by Vaclav Smil
a greater economy of conversion that has heretofore existed, to construct cheaper and more reliable and simple apparatus, and, lastly, the apparatus must be capable of easy management, and such that all danger from the use of currents of high tension, which are necessary to an economical transmission, may be avoided. (Tesla 1888:1)
Elegant design is one of the most highly valued achievements in engineering, and Tesla’s AC motor was an outstanding example of this genre. By using the rotating magnetic field (with two or more alternating currents out step with each other) produced by induction, he had eliminated the need for a commutator (used to reverse the current’s direction) and for contact brushes (allowing for the passage of the current). Westinghouse acquired all of Tesla’s AC patents in July 1888 for $5,000 in cash and 150 shares of the company’s stock, as well for the royalties for future electricity sales (the last being obviously the most rewarding deal). Tesla’s motors were first shown publicly in 1888, and Westinghouse Co. produced its first electrical household gadget, a small fan powered by a 125 W AC motor, in 1889. This was a modest beginning of a universal conquest, but by 1900 there were nearly 100,000 such fans in American households (Hunter and Bryant 1991).
FIGURE 2.18. Illustrations attached to Tesla’s U.S. Patent 391,968 for an electromagnetic motor. Drawings 1–8 show the principle of a two-phase motor’s action, and drawing 13 shows the connections of a motor to a generator. Reproduced from Tesla (1888).
As with other inventions of the remarkable 1880s, subsequent improvements further raised the already high efficiencies of induction motors (the largest machines now convert more than 95% of electricity into kinetic energy), lowered their production costs, and vastly extended their uses without departing in any fundamental way from Tesla’s basic designs. Tesla’s first patent was for a two-phase machine. Mikhail Osipovich Dolivo-Dobrowolsky (1862–1919) built the first three-phase induction motor in Germany in 1889 while he was the chief electrician for the AEG, and three-phase machines soon became very common in industrial applications: the phase offset of 120° means that at any given moment one of the three phases is near or at its peak, and this assures a more even power output than with two phases, while a four-phase machine would improve the performance only marginally but would require another wire.
Adoption of Electric Motors and Their Impact
Only a few inventions have had such a transforming impact on industrial productivity and quality of life. In less than two decades after their commercialization, electric motors surpassed lights to become the single largest consumer of electricity in the United States. After 1900 they were rapidly adopted in manufacturing and for other industrial tasks. In 1899 only 22% of about 160,000 motors produced in the United States (only about 20% of them powered by AC) were for industrial uses; a decade later the share was nearly 50% of 243,000 (more than half of them run by AC), and before the end of 1920s electric motors became by far the most important prime mover in the Western industrial production (Schurr et al. 1990).
Many reasons explain this dominance. Induction motors are among the most rugged and most efficient of all energy-converting machines (Andreas 1992; Anderson and Miller 1983; Behrend 1901). Some of them are totally enclosed for operation in extremely dusty environments or work while completely submerged. All of them can be expected to deliver years of virtually maintenance-free operation and can be mass-produced at low cost, and their unit costs decline with higher capacities. They are also fairly compact, with large machines weighing less than 5 g/W (somewhat heavier than gasoline engines). By 1914 they were made in an impressive variety of sizes ranging from the smallest units driving dental drills to nearly 1,000 machines of 20–52 kW that were used to operate lock gates, rising-stem gate valves, and chain fenders of the newly built Panama Canal (Rushmore 1912; figure 2.19). Today’s motors come in a much wider range of capacities, ranging from a fraction of a watt in electronic gadgets to more than 1 MW (and more than 300 A) for the largest synchronous motors that operate huge ore crushers.
And their scope of applications has increased to the point that everything we eat, wear, and use has been made with their help (Smil 2003). Electric motors mill our grain, weave our cloth, roll out our steel, and mold our plastics. They power diagnostic devices and are being installed every hour by tens of thousands aboard cars, planes, and ships. They do work ranging from frivolous (opening car sunroofs) to life saving (moving blood in heart-lung machines), from essential (distributing the heat from hydrocarbons burned by household furnaces) to entertaining (speeding small cars on their mad rides on roller-coasters). They also lift the increasingly urbanized humanity to high-rise destinations, move parts and products along assembly lines, and make it possible to micromachine millions of accurate components for devices ranging from giant turbofan jet engines to implantable heart pacemakers.
Perhaps the best way to appreciate their impact is to realize that modern civilization could have access to all of its fuels and even to generate all of its electricity—but it could not function so smoothly and conveniently without electric motors, these new alphas (in baby incubators) and omegas (powering compressors in morgue coolers) of our world. Not surprisingly, electric motors now consume more than two-thirds of all electricity produced in the United States, and they are doing so with increasing efficiencies (Hoshide 1994). Also not surprisingly, one design cannot accommodate all of these demands, and induction motors have evolved to include several basic kinds of machines, with three-phase and single-phase motors being by far the most commonly encountered converters of electricity to kinetic energy.
FIGURE 2.19. A set of lock gates of the Panama Canal and a plan view of mechanism for their operation. One 20-kW electric motor was needed for each leaf of 46 gate pairs, and 5.6-kW motors were used for the miter-forcing machines that made gates to come together with a tight seal and lock them in that position. Reproduced from The Illustrated London News, March 21, 1914, and from Rushmore (1912).
Three-phase motors have the most straightforward construction. Their stators are made of a steel frame that encloses a hollow cylindrical core composed of stacked laminations. Two major types of three-phase induction motors are distinguished by their rotor wiring: squirrel-cage rotors (introduced by Westinghouse Co. during the early 1890s) are made up of bars and end rings; wound rotors resemble stators. Because the three-phase current produces a rotating magnetic field, there is no need for additional windings or switches within the motor to start it. And three-phase motors have also the highest efficiencies, are the least massive per the unit of installed power, and are the least expensive to make. Not surprisingly, three-phase induction motors are the norm for most of the permanently wired industrial machinery as well as for air- and liquid-moving systems.
Diffusion of three-phase induction motors in industrial production was a much more revolutionary step than the previous epochal transition to a new prime mover that saw waterwheels replaced by steam engines in factories. The first substitution did not change the basic mode of distributing mechanical energy that was required for countless processing, machining, and assembling tasks as factory ceilings continued to be clogged by complex arrangements of iron or steel line shafts that were connected by pulleys and belts to parallel countershafts and these, in turn, were belted to individual machines. With such an arrangement, a prime mover outage or an accident at any point along the line of power distribution (a cracked shaft, or just a slipped belt) stopped the whole assembly. Conversely, even if most of the machines were not needed (during the periods of slumping demand), the entire shaft assemblies were still running.
Electric motors were used initially to drive relatively short shafts for groups of machines, but after 1900 they were increasingly used as unit drives. Their eventual universal deployment changed the modern manufacturing by establishing the pattern of production that will be dominant for as long as electric motors will remain by far the most important prime movers in modern industrial production. Schurr et al. (1990) and Hunter and Bryant (1991
) documented extensively the rapidity of this critical transition in the United States. While the total installed mechanical power in manufacturing roughly quadrupled between 1899 and 1929, capacity of electric motors grew nearly 60-fold and reached over 82% of the total available power compared to a mere 0.3% in 1890 and less than 5% at the end of the 19th century (USBC 1954; figure 2.20). Since then the electricity share has changed little: the substitution of steam and direct water-powered drive by motors was practically complete just three decades after it began during the late 1890s.
Benefits of three-phase motors as ubiquitous prime movers are manifold. There are no friction losses in energy distribution; individualized power supply allows optimal machine use and maximal productive efficiencies; they open the way for flexible plant design and easy expansion and enable precise control and any desired sequencing of tasks. Plant interiors look different because the unit electric drive did away with the overhead clutter, noise, and health risks brought by rotating shafts and tensioned belts, and the ceilings were freed for installation of better illumination and ventilation, steps that further aided in boosting labor and capital productivity.
But three-phase wiring is not normally supplied to homes and offices, and hence a large variety of machines of smaller capacity draw on single-phase supply. Tesla filed his application for a single-phase motor on October 20, 1888, which was granted on August 14, 1894 (Tesla 1894), and Langdon Davies commercialized his single-phase design in 1893. Construction of these machines is very similar to that of three-phase motors: they have the same squirrel-cage rotor and a stator whose field alternates poles as the single-phase voltage swings from positive to negative. Efficiencies of single-phase motors are lower than in three-phase motors, but they, too, can deliver years of reliable work with minimal maintenance. Induction motors using single-phase 120-V AC are most common in household and office appliances, including ceiling and table fans, food mixers, refrigerators, and air conditioners.
FIGURE 2.20. Capacity of electric motors and their share in the total power installed in the U.S. manufacturing. Plotted from data in USBC (1954).
The principle of synchronous motors was first demonstrated by Gramme’s 1873 Vienna displays: they are simply alternators connected to a three-phase supply. Because its frequency is fixed, the motor speed does not vary no matter what the load or voltage of the line. Their principal use is at low speeds (below 600 rpm) where induction motors are heavy, expensive, and inefficient. Westinghouse deployed the first synchronous motor in 1893 in Telluride, Colorado, to run an ore-crushing machine. Modern electronic converters can produce very low frequencies, and synchronous motors can be thus run at very low speeds desirable in rotary kilns in cement plants and, as in its pioneering application, in ore or rock milling and crushing. And, naturally, synchronous motors are the best choice to power clocks and tape recorders.
But the ascendance of AC motors has not led to the total displacement of their predecessors, and DC motors, which flourished for the first time during the 1870s, remain ubiquitous. Some of them get their supply from batteries; many others receive DC from an electronic rectifier fed with AC. They have been entirely displaced by AC machines in nearly all common stationary industrial applications, but because of their very high starting torque, they have been always the motors of choice for electric trains, and between 1890 and 1910, before they were displaced by internal combustion engines, they were relatively popular as energizers of electric cars.
Despite many repeated promises of an imminent comeback of electric cars, those vehicles still have not returned, but there is no shortage of electric motors in road and off-road vehicles. Their total global number in the year 2000 was well over 2 billion: in addition to their starter motors, modern cars also have an increasing variety of small DC servomotors that operate windshield wipers, windows, locks, sunroofs, or mirrors by drawing the battery-supplied DC. DC motors are also common in chairs for invalids, as well as in treadmills, garage-door openers, heavy-duty hoists, punch presses, crushers, fans, and pumps used in steel mills and mines. Another advantage of DC motors is that their power tends to be constant as changes in torque automatically produce reverse change in speed; consequently, these motors slow down as a train starts going uphill, and in cranes and hoists they lift heavy loads slower than they do the light ones.
Systems Mature and (Not Quite) Standardize
Remarkable accomplishments of the 1880s refashioned every original component of new electrical system, and introduced machines, devices and converters whose performance had soon greatly surpassed the ratings, efficiencies, and reliabilities of initial designs. But this extraordinarily creative ferment was not conducive to standardization and optimization. This dissipative design is a phenomenon that recurs with all rapidly moving innovations. Before WWI it was also experienced by the nascent automotive industry. As a result, 10 years after the first central plants began operating in the early 1880s, one could find many permutations of prime movers, generators, currents, voltages, and frequencies among the operating coal-fired and hydro-powered stations.
Prime movers were still dominated by steam engines, but steam turbines were on the threshold of rapid commercial gains. Generators were mostly dynamos coupled indirectly or directly to reciprocating engines, but again, unit arrangements of turbines and alternators were ascendant. All of the earliest systems, isolated or central, produced and transmitted DC, commonly at 105–110 V or at 200 V with a three-wire arrangement. But the first AC systems were installed within months after the commercial availability of AC transformers: Ferranti’s small installation in London’s Grosvenor Gallery in 1885, and Stanley’s town plant in Barrington in Massachusetts, which led George Westinghouse, who came to see it on April 6, 1886, to enter the AC field (Stanley 1912). And so, inconspicuously, began the famous battle of the systems.
AC versus DC
Edison, the leading proponent of DC, was not initially concerned about the competition, but he soon changed his mind and embarked on what was certainly the most controversial, indeed bizarre, episode of his life. As he led an intensive anti-AC campaign, he did not base it firmly either on scientific and economic arguments (on both counts DC had its advantages in particular settings) or on pointing out some indisputable safety concerns with the high-voltage AC (HVAC). Instead, he deliberately exaggerated life-threatening dangers of AC, during 1887 was himself involved in cruel demonstrations designed to demonize it (electrocutions of stray dogs and cats coaxed onto a sheet of metal charged with 1 kV from an AC generator), and made repeated personal attacks on George Westinghouse (1846–1914), a fellow inventor (most famous for his railway air brake patented in 1869) and industrialist and, thanks to Stanley’s influence, an early champion of AC (figure 2.21).
Perhaps worst of all, in 1888 Harold Brown, one of Edison’s former employees, became a very active participant in a movement to convince the state of New York to replace hanging by electrocution with AC. Brown was eventually selected to be the state’s electric chair technician, and he made sure that the AC was supplied by Westinghouse’s alternators (MacLeod 2001). Edison’s war on the HVAC lasted about three years. In 1889 he was still writing that laying HVAC lines underground would make them even more dangerous and that
[m]y personal desire would be to prohibit entirely the use of alternating currents. They are as unnecessary as they are dangerous…and I can therefore see no justification for the introduction of a system which has no element of permanency and every element of danger to life and property. (Edison 1889:632)
Edison’s vehement, dogmatic, and aggressive attitude was painful to accept and difficult to explain even by some of his most admiring biographers (Josephson 1959; Dyer and Martin 1929). But his opposition to HVAC stopped suddenly in 1890, and soon he was telling the Virginia legislature that debated an AC-related measure that “[y]ou want to allow high pressure wherever the conditions are such that by no possible accident could that pressure get into the houses of the consumers; you want to give them all t
he latitude you can” (quoted in Dyer and Martin 1929:418).
FIGURE 2.21. George Westinghouse had a critical role in the development of electric industry thanks to his inventiveness and entrepreneurial abilities, his unequivocal advocacy of alternating current, and his support of Tesla’s work. Photograph courtesy of Westinghouse Electric.
David (1991), in a reinterpretation of the entire affair, argues that Edison’s apparently irrational opposition had a very rational strategic goal and that its achievement explains the sudden cessation of his anti-AC campaign. By 1886 Edison became concerned about the financial problems of the remaining electric businesses he still owned (by that time he had only a small stake in the original Edison Electric Light, the principal holding company), and hence he welcomed a suggestion to consolidate all Edison-related enterprises into a single new corporation. In return for cash ($1.75 million), 10% of the shares, and a place on the company’s board, Edison’s electric businesses were taken over by the Edison General Electric Co. organized in January 1889, and a year later Edison liquidated his shares and stopped taking any active role on the company’s board. David (1991) argues that the real purpose of Edison’s anti-AC campaign was to support the perceived value of those Edison enterprises that were entirely committed to produce components of DC-based system and thus to improve the terms on which he, and his associates, could be bought out. Once this was accomplished, the conflict was over.
