Control Techniques | What is a permanent magnet motor?

What is a permanent magnet motor?

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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 , 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 s, 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 s, ferrite (ceramic) permanent magnets appeared and were used in motors for small appliances. But, in the s, 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 s, 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.

In , the Danish physicist Hans Christian Ørsted threw electromagnetic theory into a state of confusion. Natural philosophers of the day believed that electricity and magnetism were two distinct phenomena, but Ørsted suggested that the flow of electricity through a wire created a magnetic field around it. The French physicist André-Marie Ampère saw a demonstration of Ørsted's experiment in which an electric current deflected a magnetic needle, and he then developed a mathematical theory to explain the relationship.

English scientist Michael Faraday soon entered the fray, when Richard Phillips, editor of the Annals of Philosophy, asked him to write a historical account of electromagnetism, a field that was only about two years old and clearly in a state of flux.

Faraday was an interesting choice for this task, as Nancy Forbes and Basil Mahon recount in their book Faraday, Maxwell, and the Electromagnetic Field. Born in , he received only a barebones education at church school in his village of Newington, Surrey (now part of South London). At the age of 14 he was apprenticed to a bookbinder. He read many of the books he bound and continued to look for opportunities to learn more. In a fateful turn of events, just as Faraday's apprenticeship was coming to an end in , one of the bookbinder's clients offered Faraday a ticket to Humphry Davy's farewell lecture series at the Royal Institution of Great Britain.

Davy, just 13 years older than Faraday, had already made a name for himself as a chemist. He had discovered sodium, potassium, and several compounds and invented the miner's safety lamp. Plus he was a charismatic speaker. Faraday took detailed notes of the lectures and sent a copy to Davy with a request for employment. When a position opened as a chemistry assistant at the Royal Institution, Davy hired Faraday.

After Faraday [left] failed to acknowledge his mentor, Humphry Davy [right], in an paper on the electric motor, Davy accused him of plagiarism.LEFT: ULLSTEIN BILD/GETTY IMAGES; RIGHT: BETTMANN/GETTY IMAGES

Davy mentored Faraday and taught him the principles of chemistry. Faraday had an insatiable curiosity, and his reputation at the Royal Institution grew. But when Phillips asked Faraday to write the review article for the Annals, he had only dabbled in electromagnetism and was a bit daunted by Ampère's mathematics.

At heart, Faraday was an experimentalist, so in order to write a thorough account, he re-created Ørsted's experiments and tried to follow Ampère's reasoning. His "Historical Sketch of Electro-Magnetism," published anonymously in the Annals, described the state of the field, the current research questions and experimental apparatus, the theoretical developments, and the major players. (For a good summary of Faraday's article, see Aaron D. Cobb's "Michael Faraday's 'Historical Sketch of Electro-Magnetism' and the Theory-Dependence of Experimentation," in the December issue of Philosophy of Science.)

While reconstructing Ørsted's experiments, Faraday was not entirely convinced that electricity acted like a fluid, running through wires just as water runs through pipes. Instead, he thought of electricity as vibrations resulting from tension between conducting materials. These thoughts kept him experimenting.

Faraday observed the circular rotation of a wire as it was attracted and repelled by magnetic poles. "Very satisfactory," he wrote in his notebook.

On 3 September , Faraday observed the circular rotation of a wire as it was attracted and repelled by magnetic poles. He sketched in his notebook a clockwise rotation around the south pole of the magnet, and the reverse around the north pole. "Very satisfactory," he wrote in his entry on the day's experiment, "but make more sensible apparatus."

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The next day, he got it right. He took a deep glass vessel, secured a magnet upright in it with some wax, and then filled the vessel with mercury until the magnetic pole was just above the surface. He floated a stiff wire in the mercury and connected the apparatus to a battery. When a current ran through the circuit, it generated a circular magnetic field around the wire. As the current in the wire interacted with the permanent magnet fixed to the bottom of the dish, the wire rotated clockwise. On the other side of the apparatus, the wire was fixed and the magnet was allowed to move freely, which it did in a circle around the wire.

For a helpful animation of Faraday's apparatus, see this tutorial created by the National High Magnetic Field Laboratory. And if you'd like to build your own Faraday motor, this video will walk you through it:

Although a great proof of concept, Faraday's device was not exactly useful, except as a parlor trick. Soon, people were snatching up pocket-size motors as novelty gifts. Although Faraday's original motor no longer exists, one that he built the following year does; it's in the collections of the Royal Institution and pictured at top. This simple-looking contraption is the earliest example of an electric motor, the first device to turn electrical energy into mechanical motion.

The fallout from Faraday's invention

Faraday knew the power of quick publication, and in less than a month he wrote an article, "On Some New Electromagnetic Motions and the Theory of Electromagnetism," which was published in the next issue of the Quarterly Journal of Science, Literature, and the Arts. Unfortunately, Faraday did not appreciate the necessity of fully acknowledging others' contributions to the discovery.

Within a week of publication, Humphry Davy dealt his mentee a devastating blow by accusing Faraday of plagiarism.

Davy had a notoriously sensitive ego. He was also upset that Faraday failed to adequately credit his friend William Hyde Wollaston, who had been studying the problem of rotary motion with currents and magnets for more than a year. Faraday mentions both men in his article, as well as Ampère, Ørsted, and some others. But he doesn't credit anyone as a collaborator, influencer, or codiscoverer. Faraday didn't work directly with Davy and Wollaston on their experiments, but he did overhear a conversation between them and understood the direction of their work. Plus it was (and still is) a common practice to credit your adviser in early publications.

When Faraday's reputation began to eclipse that of his mentor's, Faraday made several missteps while navigating the cutthroat, time-sensitive world of academic publishing.

Faraday fought to clear his name against the charge of plagiarism and mostly succeeded, although his relationship with Davy remained strained. When Faraday was elected a fellow of the Royal Society in , the sole dissenting vote was cast by the society's president, Humphry Davy.

Faraday avoided working in the field of electromagnetism for the next few years. Whether that was his own choice or a choice thrust upon him by Davy's assigning him time-consuming duties within the Royal Institution is an open question.

One of Faraday's assignments was to salvage the finances of the Royal Institution, which he did by reinvigorating the lecture series and introducing a popular Christmas lecture. Then in the Royal Society asked him to lead the Committee for the Improvement of Glass for Optical Purposes, an attempt to revive the British glass industry, which had lost ground to French and German lens makers. This was tedious, bureaucratic work that Faraday undertook as a patriotic duty, but the drudgery and relentless failures took a mental toll.

Faraday's experiments of yield the transformer and the dynamo

In , two years after Davy's death and after the completion of Faraday's work on the glass committee, he returned to experimenting with electricity, by way of acoustics. He teamed up with Charles Wheatstone to study sound vibrations. Faraday was particularly interested in how sound vibrations could be seen when a violin bow is pulled across a metal plate lightly covered with sand, creating distinct patterns known as Chladni figures. This video shows the phenomenon in action:

Resonance Experiment! (Full Version - With Tones)www.youtube.com

Faraday looked at nonlinear standing waves that form on liquid surfaces, which are now known as Faraday waves or Faraday ripples. He published his research, "On a peculiar class of acoustical figures; and on certain forms assumed by groups of particles upon vibrating elastic surfaces," in the Royal Society's Philosophical Transactions.

Still convinced that electricity was somehow vibratory, Faraday wondered if electric current passing through a conductor could induce a current in an adjacent conductor. This led him to one of his most famous inventions and experiments: the induction ring. On 29 August , Faraday detailed in his notebook his experiment with a specially prepared iron ring. He wrapped one side of the ring with three lengths of insulated copper wire, each about 24 feet (7 meters) long. The other side, he wrapped with about 60 feet (18 meters) of insulated copper wire. (Although he only describes the assembled ring, it likely took him many days to wrap the wire. Modern experimenters who built a replica spent 10 days on it.) He then began charging one side of the ring and looking at the effects on a magnetic needle a short distance away. To his delight, he was able to induce an electric current from one set of wires to the other, thus creating the first electric transformer.

Faraday's 29 August notebook entry describes his experiment with a wire-bound iron induction ring&#;the first electric transformer.HULTON ARCHIVE/GETTY IMAGES

Faraday continued experimenting into the fall of , this time with a permanent magnet. He discovered that he could produce a constant current by rotating a copper disk between the two poles of a permanent magnet. This was the first dynamo, and the direct ancestor of truly useful electric motors.

Two hundred years after the discovery of the electric motor, Michael Faraday is rightfully remembered for all of his work in electromagnetism, as well as his skills as a chemist, lecturer, and experimentalist. But Faraday's complex relationship with Davy also speaks to the challenges of mentoring (and being mentored), publishing, and holding (or not) personal grudges. It is sometimes said that Faraday was Davy's greatest discovery, which is a little unfair to Davy, a worthy scientist in his own right. When Faraday's reputation began to eclipse that of his mentor's, Faraday made several missteps while navigating the cutthroat, time-sensitive world of academic publishing. But he continued to do his job&#;and do it well&#;creating lasting contributions to the Royal Institution. A decade after his first breakthrough in electromagnetism, he surpassed himself with another. Not bad for a self-taught man with a shaky grasp of mathematics.

Part of a continuing serieslooking at photographs of historical artifacts that embrace the boundless potential of technology.

An abridged version of this article appears in the September print issue as "The Electric Motor at 200."

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