Exploring Permanent Magnet Motors

As its name would suggest, a permanent magnet motor contains a permanent magnet that is composed of a material that can be magnetized. In a permanent magnet motor, the magnetic field of the rotor or stator is produced by this permanent magnet, generating a constant motor flux; this is unlike a temporary electromagnet, where the magnetic field depends on the electric current.

Types of Permanent Magnets

Permanent magnets have been used in electric motors for more than a century. The increasing demand for energy-efficient motors has resulted in the wider implementation of permanent magnet motors in various market segments, including Aerospace and Defense, Robotics, and Medical. Because motors are exposed to a variety of demanding environmental conditions and must meet varying torque, power, and cost requirements, selecting the right type of permanent magnet for your application becomes critical. Here are the four types of permanent magnets:

1. Ferrite Magnet. Ferrite magnets are composed of oxides like SrO, BaO, or Fe2O3. These magnets are ideal for applications prioritizing price due to the lower costs of raw materials and simple manufacturing techniques. While the mechanical strength of and energy produced by Ferrite magnets are very low, their corrosion resistance property is incomparable. This makes it an ideal motion solution for devices operating in demanding environmental conditions, such as those in automobile and power tools applications. Portescap’s 26mm to 60mm can stack stepper motors feature Ferrite magnets.
2. Alnico Magnet. Alnico magnets are primarily composed of a combination of aluminum, nickel, and cobalt, but can also include copper, iron, and titanium materials. Alnico magnets have a broad range of temperature stability and the lowest temperature coefficient of the other permanent magnets; in fact, the maximum working temperatures of an Alnico magnet can extend to 500°C/932°F. These also display a high corrosion resistance, ensuring there is no need for any surface treatment and making them an ideal solution for applications operating in severe temperature and environmental conditions. Though these magnets have low energy, they are a cost-efficient choice when compared to rare earth magnets. Keep in mind that an Alnico magnet has the lowest resistance to demagnetization; this means the magnetization and assembly of these magnets in the motor are critical. These magnets are used in Portescap’s brush DC motors like the 16DCP, 22DCP, and 22N48.
3. Samarium Cobalt Magnet. This is a rare earth magnet comprised of two basic elements: samarium and cobalt. Sm-Co magnets are extremely resistant to demagnetization, can work at higher temperatures up to 350°C/662°F, and have a good residual magnetic flux density; they also benefit from excellent coercivity and higher temperature ratings. However, because of Earth’s low samarium reserves, Sm–Co magnets are quite expensive. Samarium cobalt magnet motors are often used in the Medical industry, including surgical hand tool applications. These magnets can be found in a few of Portescap’s Disc Magnet Motors (DMM) motors and BLDC motors.
4. Neodymium Magnet. This is a rare earth magnet made from an alloy of neodymium, iron, and boron. These magnets produce the largest energy and have a high remanent magnetization, high coercive force, and comparably good mechanical strength for processing complex shapes. These magnets are widely used in various applications that require high efficiency and high torque density. However, because the material contains large amounts of neodymium and iron, the NdFeB magnets are easily corroded and thus must be coated with nickel (Ni), zinc (Zn), gold (Au), chromium (Cr), epoxy resin, etc. One of the disadvantages of these magnets is their low performance temperatures, as the magnetic loss at high temperatures is relatively large. However, most losses of rare earth magnets are reversible or recoverable. Motors with neodymium magnets deliver the best performance in a compact size; this is why they are found in a wide range of industries, including Medical, Robotic, Aerospace & Defense, and Industrial Automation. The majority of Portescap’s motors feature neodymium magnets.

Considerations When Selecting a Permanent Magnet Motor

Permanent magnet electric motors are highly desirable when compared to conventional motors with electromagnets, being characterized by the following advantages:

• Higher power/torque density
• Higher efficiency
• Possible reduction in motor weight
• Simplified stator or rotor structure
• Lower rotor inertia for the same power rating
• Improved thermal behavior due to fewer rotor/stator losses
• Improved speed torque characteristics

  There are also a few items to consider when choosing to utilize a permanent magnet motor. One of the primary considerations is cost, as permanent magnet motors tend to be more expensive than conventional motors. A second consideration is strength. The strength of an electromagnet can be adjusted by the amount of electric current; this allows it to be attuned to different strengths, which offers greater control over torque and power. However, a permanent magnet’s strength depends upon the material used in its manufacturing. It’s also possible for a permanent magnet to partially or completely lose its magnetism under certain conditions. Below are some factors that impact the magnetic properties of permanent magnets:

• Improper installation processes during assembly
• Changes in environmental conditions
• Excessive temperatures
• Strong external magnetic field
• Strong mechanical shocks

Conclusion

Selecting the right motor with the right permanent magnet can be difficult. The good news? Portescap’s entire product portfolio is comprised of permanent magnet motors, and our engineers are ready to help you select the one with the best fit for your application. Reach out to us here – we’ll be happy to help!