Electric motor

• 1. Introduction

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  On the 3rd of September 1821, in the basement of the Royal Institution, Michael Faraday dipped a free-hanging wire into a pool of mercury. A permanent magnet sat in that pool. When current flowed through the wire, the wire began to rotate around the magnet, tracing a close circular magnetic field. This was the electric motor's first demonstration with rotary motion, and Faraday mailed pocket-sized models of the device to colleagues around the world so they too could witness it. An electric motor is a machine that converts electrical energy into mechanical energy. Most do this through the interaction between a magnetic field and electric current in a wire winding, producing a force that becomes torque on the motor's shaft. How did a spinning wire over toxic mercury become the machine that, today, consumes more than half of the electricity produced in the United States? Who built the first useful versions, why did some inventors go bankrupt, and how does the same device, run in reverse, give back the energy it would otherwise waste? The answers reach from Scottish monks and Hungarian self-rotors to a 100-horsepower motor turning an artificial waterfall at a Frankfurt exhibition.

• 2. Anatomy Of Motion

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  The rotor moves and the stator does not. These are a motor's two mechanical parts, and together with the field magnets and the armature they form a magnetic circuit. The field magnet is usually on the stator and the armature on the rotor, though these may be reversed. The magnets, whether electromagnets or permanent magnets, create a field that passes through the armature and exerts force to turn the shaft. The stator core is built from many thin metal sheets, insulated from each other and called laminations. They are made of electrical steel, chosen for its specified magnetic permeability, hysteresis, and saturation. Without lamination, induced circulating eddy currents would flow through a solid core and waste energy. Mains-powered AC motors go further, immobilizing the winding wires by impregnating them with varnish in a vacuum so they cannot vibrate, abrade their insulation, and fail early. An air gap between the stator and rotor lets the rotor turn, and its width significantly affects the motor's electrical characteristics. The gap is made as small as possible, because a large one weakens performance, while a gap that is too small adds friction and noise. For decades, the failure to recognize the extreme importance of this small gap delayed efficient motor design. The St. Louis motor, long used in classrooms, is inefficient for exactly that reason. Inside, the armature carries wire windings on a ferromagnetic core, and the configurations split into two families. A salient-pole machine has projecting poles that face each other, with wire wound around each below the pole face. A nonsalient-pole machine uses a smooth cylinder with windings distributed evenly in slots around the circumference. A commutator solves a different problem entirely, and it is where the next chapter begins.

• 3. The Switch That Spins

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  A commutator is a rotary electrical switch that supplies current to the rotor and periodically reverses its direction as the shaft rotates. It is a cylinder of multiple metal contact segments on the armature. Two or more brushes, made of a soft conductive material like carbon, press against it and make sliding contact with successive segments. With each half turn of 180 degrees, the commutator reverses the current in the rotor windings, so torque always pushes in the same direction. Without that reversal, the torque on each winding would flip every half turn and stop the rotor. The brushes that make this possible are also the source of the design's troubles. They create friction, and they spark while crossing the insulating gaps between segments. That sparking limits the machine's maximum speed, because too-rapid sparking can overheat, erode, or even melt the commutator. The sparks also generate electrical noise and RFI, and the brushes eventually wear out and need replacement. On a large motor, the commutator assembly is a costly element requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it means replacing the rotor itself. Designing brushes is a constant trade-off. Large brushes maximize contact area and motor output, while small brushes have low mass for higher speed without excessive sparking, and they cost less. Because of these limits, commutated motors have been mostly replaced by brushless, permanent magnet, and induction motors.

• 4. Sparks Of Invention

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  In the 1740s and 1750s, the first electric motors were simple electrostatic devices, described in experiments by the Scottish monk Andrew Gordon and the American experimenter Benjamin Franklin. They required high voltages that were difficult to generate, so they were never used for practical purposes. The principle behind them, Coulomb's law, was discovered but not published by Henry Cavendish in 1771, then found independently and published by Charles-Augustin de Coulomb in 1785. Everything changed once persistent electric currents became possible. Alessandro Volta invented the electrochemical battery in 1799. In 1820, Hans Christian Orsted discovered that an electric current creates a magnetic field, and within a few weeks Andre-Marie Ampere produced the first formulation of the electromagnetic interaction. In 1827, the Hungarian physicist Anyos Jedlik began experimenting with electromagnetic coils and called his early machines electromagnetic self-rotors. By 1828 he had demonstrated the first device containing the three main components of a practical DC motor: stator, rotor, and commutator. It used no permanent magnets, drawing all its magnetic fields from the currents in its windings. Then came the inventors who tried to sell the idea, and nearly all of them ran into the same wall. William Sturgeon built the first commutator capable of turning machinery in 1832. Following his work, Thomas Davenport and Emily Davenport built a commutator-type DC motor, patented in 1837, that ran up to 600 revolutions per minute and powered machine tools and a printing press. The high cost of primary battery power made these motors commercially unsuccessful and bankrupted the Davenports. With no electricity distribution system yet available, no practical market could emerge. In May 1834, the German-Russian Moritz von Jacobi created the first real, useful rotating electric motor, setting a world record he improved in September 1838 with a second motor powerful enough to drive an electric boat carrying 14 people across a wide river. The path from curiosity to commerce still needed a better armature.

• 5. The DC Workhorse Arrives

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  In 1864, Antonio Pacinotti first described the ring armature, with symmetrically grouped coils closed upon themselves and connected to commutator bars whose brushes delivered practically non-fluctuating current. This was the breakthrough that made smooth DC machines possible. In 1871, Zenobe Gramme reinvented Pacinotti's design, adopted some of Werner Siemens's solutions, and produced the first commercially successful DC motors. A discovery about these machines proved as important as the machines themselves. Siemens announced the reversibility of the electric machine in 1867, and Pacinotti observed it in 1869. At the 1873 Vienna World's Fair, Gramme accidentally demonstrated it by connecting two DC devices up to 2 kilometers apart, running one as a generator and the other as a motor. Improvements then came fast. Friedrich von Hefner-Alteneck of Siemens and Halske introduced the drum rotor in 1872 to replace the ring armature, and the firm introduced the laminated rotor the following year, cutting iron losses. In 1880, Jonas Wenstrom gave the rotor slots to house the winding. In 1886, Frank Julian Sprague invented the first practical DC motor, a non-sparking device that held relatively constant speed under variable loads. Sprague did far more than build a better motor. He used his motors to invent the first electric trolley system in 1887 to 1888 in Richmond, Virginia, the electric elevator and control system in 1892, and an electric subway with independently powered, centrally controlled cars. That subway was first installed in 1892 in Chicago by the South Side Elevated Railroad, where it became popularly known as the L.

• 6. Taming Alternating Current

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  In 1824, the French physicist Francois Arago described rotating magnetic fields, later termed Arago's rotations. In 1879, Walter Baily turned switches on and off by hand to demonstrate what was in effect the first primitive induction motor. The problem of the 1880s was stark: alternating current had clear advantages for long-distance high-voltage transmission, but inventors could not yet operate motors on it. Galileo Ferraris invented the first alternating-current commutatorless induction motor in 1885 and improved it in 1886. When the Royal Academy of Science of Turin published his research in 1888, it concluded that an apparatus based on that principle could not be of any commercial importance as a motor. Nikola Tesla saw the opposite future. He invented his induction motor independently in 1887 and obtained a patent in May 1888, then presented a paper to the AIEE describing three two-phase, four-stator-pole motor types. George Westinghouse, who had already acquired rights from Ferraris for one thousand US dollars, promptly bought Tesla's patents for sixty thousand US dollars plus a royalty per horsepower sold, employed Tesla, and assigned C.F. Scott to help him before Tesla left for other pursuits in 1889. The constant-speed AC induction motor proved unsuitable for street cars, but Westinghouse engineers adapted it to power a mining operation in Telluride, Colorado in 1891. The decisive contest was over how many phases to use. Mikhail Dolivo-Dobrovolsky argued that Tesla's motor was impractical because of two-phase pulsations, and he invented the three-phase induction motor in 1889, in both cage-rotor and wound-rotor forms. He and Charles Eugene Lancelot Brown built larger models, including a 20-horsepower squirrel cage and a 100-horsepower wound rotor. At the 1891 Frankfurt International Electrotechnical Exhibition, the first long-distance three-phase system carried power rated at 15 kilovolts over 175 kilometers from the Lauffen waterfall on the Neckar river, where a step-down transformer fed a 100-horsepower induction motor driving an artificial waterfall. The efficiency gains compounded over a single century. A 100-horsepower induction motor now has the same mounting dimensions as a 7.5-horsepower motor did in 1897.

• 7. A Motor For Every Job

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  Brushless DC motors are typically more than 85 percent efficient and can reach 96.5 percent, while brushed DC motors usually manage 75 to 80 percent. In the BLDC design, the mechanical commutator is replaced by an external electronic switch synchronized to the rotor's position, often using Hall effect sensors for rotor sensing. Without brushes, these motors run cooler, last longer, make little noise, and avoid the sparking that generates ozone, which makes them suitable for audio equipment, computers, and environments with volatile chemicals. They appear in CD and CD-ROM drive spindles, laser printers, and photocopiers, and larger versions rated up to about 100 kilowatts drive electric vehicles. The universal motor takes the opposite approach, running on either AC or DC because the current in both field and armature coils reverses polarity together, keeping the force in one direction of rotation. These motors can run far faster than the power-line frequency allows for squirrel-cage induction motors. Many vacuum cleaner and weed trimmer motors exceed 10,000 rpm, and miniature grinders may exceed 30,000 rpm, which is why universal motors suit blenders, hair dryers, and portable power tools like drills and circular saws. They once formed the basis of the traditional railway traction motor, but are now rarely used there. Some specialized motors are defined by what they leave out. The switched reluctance motor has no brushes, no permanent magnets, and no rotor currents at all. Its torque comes from a slight misalignment of poles, as the rotor aligns with a stator field that is energized in sequence. The torque motor can run indefinitely while stalled, applying steady torque to a blocked rotor, which suits the supply and take-up reels in a tape drive and the force-feedback steering wheels of computer games. The coreless or ironless motor strips out the rotor's iron entirely, often reaching a mechanical time constant under one millisecond, though it has no metal mass to act as a heat sink and so must be cooled. The pancake or axial-rotor motor shapes its windings as a disc running between arrays of high-flux magnets, and one approach, Magnax, sandwiches a single stator between two rotors to reach a peak power of 15 kilowatts per kilogram. Stepper motors move in discrete steps rather than turning continuously, which makes them the smallest commonplace stepping motors in quartz analog wristwatches, where a single coil draws little power to turn a permanent magnet rotor.

• 8. The Measure Of A Motor

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  Eric Laithwaite proposed a metric to determine the goodness of an electric motor, where factors above 1 are likely to be efficient. From his goodness factor he showed that the most efficient motors are likely to have relatively large magnetic poles, though the equation relates directly only to non permanent-magnet motors. Efficiency itself spans a wide range. Shaded-pole motors sit around 15 to 20 percent, while permanent magnet motors reach up to 98 percent, with the result always dependent on load. Peak efficiency usually arrives at 75 percent of the rated load, so a 10-horsepower motor runs most efficiently when driving a load that needs 7.5 horsepower. Larger motors tend to be more efficient than small ones. Back electromotive force explains how a DC motor regulates its own speed. As the armature windings move through the magnetic field, they induce a voltage that opposes the supply and rises with running speed. When the mechanical load increases, the motor slows, the back EMF falls, and more current is drawn to provide the extra torque that balances the load. The numbers at the largest and smallest scales show the device's range. The biggest motors, used for marine propulsion, pipeline compression, and pumped-storage applications, exceed 100 megawatts of output, while electric motors can reach continuous power densities of up to 20 kilowatts per kilogram. In 2022, sales were estimated at 800 million units, increasing by 10 percent a year, and since the 1980s the market share of DC motors has declined in favor of AC. One reversal captures the whole story: in regenerative braking, a traction motor runs backward as a generator to recover energy that would otherwise be lost as heat and friction.