Permanent Rotor With Magnets For Motor
Permanent Rotors are widely used in nearly all rotating machinery in our lives, such as tiny hard drives, medium-sized brushless DC motors, massive wind turbines. As the name suggests, the rotor is the rotating part in the motor. It interacts with the stationary stator to generate a stable, powerful and continuous torque, enabling the conversion of electromagnetic energy into mechanical energy.
1. Why Use Neodymium Magnets in Rotors?
The permanent rotor consists of a rotor and permanent magnets mounted on the rotor. The magnets used on the rotor are permanent magnets. Permanent magnets include neodymium magnets, ferrite magnets, AlNiCo magnets, and SmCo magnets. Among them, neodymium magnets are the most widely used in rotors because of high energy product and coercivity, which helps to improve motor efficiency.
(I)High Remanence (Br): The higher the Remanence, the stronger the magnetic field can be generated. Under the same current, the motor can output a larger torque. Or the same torque can be output with a smaller current. This is essential for applications with strict space and weight requirements, as it can make motor lightweight and compact.
(II) High Intrinsic Coercivity (Hcj): Hcj is closely related to the stability of neodymium magnets at high temperatures. The higher the Hcj, the better the temperature stability of magnets and the stronger its resistance to demagnetization at high temperatures. This allows the motor to operate safely at higher temperatures.
(III) High Maximum Energy Product (BHmax): It means that the same magnetic field strength can be achieved with smaller magnets. This directly allows for more compact and lighter permanent rotors, reducing the moment of inertia and improving dynamic response speed (faster acceleration/deceleration). It can also reduce the costs by decreasing the amount of neodymium magnets used.
2. Why Use Arc Magnets in Rotors?
In permanent motors, arc magnets are the most commonly used in rotor. Compared to block or disc magnets, what advantages do arc magnets offer in a rotor?
- The Magnetic Field Distribution in the Air Gap is Closer to Sinusoidal Waveform
The air gap is the distance between the curved surface of the arc magnet on the rotor and the stator core. The air gap magnetic field distribution of neodymium magnets is uniform or varies according to a specific pattern. Compared to block magnets, arc magnets can produce an air gap magnetic flux density distribution that is closer to a sinusoidal waveform. It can improve the efficiency and performance of the motor, resulting in smoother operation and lower noise.
- Better Fit with the Rotor Core
The curved surface of arc magnets perfectly matches the outer diameter of the rotor core. The close fit between the two can reduce magnetic leakage inside the rotor. This allows the magnetic flux generated by the magnet to pass through the air gap into the stator more effectively, thereby improving the utilization rate of magnetic flux.
- Better Matching with the Rotating Magnetic Field
The magnetic direction of the arc magnets can better cooperate with the rotating magnetic field of the motor. This directly converts the advantages of high Br and high BHmax of neodymium magnets into powerful output torque.
- Stronger Resistance to Centrifugal Force
When the motor rotates at high speed, the rotor is subjected to enormous centrifugal force. The faster the motor spins, the greater the centrifugal force, and the higher the probability that the magnets will be thrown off. However, the design of arc magnets allows this force to actually make them adhere more firmly to the rotor, rather than being thrown off.
3. What is SPM and IPM?
In the permanent motors, the permanent magnets generate a constant magnetic field and the stator windings generate an alternative magnetic field. These two magnetic fields interact to convert the electromagnetic energy into the mechanical energy. Compared to traditional excited motors, permanent motors generate a magnetic field without consuming electrical energy in the rotor. This fundamentally reduces energy loss and improves motor efficiency. Based on how the neodymium magnets are installed on the rotor, they can be classified into surface-mounted rotors (left image) and Internal-mounted rotors (right image).
- What is SPM in Rotors?
The method of directly attaching arc neodymium magnets to the surface of the rotor core is called surface-mounted rotor (SPM). This installation method is simple and relatively low-cost, making it suitable for low-power motors, such as those used in household air conditioners and fans.
- What is IPM in Rotors?
The method of embedding magnets inside the rotor core is called internal-mounted (IPM). IPM rotors primarily use square or rectangular magnets. This installation method offers higher power density and demagnetization resistance, suitable for high-end applications such as new energy vehicle drive motors and industrial servo motors.
- What is the Difference Between SPM and IPM?
The table below shows the comparison of the characteristics of SPM and IPM for your reference.
| Surface-mounted (SPM) | Internal-mounted (IPM) | |
| Maximum speed | Low | High |
| Demagnetization resistance | Moderate | Strong |
| Heat dissipation of magnets | Good | Poor |
| Torque ripple | Low | High |
| Operating noise | Low | High |
| Cost | Low | High |
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