Читать книгу Elevator Troubleshooting & Repair - David Herres - Страница 8
ОглавлениеIt is likely that animal- and human-powered elevators predated written history. Unlike masonry and stone buildings, the cars were probably woven baskets or wooden platforms with or without guardrails, and the support structures built of wooden logs, so these remains would have decayed centuries ago. We can only surmise that they existed, powered by domesticated animals on the ground, who worked long hours at a turnstile. Alternatively, occupants of the car may have pulled a looped rope that turned a pulley with more ropes that lifted the car, as shown in Figure 1-1.
FIGURE 1-1 The rope was operated from within the car. The hoistway was primitive, but it did the job.
Vitruvius (c. 80–15 BC), a Roman author, architect, and engineer, provided the first extant written reference. He reported that the Greek mathematician Archimedes (c. 287–212 BC) built a bank of elevators operated by hoisting ropes wrapped about a drum. It was turned by humans and this torque was applied to a capstan, causing platforms to lift gladiators and fierce animals through vertical shafts into the arena. In the seventeenth century, English and French monarchs built “flying chairs” to discreetly transport their mistresses to upper palace levels. These machines, powered by humans and animals, were eventually eclipsed by steam, water, and finally electric motors.
Where it gets interesting, from our point of view, is in the nineteenth century. During this 100-year period, the elevator evolved from steam-powered platforms used to move coal in English mines, to electrically-powered elevators that lifted passengers to ever greater heights in comfortable rooms with plush furniture.
In the late 1790s, William Strutt (1756–1830), shown in Figure 1-2, assumed control of his father’s textile mills in England. Among many projects, including fireproofing and improving the heat system, he designed a combination passenger and freight elevator, known then as the crane. It was adjacent to the main stairway and was used to transport workers within the five-story building. Strutt’s elevator was powered by a flat belt, running off of power shafting that ran throughout the building, presumably powered by an outside water wheel.
The principle components were a brake wheel, two fixed and two free pulleys, two endless belts, and a belt shifter. A crossed belt permitted the direction of car motion to be reversed, as needed in any elevator.
FIGURE 1-2 William Strutt (1756–1830) (Wikipedia)
A pinion gear was attached to one end of the main shaft, and its teeth meshed with those of a spur gear attached to the hoisting pulley shaft.
This was the first in a long series of working elevators that spanned the nineteenth century. Strutt’s Teagle, as it was known, was complex in the sense that it had a lot of ropes, belts, and pulleys, but simple in that these things worked smoothly together to deliver the power to where it was needed so that the car could deliver workers throughout Strutt’s five-story textile mill.
By the 1840s, two trends in vertical transportation merged. Increasingly, elevators were optimized to carry freight exclusively or to transport only workers, residents of tall buildings, and hotel guests from ground level to the growing number of floors in taller buildings that began to crowd the cities. Also, of necessity there was greater emphasis on safety.
Previously, lower-powered lifting machines had their share of accidents, sometimes resulting in well-publicized fatalities. This was true not only in elevators, but throughout the world of increasingly mechanized, more powerful and faster machinery that characterized the new industrial age. Accidents took two forms. In one, the suspension rope and associated rigging that raised and lowered the car in a traction elevator failed, causing the car, which was slowed only a little by the air column below, to free fall to the bottom of the shaft. The inevitable result was severe injury, often fatal. The other type of accident involved the absence of reliable door interlocks, which would prevent a door from opening when the car was moving and/or prevent the car from moving when the doors were not closed and locked.
Without these interlocks, an occupant of the building could step through an open door assuming that the car was at the landing, and fall to the bottom of the shaft. Another equally great hazard was that an occupant of the car could be crushed between the car floor and the top of the door opening at any floor while the car was ascending. We shall see how mid-nineteenth century advances in elevator technology confronted these hazards and greatly reduced the number of injuries resulting from them.
Before midcentury, freight elevators were typically designed in-house to meet the needs of the many industrial facilities that were appearing, especially in England and eastern U.S. Then, beginning around 1845, industries and commercial operations such as hotels and office buildings began to look to certain emerging elevator manufacturers to meet these needs. Henry Waterman in New York City was a freight and passenger elevator manufacturer. One of his early machines, built in Manhattan for Croton Flour Mills, was operated from within the car so that an outside attendant was not required. Car motion was initiated by moving a simple iron lever, rather than tugging on the shipper rope. For passengers, the trip became smoother and more user-friendly. The control lever moved an attached chain that passed through openings in the car roof and floor, then engaged devices at the top of the shaft. The mechanism consisted of a friction clutch driven by a conventional power shaft, eliminating the need for pulleys and a belt shifter as in Strutt’s Teagle.
The operator caused the car to ascend by pulling the handle, which released the brake and engaged the clutch. Upward travel continued as long as the operator maintained pressure on the handle. The clutch disengaged and the brake was applied when the operator released the handle. To descend, the operator applied an intermediate amount of pressure on the handle, releasing the brake, and the car would descend, its speed regulated by the brake.
The innovation in Waterman’s elevator was that it was controlled from within the car by means of what we would call a joystick, rather than the bothersome shipper rope that is prohibited today.
By 1850, George H. Fox and Co., a Boston firm, was building freight elevators that were safer and more efficient. Fox replaced meshing spur gears with a worm gear attached to the winding shaft. This arrangement is superior because it is self-locking. The worm can turn the gear, but the gear cannot turn the worm. Consequently, a separate brake was not required for the hoist, which would hold its position when the driving belt was disengaged. This arrangement meant less chance of a car and occupants falling to the bottom of the elevator shaft due to mechanical failure in the drive system.
Safety was further enhanced by other innovations by Fox and Co. One was the replacement in 1852 of traditional hemp rope by stronger and more wear-resistant steel wire rope. The other innovation was a safety brake, which could stop the car from free falling in the event of rope failure. This brake, however, was not automatic and depended upon quick action by an alert operator.
Falling cars were still a severe hazard, but after 1850 new developments in elevator technology greatly reduced the number of occurrences.
In the mid-nineteenth century, William Adams and Co. manufactured freight elevators in Boston. In 1859, one of their freight platforms in a group installation dropped to the bottom of its shaft. An engineer for the firm, inspecting the damage, found that it was not as severe as might be anticipated. He concluded that the hoistway, as built, happened to be relatively airtight, and as a result, the air as it was compressed below the falling platform acted as a cushion and slowed its fall. This suggested a way to mitigate these disasters, and in fact the idea was patented and hardware developed and marketed.
Elisha Graves Otis Invents Safeties
Another very active key figure in the evolving elevator industry in mid-nineteenth century America was Elisha Graves Otis (1811–1861). His Improved Elevator of 1854 incorporated an automatic safety mechanism, which in the event of rope failure as shown in Figure 1-3, would activate automatically.
All elevators, of course, had guide rails, which were necessary to prevent the suspended car from swinging from side to side, striking the hoistway walls. The Otis Improved Elevator was a variation on existing rack-and-pinion drives, in which the rack was attached to the guide rails. In the new design, the teeth curved upward rather than extending perpendicular from the rack. The brake, relocated below the cross beam at the top of the platform or car, consisted of safety dogs connected to a spring and the hoisting rope. Because the rope, as long as it remained intact, supported the freight platform or passenger car, the spring remained compressed and held the safety dogs away from the rack and the elevator functioned as expected. In the event of a break in the rope or if for any reason it lost tension, the safety dogs would engage the upward angled rack teeth, preventing the car or platform from falling.
Otis was an accomplished mechanic and very inventive builder of elevators, always sensitive to safety issues. However, on the financial side his business failed to prosper despite the success of his Improved Elevator with its advanced safety mechanism. Beginning around 1860, nearly all traction elevators incorporated his braking system in one form or another.
FIGURE 1-3 Elisha Graves Otis cuts the hoisting rope, demonstrating his new safeties, still used, that prevent an elevator car from free falling. (Wikipedia)
Just three months after receiving his patent, Elisha Otis died of natural causes. His business flourished under the ownership of his sons, who reconstituted the firm as N.P. Otis and Brother. The company prospered under the inventive and financial skills of Norton and Charles Otis. They quickly adapted to the new post-Civil War environment, in which the focus now included passenger elevators built for the new generation of higher-rise hotels, shops, and office buildings.
At about the same time that these developments in traction elevator safety and reliability were occurring, in England and continental Europe as well as in the U.S., hydraulic elevators were emerging in low-rise applications. Here we are talking about water pistons, as opposed to the hydraulic oil machines of today. Typically, the water supply was from a high-capacity pump system or reservoir. The water pressure would cause the car to rise to the top floor or as high as required. Then, a discharge valve permitted the car to descend at a measured pace due to its own weight.
Hydraulic elevators had some intrinsic advantages in low-rise applications. Those running off a natural or impounded reservoir had no further fuel costs, and unlike steam power, there were not the tasks of moving in coal and disposing of ashes. They were simple and quiet. In the event of piston failure, the car or platform would not free fall, its speed of descent regulated by the size of the rupture.
Bedrock or a high water table could make for a difficult installation. Builders of hydraulic elevators could then, however, resort to hybrid designs, standing the cylinders vertically above grade outside the buildings or laying them down horizontally. These installations required additional wire rope and pulley mechanisms, compromising the advantages of simplicity and safety.
Just as the nineteenth century was a time in which elevators evolved from primitive lifts to becoming a defining fact in the great cities of America and Europe, so in the ninth decade of that century did the electric motor assume new forms, enabling it to replace coal-burning steam power.
Throughout the 1870s, hydraulic (water) elevators were installed in great numbers. Drive configurations and structural variations proliferated as did the number of manufacturers building them. Additionally, there were many exclusively wire-rope machines being built and installed, with great innovations that made them safer and more efficient. Still, steam power, which was noisy, hot, and required frequent human intervention, powered most elevator installations.
Then, beginning around 1880, the DC electric motor changed everything.
The first electrical distribution system was Thomas Edison’s 110-volt DC utility in lower Manhattan, intended for indoor residential and commercial use. It was energized in 1882, followed four years later when George Westinghouse began building an AC system, enabling the use of transformers to increase the voltage for efficient transmission and lower it for users. AC eventually eclipsed DC, but meanwhile Edison commenced large-scale DC motor production and for many decades these motors remained in use in many applications for which they were better suited than AC motors, notably in elevators.
DC motors could be run off an AC power supply by means of a simple motor-generator set, often in a single enclosure with no exterior shafts, and later by tube-type and inexpensive solid-state diode rectification. The reason a DC motor was at the time preferable to an AC motor was that, although both could be reversed, the speed of an AC motor could not be easily varied, as required to operate an elevator. In contrast, DC motor speed is varied simply by adjusting the voltage applied to the armature or current applied to the field circuit.
Nikola Tesla, working with George Westinghouse, developed three-phase AC power distribution and he invented the highly efficient and maintenance free three-phase induction motor, shown in Figure 1-4, which quickly permeated industrial facilities worldwide. But since it was essentially a single speed device, it was not suitable for elevator power until the 1960s, when the variable frequency drive (VFD) was introduced. This consisted of electronic circuitry that permitted users to run AC induction motors at lower (or even higher) than rated speed by means of pulse-width modulation (PWM), which we will discuss in detail in Chapter 3.
FIGURE 1-4 Tesla’s AC induction motor was not suitable for elevator use until the 1960s, when the variable frequency drive made speed control possible. (Wikipedia)
When electric motors were first suggested for elevator power, the public was skeptical. There had been a number of power line fatalities as new distribution systems were being constructed, and fire hazard was perceived to be an issue in high-rise buildings compromised by wooden hoistways piercing multiple floors. Early electric codes such as NEC, first issued in 1897, decisively confronted these hazards, and soon electric motors became part of everyday life.
The first elevator motors powered building-wide belt driving shafts in manufacturing facilities, so they were external to the elevators. But space and manufacturing costs could be saved by integrating the motor directly into the elevator assembly. That was accomplished before 1890, and is how it remains today.
Before the end of the nineteenth century and continuing to the present, electric elevators improved, with new designs becoming safer and more efficient. A key figure in this development was Frank Sprague, shown in Figure 1-5.
Electric motors and their applications in human transportation were Frank Sprague’s life. After graduating from the U.S. Naval Academy and a short stint on ship and in Europe, the young electrical engineer joined Edison’s large assembly of electricians, mechanics, and glassblowers in the lab at Menlo Park. While Edison was focused on producing a practical electric light bulb, Sprague wanted to develop a DC motor that would maintain RPM under varying loads. Edison was temperamental, but went along with this idea.
FIGURE 1-5 Frank Sprague (1857–1934) (Wikipedia)
Prior to 1880, electric motors were repurposed electric generators, then known as dynamos, which had preceded them. It had been found that voltage applied to what had been the generators’ output terminals would cause them to turn. These devices would actually run and could be configured to perform work, but they left a lot to be desired.
Sprague had some big ideas. He envisioned a DC motor that could run a loom, hoist, pump, blower, or machine tool. His highest ambition, eventually realized, was to build powerful motors that were reliable and capable of powering railroads, replacing the inefficient, smoky, and dangerous steam engines of the day.
Dynamos repurposed as motors bogged down under heavy load, and while this didn’t make much difference in some applications, in others these primitive devices were not suitable. A skilled mechanic was needed during running hours to advance or retard the brushes and adjust field strength for various loads and RPMs.
Sprague, at this juncture and throughout his life, demonstrated that Edison wasn’t the only electrical and mechanical genius. While Sprague has had less impact than Edison in the popular imagination, in many ways he was more advanced and insightful. Sprague built an electrical motor that maintained constant speed under varying load. Rather than the steam engine’s mechanical governor, Sprague’s electrical motor incorporated a reverse winding that automatically varied field strength in response to speed and loading. He solved the problem of brush position not by moving them physically, but by rotating the magnetic field to achieve the required alignment.
Since Sprague was working for Edison, the improved motor design at this point belonged to Edison. Sprague evidently saw the writing on the wall, and shortly thereafter tendered his resignation.
While Edison continued to refine his incandescent light bulbs and DC power generation and distribution system, Sprague, after eleven months working in Edison’s large organization, formed the Sprague Electric Railway and Motor Company. His lifelong project was to move rail traffic by means of electric motors. In this he was very successful and was renowned among electricians and transportation workers as “the father of electric traction.” He came to define these words to include vertical as well as horizontal traction. His elevator work was a relatively brief interlude, but its impact was enormous. After completing some difficult early streetcar and railway projects, he turned his attention in 1889 to elevator design and construction.
Sprague, together with his old friend Ed Johnson and elevator manufacturer Charles Pratt, rented a factory building and in 1892 formed Sprague Electric Elevator Company. Ed Johnson was the legal and financial specialist. Sprague provided electrical expertise, and Pratt was the mechanical engineer. Together they planned to offer two very different types of elevators. For low-rise buildings, a conventional drum-type elevator would be reconfigured with a reversible, adjustable-speed electric motor replacing the steam engine.
For high-rise applications, a faster machine would consist of a large, threaded-steel shaft placed horizontally and powered through a gearbox by an electric motor. A large nut would move along the turning shaft, driving a cable pulley. The contraption actually worked, and in fact dominated the industry until shortly before the turn of the century.
After constructing a small prototype in their new Manhattan facility, the firm secured a contract to install a similar elevator in the Grand Hotel in New York City.
There were problems in this installation. The control system, which had been satisfactory in Sprague’s electric trolleys, did not provide the smooth performance required in an elevator.
Sprague’s electrical expertise was severely challenged. First, he built an improved resistance network, known as the grid, for the controller. This smoothed out the elevator motion, but the resistance network and controller mechanism heated and contacts had to be replaced.
The elevator was put back in operation, but after a few days at an upper floor the ascending car suddenly dropped, its speed doubling, coming to a stop after striking the bumpers at the bottom level.
Fortunately, there were no injuries. The cause was determined to be a defective motor that ran the reversing lever in the controller. The sudden reversal damaged the safeties, permitting the car to drop.
Soon redesigned safeties and controls were in place and the elevator resumed normal operation. This did not solve the network problem. Eventually, the firm built and installed a new controller with heavier contacts, but the problem persisted. Sprague favored a cast iron grid, which turned out to work on a long-term basis. The Grand Hotel signed off on the project and Sprague Electric Elevator Company moved on to another project, the Postal Telegraph Building. This was to be located close by on Broadway and be far bigger and faster than the Grand Hotel installation. There would be four local and two express elevators, rising 14 stories above street level.
The immense screw and traveling nut mechanisms resulted in heavy loading and increased friction, which Sprague intended to mitigate by incorporating captive steel balls within the nuts.
After mishaps and delays, the Postal Telegraph Building installation performed flawlessly in tests and was placed in operation. It was a prestigious project, and the Postal Telegraph Company Building Committee was well satisfied. But unfortunately, the country was enduring a severe financial depression. New building had halted, and orders were not coming in. Sprague, never one to let up, had some ideas for improving elevator safety and efficiency, and the business slowdown allowed time to work on them. One innovation was the self-centering “dead man’s control,” which stopped the car if for any reason the control was released by the operator. Another innovation was an automatic elevator that incorporated door interlocks and floor alignment. Sprague stuck with the screw and nut design, rather than going with an improved traction drive, which together with his electric motor became the wave of the future. The business climate improved and by 1895 new orders soared and the company moved to a larger facility across the Hudson River, in New Jersey.
In 1898, Sprague decided to return to his first love, railroad electrification. He sold the elevator business to Otis for $1,000,000, retaining royalty rights to two thirds of foreign business and rights to lease back plant and equipment for five years. With that transaction Sprague became much less of a presence in the elevator world, in which Otis was now the most prominent player.
Elisha Graves Otis (1811–1861), a skilled builder and mechanic, in 1850 found his way to Albany, New York, where he was employed to manage a bedstead factory. Relocating in nearby Bergen, New Jersey, in 1851 and then in Yonkers, New York, he organized and managed successive bedstead facilities, and in the Yonkers factory he built a freight elevator. Soon, he established an independent company, which by 1853 was building and installing freight hoists in nearby manufacturing facilities.
In early 1854, as we have seen, one of the defining events in elevator history occurred. At an exhibition in New York’s Crystal Palace, Otis ascended in one of his fully-loaded open freight hoists, and in the presence of astonished onlookers, cut the hoisting rope. Rather than crashing to the floor, a frequent cause of fatal elevator accidents, the platform dropped a short distance and then stopped.
Otis had demonstrated the effectiveness of his great invention. Safeties, as they were (and still are) called, in response to breakage or loss of tension in the rope, automatically, which is to say without human intervention, gripped racks attached to guides, bringing the car or platform to a stop. These safeties, in one form or another, were universally adopted in the elevator industry and have saved many lives.
Otis, in 1855, established the Union Elevator Works. The firm sold a gradually increasing number of hoists in the years that followed. They were powered by water or steam engines, which were optionally furnished, and always with the new safeties.
Elisha Otis was a great mechanic and inventor, if not always successful financially. Following his death in 1861, his sons, Norton and Charles Otis, took over the firm, renaming it N.P. Otis and Brother. As inventors and builders, their skills equaled Elisha’s, and financially they succeeded where their father had been challenged.
After the American Civil War, in 1867, Norton and Charles again renamed the firm Otis Brothers and Company. In the years that followed, the organization, by means of intense research and development and aggressive marketing, became the preeminent powerhouse that it is today. By the end of the nineteenth century, through a series of mergers, stock acquisitions, and purchases, Otis absorbed its major competitors.
First Edison, and then Tesla and Westinghouse had built electrical distribution systems capable of supplying power where needed. Clearly, the electric motor was the wave of the future. At first, elevators moved from steam engine to electric power merely by substituting an electric motor for the connection to mill shafting or the steam engine. This arrangement worked reasonably well, but Otis Brothers and Company between 1887 and 1889 realized further advantages in fully integrating the motor into the elevator mechanism. This development occurred at a time when new, taller buildings were proliferating in New York City and other urban areas. Otis began selling passenger elevators for the new generation of hotel and office buildings. An operator in the car controlled a reversible, multispeed DC motor from inside the cab, at first by means of a shipper rope and later a dead man’s rheostat. A separate worker tending a steam engine was no longer required.
In 1892, Otis Electric Co., jointly owned by Otis Brothers and General Electric, was created for the purpose of designing and building elevator-specific electric motors for Otis Brothers and Company.
In the years that followed, Otis Brothers continued to acquire competing elevator manufacturers, and in 1898 they created a gigantic umbrella entity known as Otis Elevator Co., which is now well into its second century of operation.
From 1900 to the present, there have been numerous refinements in elevator technology, some incremental, some revolutionary. The trend has been to make vertical transportation safer, faster, more efficient, and more reliable. Three principle developments that revolutionized elevators in the twentieth century are:
■ The automatic elevator
■ The low-rise hydraulic elevator
■ The VFD, which enables use of AC induction motors to power elevators
We will discuss each of these.
At first elevators were operated by means of clumsy and not always reliable shipper ropes that passed through holes in the car floor and ceiling. (They are prohibited in the current ASME A17 Safety Code for Existing Elevators and Escalators.) Car and hoistway doors had to be opened and closed manually, and until the development of the door interlock, a door could be left open, sometimes resulting in fatal accidents.
The first automatic door mechanism was built and patented in 1887 by Alexander Miles, an African-American inventor in Duluth, Minnesota. The door opens and closes by means of a series of rollers and levers. After a car has stopped at a floor, a flexible belt extending the length of the shaft opens the shaft door. The car door also opens automatically. Both doors close before the car proceeds to the next stop.
Still, a human operator started and stopped the car by means of a controller inside the car, as shown in Figure 1-6.
FIGURE 1-6 Otis manual elevator controller. If for any reason the operator released the handle, it would return to the neutral position and the car would stop. (Wikipedia)
Rudimentary elevator automation first appeared in the 1920s. At the time there were no microprocessors or solid-state components, but digital logic could be accomplished by means of mechanical relays. These were cumbersome by today’s standards, and had some disadvantages. They were slower-acting, consumed more energy, were less reliable, and more costly. Still, they worked surprisingly well in elevator applications, and in contrast to today’s computer-controlled devices, there were never system-wide crashes.
The first relay version was complex and had few of the features we expect in fully automatic elevators. It was known as the selector. It had numerous mechanical parts including a magnetic tape attached to the top of the car. This tape, as the car traveled, caused mechanical gears to move in response. The gears controlled speed, position, and door operation. A human operator was still required, but car leveling and stopping were simplified.
In 1924 Otis introduced Signal Control, which was a fully automatic elevator system, still with mechanical relays. Throughout the 1940s and 50s, other manufacturers introduced enhanced relay-controlled automation, permitting the car to bypass floors when fully loaded.
Microprocessor-based controllers were introduced by Otis in 1979. The Elevonic 101 was a true motion controller, overseeing all aspects of elevator operation. Another Otis product, Elevonic 401, offered in 1981, was fully computerized.
All-in-one, microprocessor-based controllers, which are a product of China, are currently widely used. They are compact and consume far less energy than previous motion controllers. These units sense car position and door status and are capable of managing large group installations, with human intervention necessary only rarely in the event of sensor, termination, or wiring failure.
Software as a Service (SaaS) enables remote monitoring of group installations via web browser, and it will alert technicians and building managers of imminent or actual malfunctions. Remote monitoring systems are offered by Otis (REM), ThyssenKrupp (Vista), Schindler (Servitel), Kone (KRM) and Mitsubishi (ELE-FIRST).
We will discuss the inner workings of the contemporary motion controller in Chapter 9.
Speaking now of hydraulic elevators, we are no longer concerned with the water-powered affairs that were prominent in the late nineteenth century. The newer hydraulic elevators had some important differences. For one thing, the fluid that characterized them was not water, discarded after each cycle, but hydraulic oil, which, with an anti-foaming additive, resembles a lightweight non-detergent motor oil. It is never discharged into the ground, a water body, or into a waste disposal system, but instead is reused until with multiple heatings, it eventually breaks down and must be changed like automatic transmission fluid in a motor vehicle. When the hydraulic piston is retracted, the oil returns to a steel tank located in the machine room, a reservoir where it cools. When the hydraulic piston is fully extended, enough oil remains in the reservoir so that the submersible hydraulic pump is covered.
We’ll have a lot to say about different hydraulic elevator configurations in Chapter 2, Types of Elevators, and about troubleshooting them in Chapter 5, Troubleshooting Elevator Systems.
As pointed out earlier in this chapter, Nikola Tesla’s brilliant invention of the AC induction motor in conjunction with three-phase power was not much help for the elevator until the development of the VFD in the 1960s.
Also called adjustable-frequency drive, variable-voltage/variable-frequency drive, variable-speed drive, AC drive, micro drive, and inverter drive, the VFD was developed in response to the need to enable the very efficient, reliable, and inexpensive AC induction motor to run at infinitely variable speed and torque levels without wasteful heat accompanying rheostat-controlled voltage as in the DC motor.
To run a DC motor off the usual AC power supply required a motor-generator set or diode rectifier. This was not as great a problem as one might think. After all, a VFD requires high-power DC for the solid-state inverters in the output stage, so this rectification is provided in the front end. Virtually all elevator motors, AC or DC, use full-power rectification somewhere along the line. But the DC motor was more expensive to manufacture and the brushes and commutator required regular maintenance.
There were some early relatively crude VFDs, such as the rotary machines patented by General Electric in the early twentieth century, but they were not generally used in elevator applications. VFD technology improved in stages over the years. Before 1958 there had been various mechanical systems, but VFDs were not widely used until the introduction of silicon-controlled rectifiers (SCRs), which set the stage for subsequent improvements. In the early 1960s, the cost of SCRs dropped and VFDs became available for manufacturing applications. After the late 1960s, analog control circuitry with digital input was introduced. With phase-locked loops for synchronization, motor drives became less subject to noise and hence more reliable.
Throughout the 1970s, large-scale integration (LSI) improved VFD reliability, and cost further declined. In the late 1980s, bipolar pulse-width modulation (PWM) came on the scene and switching frequency increased. Insulated gate bipolar transistors (IGBTs) emerged.
Today, VFD three-phase (and for small applications, single-phase) drives are widely used wherever AC motor speed control is required. VFDs are marketed with dedicated AC induction motors, or off-the-shelf motors with suitable bearings and cooling may be obtained.
VFDs will be covered in more detail in Chapter 3, AC and DC Electric Motor Maintenance, VFD Troubleshooting, and Diagnostic Procedures, Chapter 4, Advanced Motor Repair, and Chapter 5, Troubleshooting Elevator Systems.
Regenerative braking is a valuable energy recovery strategy. It has been used by railroads in long downhill grades, and it is a logical solution for elevators, which do a lot of braking.
Electric motors operate as or can be configured to operate as generators. In this mode, energy that would otherwise have to be dissipated as heat is fed back into the building to power other loads, and/or returned via reverse metering to the utility where it is credited to the customer’s account.
Virtually all traction elevators have counterweights, so energy consumption is divided equally between upward trips when the car is heavily loaded and downward trips when the car is lightly loaded. Actually, regenerative braking was used over a hundred years ago in elevators as well as cranes, which also work in a downward-going mode.
Regenerative braking benefits the building owner by reducing utility bills, and it benefits our planet by reducing carbon emissions inherent in electrical generation that relies on fossil fuels.
Another benefit is that in hot weather the heat generated by purely dynamic braking does not have to be offset by motorized fans or by the building’s air-conditioning system.
There are other energy-saving strategies that have emerged. Replacing relays, solid-state controls dissipate less heat. Sensors in conjunction with software cause the elevator to enter a sleep mode, temporarily switching off in-car lighting and ventilation when not in use. Car stops are batched, reducing waiting time. Double-deck cars, one above the other, stop at even- and odd-numbered floors simultaneously, saving energy and reducing the size of group installations. LED lighting cuts energy costs dramatically, and it can be retrofitted without even changing the fixtures.
Two important contemporary concerns in elevator technology are reducing energy consumption and dealing with ever-greater heights in the latest high-rise buildings.
Buildings consume about 40 percent of the world’s energy, and of this, elevators require between two and ten percent. Obviously, if ways can be found to reduce elevator energy consumption, building owners and indirectly individual tenants will realize capital savings. And more important, significant progress will be achieved in reversing climate change.
The shift from DC to VFD AC induction motors, regenerative braking, better software, more efficient cable, and counterweight systems and LED lighting are examples of energy-saving measures already in place, though not fully realized.
Hydraulic elevators offer advantages in low-rise applications, but for anything over five stories, traction elevators with the exception of some new alternate designs continue to be the focus. In cities and in widening circles around them, enormous high-rise projects are causing us to rethink traction elevator design.
First we need to consider, in the context of energy efficiency, geared versus gear-less motors. In geared elevators, the motor drives a gearbox, which turns the sheave at a substantially lower speed than the input shaft RPM. In gearless elevators, the motor turns the sheave directly, eliminating gearbox loss in heating the oil. Gearless drives reduce energy loss substantially, and moreover gearless motors, substituting torque for RPM, have a longer service life. The initial cost is greater, but long-term savings are substantial.
Another area of concern, particularly in taller buildings, is elevator rope. Longer steel rope means more weight, sometimes to a point where the rope has difficulty holding its own weight. In the tallest buildings, wire rope may weigh several tons, comprising 70 percent of the total load.
Elevator manufacturers have been confronting this problem by developing lighter-weight alternatives. For example, Schindler has introduced Aramid fiber rope, which is both stronger and lighter. Gen2, offered by Otis, consists of thin cables enclosed within a polyurethane outer sheath. Mitsubishi has introduced a stronger, denser rope composed of steel wire arranged in concentric layers. Kone’s UltraRope consists of a carbon-fiber core. The rope is only ten percent the weight of conventional wire rope.
Since these ropes are stronger and lighter with smaller profile, power requirements are reduced.
Another very significant innovation, by ThyssenKrupp, is the Twin System. Two cars travel independently in a single shaft. Besides moving passengers more efficiently, the number of shafts is reduced, saving space on each floor in a group installation, which can now be rented.
Elevators, beginning in the nineteenth century and continuing to the present, have evolved from primitive platforms that lifted freight in factories and mines, to powerful multi-car systems that convey materials and human passengers to offices and residences approaching a mile above street level. There is every reason to believe that they will continue to grow. A space elevator, shown in Figure 1-7, has even been envisioned. It would transport freight and humans from the surface of the earth to geocentric orbit and beyond.
FIGURE 1-7 Idealized diagram of a space elevator, not to scale. (Wikipedia)
A cable or tether would be anchored to the earth and extend into space. Vehicles would climb this cable, lifting passengers and freight to orbiting space stations without benefit of huge shuttles. The cable would extend well beyond the system’s center of mass, to a heavy counterweight, which due to its position well beyond the geostationary orbit, would exert a powerful counter-gravitational force, pulling the cable tight.
A continuous stream of climbers (not human, but mechanical) could ascend the cable, passing others returning in the opposite direction to earth. Escape velocity would not have to be attained. It would be a relatively slow journey, requiring about a week. Konstantin Tsiolkovsky conceived the space elevator in 1895, near the end of the century that defined elevator technology. Rather than a cable under tension because it was hanging from the counterweight, it was a tower, sitting on the ground and subject to great compressive force.
There are some major difficulties in building the space elevator, primarily having to do with fabricating the cable, which has to support its own enormous weight without breaking. As of now, no known material can do that. But carbon nanotubes should work. Development costs are high, but once built, the space elevator will transport passengers and freight quite economically. Space elevators will eventually be constructed on other planets, moons, and asteroids. For these lighter bodies, the challenges are less formidable but the rewards are not as great.
Lighter materials such as Kevlar would be suitable for constructing extraterrestrial cables. In 1975, Jerome Pearson introduced the idea of a tapered cable. Maximum tension on a space elevator cable would be at geosynchronous altitude, so the cable would have to be thickest there and taper carefully as it approaches earth. The concept of a space elevator became more realistic after the development in 1990 of carbon nanotubes. In 2000, Bradley Edwards proposed a flat ribbon rather than the round cable of previous designs, because it would be less vulnerable to damage from meteors and space debris. Moreover, climbers could use rollers to travel upward. Since then, numerous feasibility studies have concluded that the space elevator is a valid concept, and it will profoundly affect human history as we continue on our greatest journey.
1. Who made the first extant reference to an elevator?
A. Archimedes
B. Vitruvius
C. Plato
D. Aristotle
2. Early nineteenth century elevators:
A. were powered by work horses
B. were steam-powered
C. were powered by electric motors
D. rose to a height of 20 stories
3. William Strutt’s elevator:
A. was in a coal mine
B. had no pulleys
C. ran off a flat belt
D. carried passengers and freight
4. Hotels and office buildings used elevators beginning around:
A. 1875
B. 1865
C. 1855
D. 1845
5. Henry Waterman’s elevator in Manhattan:
A. did not require an outside attendant
B. was powered by an electric motor
C. was a hydraulic elevator
D. had a clutch that disengaged when the operator released the handle
6. In George Fox and Co.’s freight elevators:
A. there were frequent mechanical failures
B. meshing spur gears with a worm gear became obsolete
C. a separate brake for the hoist was required
D. wire rope replaced hemp rope
7. Elisha Graves Otis:
A. was enormously successful financially
B. pioneered the use of safeties
C. specialized in hydraulic elevators
D. built traction engines throughout the United States
8. In a hydraulic elevator installation:
A. if bedrock was encountered, a hybrid design was needed
B. cylinders could be installed vertically only
C. noise was a severe problem
D. complex rope and pulley assemblies were required
9. Nineteenth century hydraulic elevators:
A. used no coal
B. used oil for hydraulic fluid
C. rose to unprecedented heights
D. would free fall if the piston failed
10. Electric motors replaced steam in elevators:
A. before the American Civil War
B. in the 1920s
C. after 1900
D. beginning around 1880
For answers, go to Appendix A.
