Permanent Magnetic Synchronous Motors

What Are Permanent Magnet Synchronous Motors?

The permanent magnet synchronous motors (PMSM) share similarities with traditional AC synchronous motors, but they differ in that the magnetic field is excited by permanent magnets, generating sinusoidal back EMF. They have components resembling typical rotating electric motors with a rotor and a stator, but PMSMs uniquely utilize just a rotor to create the magnetic field.

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Permanent magnet synchronous motors exhibit high efficiency, are brushless, secure, fast, and provide high dynamic performance. Their advantages have led to widespread adoption in various industries, with numerous manufacturers producing PMSMs with diverse specifications. AC synchronous motors are the predominant type of steady-state 3-phase AC motor in electrical systems, converting electrical energy into mechanical energy. They maintain a synchronous speed aligned with the AC supply frequency, improving the overall power factor within a system.

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How Permanent Magnetic Synchronous Motors Work

The operational principle of permanent magnet synchronous motors is based on the interaction between the stator's rotating magnetic field and the rotor's constant magnetic field. PMSMs offer excellent efficiency, being brushless, very fast, safe, and providing high dynamic performance compared to conventional motors. Instead of rotor windings, permanent magnets are used to create a rotating magnetic field, making these motors simple and cost-effective. A stator with three windings receives a 3-phase AC supply to initiate operation. PMSMs are characterized as AC synchronous motors whose field excitation relies on permanent magnets and features a sinusoidal back EMF waveform. With the inclusion of permanent magnets, PMSMs can generate torque even at zero speed.

Types of PMSM

The types of PMSM are categorized based on how magnets are affixed to the rotor and the rotor's design, classifying them into:

• Surface permanent magnet synchronous motor (SPMSM)
• Interior permanent magnet synchronous motor (IPMSM).
• SPMSM mounts all magnet pieces on the surface
• IPMSM places magnets inside the rotor

Moreover, depending on the stator design, PMSMs can further be classified as:

• With distributed winding
• With concentrated winding

Properties of Permanent Magnetic Synchronous Motors

• PMSM do not emit sparks and are safer in explosive environments.
• They are clean, very fast, and efficient.
• They are more compact and lighter than an AC induction motor (ACIM).
• PMSM can operate with or without position encoders.
• Smooth low- and high-speed performance, suitable for high-performance applications.
• Produce low audible noise and electromagnetic interference (EMI).
• Generate high torque when coupled with Field-Oriented Control (FOC).

Core Advantages

Key advantages of permanent magnet synchronous motors include:

• High power density
• Reduced noise
• Dynamic performance across both high and low-speed operations
• High efficiency at elevated speeds
• Efficient heat dissipation
• Low rotor inertia enhances control
• No torque ripple during motor commutation
• High and smooth torque and dynamic performance
• Durability against wear and tear
• Available in compact sizes
• Easier maintenance and installation compared to induction motors
• Retain full torque at low speeds
• High reliability

Further Reading: Advantages and Applications of Permanent Magnet Synchronous Motor

Disadvantages

Despite their benefits, the main disadvantages of permanent magnet synchronous motors are:

• Higher initial costs, making them more expensive compared to induction motors.
• Start-up can be challenging since they are not self-starting motors.

Applications of Permanent Magnetic Synchronous Motors

Permanent magnet synchronous motors find applications in various fields, including:

• AC compressors
• Refrigerators
• Direct-drive washing machines
• Automotive electrical power steering
• Machine tools
• Air conditioners
• Control of traction
• Data storage devices
• Servo drives
• Electric vehicle drivetrains
• Large power systems needing improved power factors
• Robotics
• Aerospace

Conclusion

PMSM serves as a 3-phase AC synchronous motor operating at synchronous speed with the AC source, offering smooth torque and low noise. Primarily utilized for high-speed applications like robotics, their operation hinges on the interaction between the stator's rotating magnetic field and the rotor's constant magnetic field. We appreciate your interest in our article and hope it enhances your understanding of permanent magnet synchronous motors.

For additional insights on magnet applications, consider visiting Stanford Magnets for further information. As a prominent global magnet supplier, Stanford Magnets has been active in R&D, manufacturing, and selling magnets since the early 2000s, providing customers with top-quality permanent magnets, including SmCo magnets, neodymium magnets, AlNiCo magnets, and ferrite magnets, at competitive prices.

Induction vs. Permanent Magnet Motor Efficiency

Motor efficiency is vital across industrial, consumer, and automotive sectors. Enhancing efficiency leads to lower greenhouse gas emissions by reducing power consumption and extending range between charges—applicable to everything from electric vehicles to power tools. As electrification continues to advance in our daily lives, it raises questions about the most suitable motor type to meet modern demands.

Historically, the induction motor has been favored for its availability and established technology. However, the induction motor's design, which necessitates rotor and stator slip, inherently caps its efficiency. Recent developments in permanent magnet materials (energy density) and manufacturing have enabled today's permanent magnet motor designs to achieve levels of performance and energy efficiency unattainable with induction motors.

Let’s delve deeper into both motor designs, providing insights into why permanent magnet designs are preferred over induction motors. Additionally, we'll explore how Soft Magnetic Composites (SMC) are revolutionizing traditional radial flux designs and contributing to next-generation performance while minimizing heat generation and optimizing copper and magnet material usage.

Consider the following when comparing induction and permanent magnet motors:

• Cost

• Efficiency—encompassing torque, core losses, frequency, and motor speed control

• Material considerations

• Applications

Permanent Magnet Motor vs. Induction Motor Efficiency

The layout of a permanent magnet motor (left) and an induction motor (right) reveals significant differences. In the permanent magnet design, the rotor houses a series of magnets positioned either internally or externally. The stator is wound with copper wire to create a magnetic field that interacts with the rotor’s permanent magnets, leading to rotation and torque. In contrast, the induction motor features a stamped lamination steel rotor and stator windings that generate an opposing magnetic field, inducing rotor rotation.

For further information, please visit our website PMSM Motor Manufacturers.

(Comparison of AC induction motor design vs. permanent magnet motor)

Modern high-torque motors, whether permanent magnet or induction, employ three-phase applied current for enhanced efficiency and self-starting capabilities. For fixed-speed applications, stator poles can be adjusted to achieve desired operational speeds at typical 50 or 60 Hz frequencies. In this scenario, laminated induction motors are often chosen. However, for variable-speed motors, a variable frequency power supply is needed. While induction motors can perform, permanent magnet designs typically yield better performance and flexibility.

While the intricacies of electric motor design are detailed, this overview serves as a useful starting point for weighing the merits of induction vs. permanent magnet motors.

Permanent Magnet Motor Efficiency

Permanent magnet motors inherently exhibit higher efficiency than induction motors, negating the delay associated with applied and induced fields. They operate synchronously with the applied frequency, meaning their speed is defined by frequency drives. As frequency increases, the overall losses in induction motors surpass those in permanent magnet motors, with efficiencies reaching up to 97.5%.

A 50 kW (approximately 70 HP) permanent magnet motor generally weighs under 30 lbs. Its rotational speed at any given frequency consistently surpasses that of its induction counterpart due to the obligatory slip characteristic of induction designs. The synchronous speed can be described by the equation:

Ns = 120 * frequency / pole count

(Ns signifies synchronous speed while pole count indicates total poles per phase, covering both north and south poles)

Currently, permanent magnet motors are utilized in high-efficiency applications, including the Ford Mustang Mach-E, BMW platforms, Tesla vehicles, energy-efficient variable frequency HVAC motors, battery-operated tools, and drones. This illustrates a prevailing trend—every battery-powered or high-efficiency dependent device utilizes a 3-phase permanent magnet motor.

Induction Motors:

As previously indicated, an induction motor’s operation stems from stator windings inducing an opposed current in the rotor, resulting in a magnetic field that facilitates rotor rotation. The inherent lag between stator current and resultant rotor field results in slip. Therefore, the maximum speed of an induction motor is governed by the same equation applicable to permanent magnet motors, with the added challenge of slip (asynchronous operation). As demonstrated below, as the slip approaches zero, the torque generated also wanes, making synchronous operation impossible. For instance, a two-pole AC induction motor operating at 60 Hz may have a synchronous speed of 3600 RPM, but a typical 5% speed loss due to slip reduces the maximum speed to around 3450 RPM, capping the maximum efficiency of induction motors to approximately 90-93%.

In contrast, the maximum efficiency of induction motors is confined to 90-93%, while permanent magnet motors consistently achieve upwards of 97%. Although a 4-7% enhancement might seem minimal, the operational cost savings over a decade or more reflect significant energy savings alongside reduced greenhouse emissions.

When we reference that a typical 50 kW (70 HP) permanent magnet motor weighs under 30 lbs?—the weight of a commonplace 75 horsepower induction motor can exceed 500 pounds, demonstrating considerable benefits for automotive designs due to weight reduction implications.

Cost Vs. Performance

One crucial factor in the choice of permanent magnet motors is the cost of magnets. Users of high-energy neodymium magnets can feel the financial burden (be it on their own budget or their supervisor's). Stamping lamination material can exacerbate costs further.

Powder metallurgy offers significant opportunities in creating these motors. Permanent magnet motor rotors can be constructed using sintered powder metal, whether through an internal or external design approach. The stator can also utilize soft magnetic composites, which, at high switching frequencies, surpass the losses of laminated 3% silicon iron, thereby enhancing design efficiency. Soft magnetic composites excel in high-frequency applications.

Powdered metal presents additional efficiency enhancements for permanent magnet motors over induction motors. The 3D fabrication capabilities of powder metallurgy enable the design of stators that encapsulate all wiring in soft magnetic composite materials, effectively removing end-turn losses.

These aspects highlight the many benefits that powder metallurgy—encompassing both sintered soft magnetic materials and SMCs—contributes to motor efficiency.

Induction Vs. Permanent Magnet Motor Efficiency: The Winner Is...

Ultimately, permanent magnet motors emerge as the superior choice. When integrated with a distinctive topology made possible by Soft Magnetic Composite (SMC) technology, your motor becomes lighter and more efficient, boasting higher torque density and reduced material costs, while also simplifying supply chain operations and supporting sustainable manufacturing processes.

Should you need assistance in developing components optimized for AC or DC magnetic applications through powder metallurgy, do not hesitate to reach out. For further resources, visit our resource hub:

(Editor's note: This article was originally published in April 2023 and was updated on November 29, 2023, and July 27, 2023)

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