Elevator Troubleshooting & Repair. David Herres

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СКАЧАТЬ 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.

Hydraulic Elevators

      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.

Edison and Westinghouse

      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.

Frank Sprague

      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, СКАЧАТЬ