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.
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