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
