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PM Motors, commonly referred to as Permanent Magnet Motors play a role, in industrial applications. In contrast, to Induction Motors (IMs) PM motors incorporate magnets within. Attached to the rotor to produce a field during operation. The purpose of this article is to provide an understanding of the concepts, principles and essential elements associated with PM motors.
The key difference, between PM motors and induction motors lies in how the magnetic field’s generated. Induction motors rely on a rotating field produced by the stator windings to induce currents in the rotor, which then interacts with the stators field to generate driving force. A notable feature of induction motors is that there needs to be a speed difference between the rotor and the magnetic field to induce current making them suitable for use in combination with Variable Frequency Drives (VFDs), for speed control.
Magnet (PM) motors generate fields, with magnets, inside the rotor eliminating the need to connect flux through the stator field. This configuration enhances efficiency in applications that require speed control. Permanent magnet motors are classified as Surface Permanent Magnet Motors (SPM) and Interior Permanent Magnet Motors (IPM) depending on the placement of the magnets. These types differ in terms of durability, magnetic power and electromagnetic characteristics.
In order to understand the functioning of PM motors it is important to grasp the concepts related to flux flux linkage and magnetic fields.
When a current flows through a conductor it generates a field. Flux is the measurement of the movement of a characteristic, over an area. In motors flux indicates how rapidly the magnetic field expands across the wires surface.
When a magnetic field interacts with a material, such, as passing through a coil flux linkage occurs. This is calculated based on the number of turns in the winding and the magnetic flux often represented by the symbol ϕ to indicate the value of the flux, over time. The equation used to calculate flux linkage is λ = N × ϕ, where λ represents flux linkage N denotes the number of turns and ϕ signifies the flux.
The magnetic field illustrates how magnetism moves within a conductors space. In magnet motors magnets are positioned on. Affixed to the rotors surface to generate the field.
When discussing the characteristics of PM motors it’s important to take into account inductance and EMF as concepts.
Inductance (L): Inductance is defined as the proportionality constant of the induced voltage when current changes. In other words, inductance is the flux linkage per unit current. It is a geometric property related to the path of the current and is measured in Henrys (H). In PM motors, inductance can be divided into d-axis inductance and q-axis inductance based on the position of the rotor and the magnetic poles.
Back EMF: Back EMF refers to the voltage induced in the stator windings due to the relative motion between the rotor’s magnetic field and the stator windings during motor rotation. In PM motors, the rotor’s magnetic field is generated by permanent magnets, so as long as the rotor is moving, voltage will be induced in the stator windings. Back EMF increases linearly with motor speed and is a key factor in determining the motor’s maximum operating speed.
The d-axis and q-axis are two key axes used to describe the electromagnetic characteristics of PM motors.
d-axis (Direct Axis): This is aligned with the main flux direction of the motor. The d-axis inductance corresponds to the inductance value when the flux passes through the magnetic pole.
q-axis (Quadrature Axis): This is aligned with the main torque generation direction of the motor. The q-axis inductance corresponds to the inductance value when the flux flows between the magnetic poles.
For interior magnet PM motors, the d-axis and q-axis inductance values differ because the presence of magnets reduces the core material along the d-axis, affecting the inductance. In contrast, surface PM motors have almost identical d-axis and q-axis inductance values, as the magnets are on the rotor’s exterior and do not affect the stator magnetic field’s connection to the core.
Magnetic saliency describes the variation in d-axis and q-axis inductance at different rotor positions. This characteristic is crucial for the design and optimization of PM motors. Generally, magnetic saliency reaches its peak at an electrical angle of 90 degrees, where the difference between q-axis and d-axis inductance is greatest.
Magnetic Torque and Reluctance Torque are the two main components of the torque produced by PM motors. Magnetic torque is generated by the interaction between the rotor’s magnetic flux and the stator winding current, while reluctance torque arises from the alignment of the rotor axis with the stator flux field. The combination of these two determines the motor’s output torque.
In PM motors, the inductance values of the d-axis and q-axis decrease as the load current increases. This phenomenon is due to the magnetic saturation of the core material. When the flux reaches a certain level, the inductance of the core will no longer increase and may even decrease.
Flux weakening is a method of reducing the flux field to lower the back EMF, allowing the motor to operate at higher speeds. This operation typically requires additional motor current, and by adjusting the current direction on the d-axis, the motor can switch between flux strengthening or weakening to meet different operational demands.
PM motors can be classified into interior magnet and surface magnet types based on their structure. Each structural type has its pros and cons, and the specific design choice often depends on the application requirements. For example, interior magnet motors, with magnets embedded inside the rotor, have higher mechanical strength and are suitable for high-speed operation, while surface magnet motors are easier to manufacture and have lower costs.
The magnetic materials used in PM motors directly impact the motor’s performance. Commonly used permanent magnetic materials include neodymium iron boron (NdFeB) and samarium cobalt (SmCo), which exhibit different characteristics in terms of magnetic performance and high-temperature resistance. Therefore, selecting the appropriate magnetic material is crucial in motor design, depending on the specific application scenario.
With the advancement of drive technology, modern AC drives can achieve self-sensing and closed-loop control. By detecting and tracking the motor’s pole position, the drive can optimize the motor’s torque output and efficiency. This control method is widely used in servo motors, especially in applications requiring precise position control and high-speed response.
Servo motors often adopt an interior PM design and are paired with specific amplifiers. This combination, optimized and tuned by the manufacturer, ensures optimal operational performance. In practical applications, servo motors are commonly used in CNC machines, robotics, and automation equipment.
Although known as “permanent magnets,” these materials are not truly permanent. Their magnetism can weaken or fail due to changes in external conditions, such as mechanical stress, high temperature, or strong electromagnetic interference.
Mechanical Stress: Permanent magnets may lose their magnetism due to internal structural changes when subjected to severe impact or dropping.
The effect of temperature, on materials is that each one has a temperature called the “Curie temperature,” where it stops being magnetic.
The magnetic properties of magnets can be affected by interference, which could lead to a loss of their magnetism.
Therefore it is crucial to consider these demagnetization aspects and incorporate strategies when developing and utilizing PM motors.
X.Enneng：Advancing the Development of High Efficiency Permanent Magnet Motors
Enneng, a company named ENPMSM operates as a supplier of magnet motors based in Qingdao, China. They specialize in manufacturing types of magnet motors including standard, general and customized models offering both drive and gearless options. These motors are used in a range of industries such, as power plants, metallurgy, chemicals, mining and oil fields. Enneng is known for its focus on innovation and houses a research and development team that has secured technical patents. Their commitment, to progress has led to them being recognized as one of the standout “Hundred Innovative Enterprises” in Qingdao.
PM motors are recognized for their effectiveness and precision making them highly sought after in consumer applications. Understanding the principles and concepts of these motors is essential, for enhancing design and guaranteeing performance. As technology advances the utilization of PM motors is expected to expand contributing significantly to power systems.
