9.1.1 Linear-Motor-Driven Elevator Cars

The earliest patent concerning a linear motor in elevators was granted to K. Kudermann in 1970 [P211]. The principle of that patent is illustrated in Fig. 9.1. The proposed system consists of a counterweight and two linear motor armatures on both sides of the car. The disadvantage of this setup is that the motor must also lift its own weight, thus increasing the demand for power supply during acceleration.

The best linear motor drive is a combination of the double-sided motor with the guide rail, which provides an advantage in balancing the attractive forces. The linear motor must be distributed on both sides of the car to obtain a symmetrical hoisting. The analysis of a few types of motors indicates that the LSM with ferrite PMs can offer the smallest cost-related mass, as shown in Table 9.1 [78]. The relative power used in Table 9.1 is defined as

where Phoist = mlgv is the hoisting power, v is the rated speed at rated load of the elevator, P_inm is the power supplied by the hoisting machinery, and ΔP is the power dissipated in the hoisting system. The relative power loss is the ratio of losses to the hoisting power

The Phoist is not a theoretical minimum for the power of the hoisting machine because the counterweight does a part of the hoisting work, and the rest must by supplied by the hoisting machinery.

Table 9.1. Comparison of different linear motors applied in 1000 kg car traveling at 2 m/s for mid-rise elevator. Cost of related motor masses reflects material cost in 2010

The relative masses are moving masses or stationary masses related to the mass of the rated load of the elevator m_l.

The relative drive size is defined as the ratio of the maximum electrical current demand I_max to the current corresponding to the hoisting power Ihoist.

The relative cost represents the weighted prices of the active material used for the elevator, excluding all the supportive structures and scaled to the mass of the whole hoisting system. The cost weighting coefficients have been assumed as follows: 2 for steel, 5 for aluminum, 8 for copper, 4 for ferrite PMs, and 40 for NdFeB PMs.

Coefficients listed in Table 9.1 lead to the conclusion that the PM LSM can offer the smallest cost-related mass, thus being the most promising alternative. On the other hand, it still remains uncertain whether the benefit of eliminating the rotary machine can be favorably compared to the increased cost of the linear motor.

9.1.2 Elevator with Linear Motor in the Pit

The alternative for a hydraulic elevator can be a linear motor with moving reaction rail as shown in Fig. 9.2.

The idea is to replace the piston of hydraulic machinery with the reaction rail, and the cylinder with the armature of the linear motor. Power can be supplied to the stationary winding. However, either a pit in the basement or the 1:2 roping ratio is required to obtain the required doubled force. Furthermore, the car travel distance is limited to maximum 20 m. Comparison of different linear motors installed in the pit for low-rise elevators is given in Table 9.2.

The structure has the same features as the hydraulic set-up but with a few additional advantages, i.e.,

• no oil and oil tank,

• all machinery is located in the shaft, so that a separate room is not needed,

• the regenerative braking can be applied for the car going downward, so that the lack of counterweight can be partially compensated.

The performance of this elevator with PM LSM compares well with the conventional hydraulic system, but the amount of magnetic material required makes this structure commercially unattainable.

Table 9.2. Comparison of different linear motors installed in the pit for low-rise ropeless elevator (1600 kg car, 4 m/s). Cost of related motor masses reflects material cost in 2010 [78]

9.1.3 Linear Motor in Counterweight

The counterweight is the most natural place for the linear motor in a traction type elevator. In this way, the mass of the motor can be utilized as a part of the balance. However, the energy must be supplied to the motor through a cable, the length of which on the balance side varies with the counterweight position. Proper measures should then be taken to compensate for this variation of the balance weight. This counterbalance motor placement has been well known for some time and even commercially explored [P14, P50, 202]. Although, similar to the motor installation in the car, when the motor is installed in the counterbalance, the difference in the mass of the supply cable must be taken into account (Fig. 9.3a).

The mass of the counterweight for optimum balancing should be

and the mass of balancing ropes (per unit length)

where m_c is the mass of the counterweight, m is the mass of the car, m_l is the mass of the rated load, m_ec is the mass per unit length (kg/m) of the traveling electric cable on the counterweight side, m_e is the mass per unit (kg/m) of the traveling electric cable on the car side, m_rope is the mass of rope per unit length (kg/m), and H is the total hoisting height. The possible linear motor placement in the counterweight is shown in Fig. 9.3b. The system can be designed with the armature in the counterweight and reaction rail on the wall, as well as with the reaction rail in the moving counterweight and the armature on the wall.

Table 9.3 shows a comparison of different linear motors mounted in the counterweight of a low-rise elevator. PM LSMs require less power than conventional hoisting motors. Further analysis shows that the performance of a reluctance motor improves at higher speeds.

9.1.4 Conventional versus Linear-Motor-Driven Elevator

Table 9.4 illustrates the major parameters of a mid-rise elevator system with different motor types [78]. Both the conventional traction motor and linear motor placed either in the car or in the counterweight offer approximately the same sizing of a hoisting machinery and the drive.

Table 9.3. Comparison of different linear motors mounted in the counterweight of a low-rise elevator (630 kg car, 1 m/s). Cost of related motor masses reflects material cost in 2010 [78]

Table 9.4. Rotary motors versus linear motors for mid-rise elevators (1000 kg car, 2 m/s). Cost of related motor masses reflects material cost in 2010

9.2.1 Vertical Transport in Ultrahigh Buildings

Land in the world’s biggest cities, e.g., New York or Tokyo, is extremely expensive, which drives the expansion of rentable spaces into higher and higher buildings and underground areas. However, the larger and taller the buildings, the more elevators are required to keep acceptable waiting time for dispatching. The increasing interest in ultrahigh buildings, also called hyper buildings or vertical cities that exceed 600 m in their height inspires elevator companies to intensify their research effort in alternative technologies for vertical transportation. This becomes apparent in Japan where building contractors, elevator companies, and other institutions are spending considerable effort in developing transportation concepts for these hyper buildings. Vertical transportation systems in ultrahigh buildings must address many technical issues, amongst others

• transport configuration with traffic flow within and between buildings;

• diverse building capacity incorporating residential, commercial, and service functions;

• use of alternative building transportation systems (roped elevators, ropeless elevators, escalators, people movers);

• the highest levels of reliability, safety, and passenger rescue;

• comfort of travel (air pressure changes, vibration, vertical and horizontal motions, travel time);

• elevator propulsion, guidance, brakes, power consumption, control, and communication.

Ultrahigh buildings pose new problems in the construction of high-speed elevator systems, i.e., vertical oscillations, horizontal swing, car noise, and cable length limitations. Because the ropes (steel cables supporting the car) are very long and usually have a low dumping coefficient, even small disturbances, e.g., traction machine torque ripple, can cause car oscillations. This vibration can further be amplified when the disturbance frequency coincides with the car’s natural frequency. In some instances, this bounce may destabilize the system that controls the elevator speed. The horizontal swing of the elevator car can be caused by curvatures of the guiding rails or by imperfect rail segment junctions. Car noise is due to the roller-guides tracking the rails and by wind noise (air passing through the traveling car).

The car rise limit is imposed by the cable weight and strength and can be considered as a function of five variables: (a) safety factor, (b) rope strength, (c) rope mass per unit length, (d) number of ropes, and (e) mass of the car. Under the most favorable conditions, a cable-based elevator can achieve a rise of approximately 1200 m, based on 10 commonly used steel cables with 320 kN strength, mass per unit length of 2.14 kg/m, and safety factor of 10 [95].

Ropeless elevators with multiple cars in one shaft may be perceived as practical solution to ultratall buildings above 1000 m. The primary concern is that the roping technology may not be extensible to hoistways of that height, both from a rope strength standpoint, and safety margin considerations. Another problem is that roped elevator systems, understood to be based on sky lobbies, would consume too much space to make such a building financially viable. An analysis conducted by Mitsubishi Corporation [95] found that almost 30% of the total space in a 100-floor skyscraper must be devoted to elevators, including their hoistways, halls, and machine rooms (Fig. 9.4). Recently, the peak rents for Tokyo skyscrapers were estimated about US$ 1000/m²/year. Clearly, the elevator space occupancy ratio has a significant financial impact on building utilization.

The one-shaft, multicar, ropeless elevator system is considered to be the most promising answer to these problems. It eliminates the suspension cables, and with them, the rope-strength and vertical-oscillation problems. Use of multiple cars in a single hoistway improves the space occupancy factor even when compared with roped double-deck elevators. For example, the required number of hoistways can be reduced by 65% to 80% when using a ropeless elevator with multiple cars versus traditional roped system (Fig. 9.5).

The mining industry is also interested in alternative vertical transportation systems with ropeless elevators [43]. South African ultra-deep gold mines (exceeding 3500 m) are considering the implementation of PM-LSM-driven hoisting systems because the maximum depth achievable by a roped hoist system is approximately 2800 m. After this depth, the hoisting rope can no longer support its own weight and the payload. To overcome this problem, at the present time, subvertical shafts are sunk, i.e., additional roped hoist system are installed underground at a depth of about 2000 m [43].

9.2.2 Assessment of Hoist Performance

The hoist efficiency without the rope is independent of the height (in a mine independent of the depth), and therefore, the operation of the lifting machinery is not limited by the rope mass.

The successful implementation of the linear motor hoist depends on two main factors: (a) the ratio of the motor weight–to–thrust that it can produce, and (b) the motor size (cost).

One of the main criterion for assessing the performance of the hoist is the efficiency of the overall system [44]:

where P_outm is the mechanical power required to lift the payload, P_inm is the total mechanical power required to operate the system, m_l is the mass of the payload, m is the mass of the car, m_rope is the mass of ropes, g is the acceleration of gravity, and v is the linear speed.

Assuming a constant speed of operation and neglecting the friction, the efficiency of the hoist system without a counterweight can be expressed as follows [44]:

• for a conventional roped hoist

  η=mlml+m+mrope⁢(9.6)

• for a ropeless hoist

  η=mlml+m⁢(9.7)

Assuming that the mass of the car and payload are the same in both cases, the system efficiency becomes entirely dependent on the mass of ropes.

The taller (deeper) the hoist shaft, the longer and therefore the heavier the rope since mrope=0.25πρnropedrope2lrope where ρ is the specific mass density of steel and n_rope, d_rope, and l_rope are the number of ropes, diameter, and length of the rope, respectively. Heavier ropes have larger diameter to withstand the increased tensile stresses. As a result, the increased cross-sectional area of ropes contributes further to their mass, affecting the overall system efficiency (Fig. 9.6a) [43].

The efficiency of roped hoist without counterweight tends to zero as the height (or depth) approaches the operating limit (about 2800 m for steel ropes).

Technologies crucial to the successful development of ropeless elevators can be divided into four major categories:

• system configuration and energy management

• propulsion and guidance

• safety and brakes

• control and communication

9.2.3 Construction of Ropeless Elevators

The unique features of the ropeless elevator include

• unlimited rise,

• vertical and horizontal motion,

• multiple cars in the same hoistway,

• no traveling cable,

• high traffic-handling capacity with minimum core space.

The ropeless elevator with multiple cars in one shaft can be built by hanging each cab from a bar fixed to the movers of a pair of LSMs (Fig. 9.7). Long armature windings are stationary, and PMs are integrated with movers. Each car hangs from a shaft fixed to the PM movers. The mover is a steel rail with PMs that is installed between two parallel stationary armature systems. Armature units are segmented into blocks (Fig. 9.8). The guidance system maintains a small and constant air gap between the armature cores and PMs. When the armature windings are excited, the interaction between the excitation flux and the flux of PMs produces linear thrust moving the car up or down. The long stationary armature is divided into sections, the minimum number of which is equal to the number of cars. Only one car can be permitted in each section at a time so that each car may be controlled independently of the others. The more sections there are, the more precisely the system can be controlled; however, the construction cost is higher since each section requires its own high-power converters.

9.2.4 Operation

The LSM-powered ropeless elevator consumes significantly more energy than an equivalent roped elevator to move a given number of passengers to a given distance. The reason for this is that a typical counterweighted elevator with ropes needs to drive only the offset load (typically only 12% to 14% of the total moving mass), while the ropeless elevator must lift the entire combined mass of the car, the frame, passengers, and the PM movers. Assuming motors and converters of similar efficiency, the ropeless elevator requires power supply 7 to 8 times that of its roped counterpart.

An ascending ropeless elevator requires almost constant power for a given load as it rises in the hoistway. When moving at constant speed, the power is consumed to overcome the force of gravity. Peak acceleration or deceleration, when limited to 0.11g increases or decreases the power required by only about 10%. During the descent, a significant portion of energy is recovered as the LSM is able to regenerate it from the passage of the PM excitation system through the armature windings. However, the recovered energy is always smaller than the propulsion energy because regeneration occurs when the descending car travels with speed exciting higher EMF in the winding than the drop voltage on the winding impedance. Fig. 9.9b shows that the car absorbs the power taken from the electric supply system as it starts traveling downward. A freely falling car would develop the acceleration equal to 1.0g, that is too high for the passengers riding inside. To limit the acceleration to 0.11g the LSMs must develop braking thrust (directed upward) slightly smaller than the gravitational force.

9.2.5 First Prototypes

Completing a study on very large drives in the early 1990s, the Underground Development Utilization Research Center of the Engineering Advancement Association of Japan (ENNA) is seriously considering the application of linear motors to ropeless elevators. This demonstrative research was conducted by a group of five companies: Fuji Electric, Ishikawajima–Harima Heavy Industries, Kawasaki Heavy Industries, Simizu Corporation, and Fujitec. ENNA has already built a working model of a horizontal and vertical ropeless transport system [121]. This scaled installation serves as a model for future hyper building transportation systems. Specifications are the following:

• maximum thrust 3,000 N

• vertical travel distance 8 m

• horizontal travel distance 1 m

• velocity from 0 to 1 m/s

• acceleration 1 m/s²

• moving mass 270 kg (including payload)

• car dimensions: 1.0 W ×0.9 D ×1.8 H m

• power and control: IGBT inverter

• position encoder: optical pulse generator

The overall view of the test system is shown in Fig. 9.10. The car is suspended as a pendulum from the traveling carriage. To prevent the car from swaying at the time of acceleration or when passing through a curved rail, a friction plate has been set between the carriage structure and the car. The car is guided by two rails: (a) a rigid H-shaped beam, which supports and guides the car (travel rail), and (b) a motor guidance maintaining a constant linear motor air gap. The system is capable of moving the car both along the straight and curved path. Such a transition takes approximately 10 s.

To meet the most important propulsion system requirements, i.e., lightweight and large motive force, an LSM with PM mover installed on the car and the armature on the track has been adopted. The mover is placed between two sets of stationary armature systems. High-energy N32 NdFeB PMs (energy product 255 kJ/m³) have been used. PMs are fixed to both sides of 10 mm thick steel rails and skewed by one slot pitch of armature core to reduce the thrust ripple (Fig. 9.11a). To minimize the attractive forces between PMs and armature cores, a double-sided armature configuration has been chosen (Fig. 9.11). This motor topology is referred to as a U-type LSM with short PM reaction rail and two long armature systems.

The armature of LSM is divided into 10 straight and curved sections, each about 2 m long. The segments are powered from sinusoidal IGBT inverters with low noise emission levels arranged in a tandem. The power from inverters is distributed by means of switches connecting individual armature sections in a sequence (Fig. 9.12). When the mover travels between the segments, both inverters operate; otherwise, only one is used. The presented system is able to fully control the movement of a single carriage. For higher number of cars, a more complicated power distribution scheme is needed. The speed and position are detected by a slotted plate on the car and photodetectors mounted on the track.

The safety of the system is guaranteed by a braking device activated by a hydraulic system that is mounted at each end of the trace ends to hold the traveling carriage in position. Oil buffers are mounted to minimize a sudden impact in the case of an improperly decelerating carriage. An electrical dynamic braking is applied in the case of power failure to prevent a free fall of the car. The armature windings are automatically connected (shunted) to the external resistors. The power dissipated in the braking resistor provides smooth, controllable braking force, allowing the car to descend at stable crawl speed even when all safety monitoring equipment fail.

The ENNA project, completed in 1995, shows that, in the near future, the vertical transportation system would evolve into arteries in which cars could move three-dimensionally. Travel will not be limited to enclosed spaces, like the conventional elevator systems in use today, but multiple passenger cars will be moving freely between floors, buildings, in underground or in any subterranean space.

Fig. 9.13 shows an artist’s impression of a future high-rise building district with vertical–horizontal ropeless elevators propelled by linear motors.

9.2.6 Brakes

One of the most important elevator subcomponents is a braking system. The brake must:

• maintain the car in a fixed vertical and horizontal (side-to-side and front-to-back) position while passengers are boarding and exiting;

• operate to stop the vertical motion of the car under complete power failure;

• have the capability of producing at least 1.0 m/s² deceleration for periods above 10 s when the car is traveling downward;

• supply auxiliary guidance to the car in the case of a complete power failure. This requirement is important for elevators with active magnetic guidance (AMG) [156].

The primary function of a brake is to hold the car in a fixed position when passengers are entering and exiting. While this function could be accomplished using an electric motor, it is more efficient to use a friction brake to hold the car in position. In a roped elevator, this brake generally acts on a drum attached to the sheave. Therefore, a releveling process is required as the car load changes and the cable length between the brake and the car stretches or contracts. For the brake mounted directly on the car (ropeless design), the need for releveling is eliminated as there is no cable located between the brake and car stretch. The second function of the brake is to provide stopping the car in the case of a power failure. The braking force in a conventional elevator is generally applied by a spring pressure and released using an electrical actuator (solenoid). In the case of an electrical outage, the braking force is automatically produced by the spring pressure. Usually, elevator brakes are either fully applied or fully disengaged. Normally they would not be modulated or applied gently to generate, for example 50%, of possible braking force.

The brake in a standard roped elevator is used under steady-state conditions. It is applied after the car has completely stopped, and released only after the motor has been energized to support the car. This is done to prevent the car from lurching either up or down at the brake release. Conventional elevator brakes may also operate in a dynamic mode if power is lost to the hoistway or safety limits are exceeded. In this mode, the brake is required to stop the car safely while it is moving up to 120% of the contract speed, i.e., speed negotiated between the manufacturer and customer.

Brakes for ropeless elevators will have to meet more severe requirements. With ropeless elevators, it is possible to use a caliper friction brake, similar to the standard elevator brake. The caliper and brake assemblies should be attached to and move with the car. These brakes can be actuated by a spring and retracted by an electromagnetic actuator. There should be one brake assembly on each side of the car, acting against the same rails (T-rails) as safety devices. Under normal operation, the car comes to a stop being driven by the LSM, and the brake is only deployed after car has stopped. In case of emergency, the brake needs to be capable of stopping the moving car. An example of caliper brake installation on the ropeless elevator car is shown in Fig. 9.14.

9.3.1 Guidelines for Installation

Linear motors can simplify the transfer bulk and loose materials, small containers, pallets, bottled liquids, parts, hand tools, documents, etc., in building and factory transportation systems. It is easy to adopt the linear motor system to the allowable space as linear motors are compact machines, do not have rotating parts, do not need maintenance and allow design of a transfer system that is in harmony with surrounding equipment. Linear motors are silent, and the noise emitted does not exceed 65 dB. Magnetic fields emitted by armature windings and PM excitation system are not dangerous to human organisms. Well-designed transportation systems emit very low level of RFI. High-intensity magnetic fields exist only in the air gap between the armature and reaction rail.

It is possible to design both on-floor and overhead transportation systems. Moreover, the linear motor transportation line can be sloped to create a link between two or more floors. Parallel transportation lines or line for double-deck carriages can also be designed. The rule is that transportation lines must never cross. If, say, a loop like “8” shaped line is needed, the crossing must be designed on two different levels. Possible designs are shown in Fig. 9.15 [106].

Stations for loading and unloading materials must be carefully planned, and additional space must be reserved. Linear motors cannot be installed near inlets and exhausts of air-conditioning systems because airborne dirty particles could deteriorate the sensors’ performance and sometimes contaminate the air gap between the armature and reaction rail. In the case of PM excitation, the space around the reaction rail must be free of any ferromagnetic particles.

9.3.2 Construction

There are two basic constructions of building or factory horizontal transportation systems with LSMs:

• Long stationary reaction rail with PMs and moving short armature (Fig. 9.16)

• Stationary armature or armatures and moving reaction rail with PMs (Fig. 9.17) [184, 185]

The stationary long reaction rail is expensive, and the moving armature winding needs a contactless energy transfer [201]. It is more economical to use stationary short armature units distributed along the track. The mover is accelerated by the short armature unit and is then driven by its own inertia. Since the speed decreases, the next unit must be installed at a certain distance to reaccelerate the mover (Fig. 9.18). The longer the travel distance, the more armature units need to be used. Such discontinuous arrangement of LSMs requires synchronization of the speed of the mover (PM reaction rail) and traveling magnetic field. The method of synchronization can use either feedback signals from position sensors or open-loop control of synchronization [184, 185]. The first methods are very reliable; however, they are costly. Open-loop methods are more economical (lower cost of sensors).

9.3.3 Applications

The horizontal transportation system with stationary short armature units can be used for transfer of containers in storage areas.

The system developed by Preussag Noell, GmbH, Würzberg, in cooperation with the Technical University of Braunschweig, Germany, consists of track-mounted, single-sided, iron-cored armature units and car-mounted movable PM excitation systems [93]. The length of the PM excitation system covers at least two armature units, including the distance between them. A group of armature units connected in series is fed from an IGBT VVVF inverter. The position of cars is detected with the aid of special position sensors. Cars of length up to 15 m run on standard gauge railway track. The air gap between the armature core and PMs is 13 to 15 mm. With the pole pitch from 80 to 100 mm and PM height from 9 to 15 mm, thrust density of 30 to 40 kN/m² can be obtained [93].

Another LSM system for container transportation has been developed at Kyushu University, Fukuoka, Japan [246]. The air-cored armature coils are distributed along the guideway beneath the container carrier, and the PM excitation system is integrated with the carrier. The air-cored LSM produces both the propulsion force (thrust) and normal repulsive force (no armature ferromagnetic core). The normal repulsive force (electrodynamic levitation) helps to reduce the mass of the payload by about 85% [246]. By applying the decoupled control method, the thrust and normal force can be controlled independently [246]. It is possible not only to reduce the mass of payloads but also to reduce friction, vibration, and noise caused by heavy goods.

Horizontal linear motor transportation systems can alleviate and simplify duties of medical personnel in hospitals. Clinical charts, x-ray films, chemicals, specimens and documents can be transported by linear motor conveyance systems in special containers. Shinko Electric Co. Ltd., Tokyo, Japan has developed linear motor conveyance technology for hospital staff [120]. This system, called LimLinear, can travel both vertically (5 m/s) and horizontally (3 m/s), connects several floors, and is controlled by computer.