What is a permanent magnet motor?
15 listopada 2016

Leroy-Somer’s Mickael Villeger looks in detail at what is a permanent magnet motor, including a look back at how these useful industrial workhorses came back into mainstream use. 

Any device that turns electricity into motion, meaning electrical energy into mechanical energy, is called an electric motor. Due to the continuous need for increased power density and high efficiency levels, PM motors (permanent magnet) are now common among today’s motor market.

The first electric motors used bar magnets, and were more or less a ‘laboratory gadget’. These types of magnets were of a poor quality, and were generally considered no good for industrial applications. This limitation led numerous inventors to experiment with magnets of different sizes, shapes, configurations and materials, which resulted in the powerful and compact magnets used in today’s PM motors.

The creation of one of the first PM motors was by an inventor called Michael Faraday, who experimented with electrical fields and electromagnetism.  He built a rotating electrical machine that is commonly recognised as the world’s first electric motor. Faraday built a device that converted electrical energy into mechanical motion. This device used both fixed and rotating permanent magnets with wires attached to bowls of mercury and to a battery. When a battery was connected to the wires, current flowed in the circuit and the generated electromagnetic field interacted with the permanent magnets to produce torque and cause mechanical motion.

Even though motion had been created by the use of electromagnetic fields and magnets, inventors of electric motors knew quite early that permanent magnetic motors had severe limitations as far as practical applications were concerned. In 1882, electrician John Urquhart surmised that when the electro-motive machine is intended to exert any considerable amount of energy, it is advisable to replace the permanent magnets by electro-magnets. A considerable increase of power is yielded by motors when furnished with electro-magnets in place of PMs.

At the start of the 19th century, the world saw a renaissance in the discovery of new types of magnetic materials such as carbon, cobalt and wolfram steel. However, these first new magnetic materials were still of low quality. It wasn’t until the development of certain new hybrid magnets that the world would have a high quality resource that could be used for a lot of applications. This opened the door for the return of PM motors.

After extensive research in the 1930s, it was discovered that significant additions of aluminium, nickel and cobalt, combined with iron, produced a highly effective and commercially viable PM produced by conventional ingot casting. Called ‘alnico’ magnets, they were 100 times stronger than any lodestone. In the 1950s, ferrite (ceramic) permanent magnets appeared and were used in motors for small appliances. But, in the 1960s, another significant step occurred in the expanded use of PMs in electric motors when the compounds of rare-earth metals (samarium) and cobalt were invented. These PM materials were significant in and of themselves. Yet, they were soon overshadowed by the invention of neodymium-iron-boron PMs in the 1980s, which yielded both a higher energy product and were more common than the rare samarium and cobalt. The delay was due not only to the development of high energy PMs, but also the development of power devices and electronic controllers that could replace mechanical commutation with electronic commutation.

Unlike to induction motors, PM motors do not rely entirely on current for magnetization. Instead, magnets mounted on, or embedded in, the rotor couple with the motor’s current induced, internal magnetic fields, which are generated by electrical input to the stator. More specifically, the rotor itself contains permanent magnets, which are either surface-mounted to the rotor lamination stack or embedded within the rotor laminations.

As in common AC induction motors, electrical power is supplied through the stator windings. Permanent-magnet fields are, by definition, constant and not subject to failure, except in extreme cases of magnet abuse and demagnetization by overheating.

Although PM motors are more expensive than induction motors, they offer a longer operating life, improved efficiency, better thermal resistance, reduced size and weight. Due to these advantages, PM motors for industrial use are particularly favoured in pumps, fans, compressors and traction applications. They are one of the most common electrical components in use today.

Today Permanent Magnet Assisted Synchronous Reluctance Motor (PMASynRM) & nanocomposite permanent magnets are used. The advantages of adding permanent magnets to the synchronous reluctance motor rotor construction are the increased motor power factor and thus reduced motor stator Ohmic losses. The Ohmic losses represent the majority of the motor total losses. The advantage of reluctance torque is the decreased need of expensive permanent magnet material, which makes this solution thus cheaper than the respective PM motor.

Use of PM motors will continue to grow as they are used in new applications. There are also new innovations in the area of high energy permanent magnets. One of these innovations is nanocomposite permanent magnets. These magnets are artificially constructed magnetic structures (referred to as metamaterials) that produce strong permanent magnets by fabricating nanostructured hard/soft phase composite materials with dimensions less than a micrometer. Currently, they are being used in biomedicine, magnetic storage media, magnetic particle separation, sensors, catalysts and pigments. Indeed, in the future, the world may see nanocomposite magnetic materials finding use in future generations of PM electric motors.

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