar and the rotating shaft, and the starting performance is good. Most permanent magnet synchronous motors use this structure.

2. How Does a Permanent Magnet Synchronous Motor Work

2.1 Working Process of Permanent Magnet Synchronous Motor

In a permanent magnet synchronous motor, when the stator is energized, the stator winding becomes a group of electromagnets, and the N pole and S pole of the electromagnet are evenly distributed along the inner side of the stator. The permanent magnet rotor itself has stable N pole and S pole. Therefore, there is an electromagnetic force between the stator and the rotor.
When the stator magnetic field rotates at a certain speed, the electromagnetic force pulls the rotor, and makes it follow the movement of the magnetic field, and try its best to keep the trend of synchronization with the magnetic field.

How does the stator magnetic field of a permanent magnet motor turn?/How does stator produce rotating magnetic field? The permanent magnet synchronous motor rotates the stator magnetic field by converting electrical energy into magnetic energy, instead of rotating the outer ring mechanically. As shown in the figure below, the coil on the stator is divided into several parts, for example, divided into 3 parts, and each part is energized in turn, so that a rotating magnetic field is available.

Figure 5: 3 sets of stator coils.

2.2 Working Modes of Permanent Magnet Synchronous Motor

There are two working modes of permanent magnet synchronous motor: one is to control the motor to achieve synchronization through a variable frequency speed controller, and the other is to achieve synchronization through asynchronous starting.
The permanent magnet synchronous motor cannot be started directly through three-phase AC, because the rotor inertia is large and the magnetic field rotates too fast, and the stationary rotor cannot start to rotate with the magnetic field at all.

Inverter Mode
The power supply of the permanent magnet synchronous motor is provided by the frequency converter. When starting, the output frequency of the frequency converter continuously rises from 0 to the working frequency, and the motor speed increases synchronously with the output frequency. Changing the output frequency of the frequency converter can change the motor speed. It is a very good frequency conversion motor.

Asynchronous Starting Mode
This starting and running method is formed by the interaction of the magnetic fields generated by the stator winding, the rotor squirrel cage winding and the permanent magnet. The method of directly supplying power with three-phase alternating current in occasions where speed regulation is not required is to add cage windings on the permanent magnet rotor.

When in the stationary state, the stator winding is fed with three-phase alternating current to generate the stator rotating magnetic field. The rotating magnetic field of the stator rotates, and an induced current is generated in the cage winding to form the rotating magnetic field of the rotor. The two magnetic fields interact to generate torque that causes the rotor to rotate from rest.

At the beginning of rotation, the rotational speed of the rotor's rotating magnetic field is not equal to that of the stator's rotating magnetic field, which will generate alternating torque.
When the rotating magnetic field of the rotor is almost synchronized with the rotating magnetic field of the stator, the rotor winding does not generate an induced current, and only permanent magnets on the rotor generate a magnetic field to produce driving torque.

Therefore, the rotor winding is used to realize a startup. After the startup is completed, the rotor winding no longer works, and the magnetic field of the permanent magnet and the stator winding interact to generate torque.

Figure 6: Rotor structure of permanent magnet synchronous motor.

3. Types of Permanent Magnet Synchronous Motor

For permanent magnet synchronous motors, permanent magnets are generally used, so excitation is generally not needed. Ordinary permanent magnet motors use permanent magnets as excitation, instead of the excitation coils in ordinary motors. Hybrid excitation motor has both permanent magnet and excitation coil, the role of this excitation coil is used to adjust the excitation field generated by the original permanent magnet.

1. Classification by power supply frequency
Permanent magnet brushless motors include permanent magnet brushless DC motors (supplied by square wave inverters) and permanent magnet brushless AC motors (supplied by sine wave inverters).

2. Classification by air gap magnetic field distribution
a) Sine wave permanent magnet synchronous motor: the magnetic poles are made of permanent magnet material. When a three-phase sine wave current is input, the air gap magnetic field is distributed according to a sinusoidal law, referred to as a permanent magnet synchronous motor for short.
b) Trapezoidal wave permanent magnet synchronous motor: The magnetic poles are still made of permanent magnet material, but when a square wave current is input, the air gap magnetic field is distributed in a trapezoidal wave shape, and the performance is closer to that of a DC motor. The self-controlled variable frequency synchronous motor composed of trapezoidal wave permanent magnet synchronous motor is also called brushless DC motor.

3. Classification according to the position of the permanent magnet on the rotor
a) Surface permanent magnet synchronous motor (SPMSM): The permanent magnets are usually tile-shaped and located on the outer surface of the rotor core. The important feature of this motor is that the main inductances of the straight and quadrature axes are equal.
b) Interior permanent magnet synchronous motor (IPMSM): The permanent magnets are located inside the rotor. There are pole shoes made of ferromagnetic material on the outer surface of the permanent magnet and inside the stator core, which can protect the permanent magnet. The important points of this type of motor is that the main inductance of the straight and quadrature axes are not equal.

Figure 7: Interior permanent magnet synchronous motor.

4. What are the Advantages of Permanent Magnet Synchronous Motor

The permanent magnet synchronous motor can integrally be installed on the axle to form an integral direct drive system, that is, one axle is one drive unit, eliminating the need for a gearbox. The advantages of permanent magnet synchronous motor are as follows:
1. The permanent magnet synchronous motor itself has high power efficiency and high power factor.
2. The permanent magnet synchronous motor generates less heat, so the motor cooling system is simple in structure, small in size and low in noise.
3. The system adopts a fully enclosed structure, no transmission gear wear, no transmission gear noise, free of lubricating oil and maintenance.
4. The permissible overload current of the permanent magnet synchronous motor is large, and the reliability is significantly improved.
5. The whole transmission system is light in weight, its unsprung weight is also lighter than that of the traditional axle transmission, and the power per unit weight is large.
6. Since there is no gearbox, the bogie system can be designed at will: such as flexible bogies and single-axle bogies, which greatly improves the power performance of the train.

7. Due to the use of permanent magnet material poles, especially the use of rare earth metal permanent magnets (such as NdFeB, etc.), its magnetic energy product is high, and a higher air gap magnetic flux density can be obtained. So in the same capacity, the volume of the motor is small and lightweight.
8. The rotor has no copper loss and iron loss, and no friction loss of collector rings and brushes, and has high operating efficiency.
9. It has small rotational inertia, large permissible pulse torque, high acceleration, good dynamic performance, compact structure, and reliable operation.

5. Disadvantages of Permanent Magnet Synchronous Motor

When the permanent magnet material is subjected to vibration, high temperature and overload current, its magnetic permeability may decrease, or demagnetization may occur, which may reduce the performance of the permanent magnet motor. In addition, the rare earth permanent magnet synchronous motor uses rare earth materials, and the manufacturing cost is not stable.

6. Application

Permanent magnet synchronous motors can be used in various fields such as aviation, national defense, industrial and agricultural production and daily life.

Industrial support: industrial drive devices, such as textile machinery, reducer support, pump support, fan support, mining industry equipment, material processing systems, automation equipment, robots, etc.
Transportation: electric cars, trams, aircraft auxiliary equipment, ships, etc.
Space field: rocket, aircraft, spacecraft, space shuttle, etc.
Defense field: tanks, missiles, submarines, aircraft, etc.
Industrial power generation: wind power generation, waste heat power generation, hydroelectric power generation, and generators for internal combustion generating sets, etc.

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