The present invention concerns a power supply system for rack and pinion lifts according to the introduction to claim 1. The present invention concerns also a method for the power supply for rack and pinion lifts according to the introduction to claim 12. The invention includes also the use of a DC bus for lifts as specified in claim 20.

The power that is required to drive transport means, such as lifts, for persons or goods between floors in buildings varies, depending on a number of factors such as, for example, the instantaneous load on the lift, its speed, the direction of travel and in which part of the transport cycle the lift is currently operating. It is important that the power requirement be reduced as far as possible not only to reduce the installation and operating costs of the lift, but also to reduce the dimensions and space required for the power supply system of the lift.

A rack and pinion lift comprises in general a load carrier such as a lift car that can be driven along a track by means of electric motors and cogged wheels, which track is normally in the form of a mast provided with a cogged rod. The electric motors that are referred to are normally of three-phase type with a rated voltage of 380-500 V and a frequency of 50 or 60 Hz. The motors offer soft starting and stopping by the use of frequency control. The electric motor for a rack and pinion lift is supported immediately by the lift car, as is also the control and operating unit for controlling the lift. This means that the rack and pinion lifts differ from conventional lifts in that the lift car carries its own drive unit and the associated control and operating unit for the control of the electric motor. As a consequence of this, the distance between the power supply system and the drive machinery of the lift will vary as the lift car moves upwards and downwards along the mast. An electrical power line, such as electrical conductors surrounded by insulating material, passes from a unit at ground level up to the lift car for power supply to the electric motors. The electrical power line is arranged, with the aid of a cable trolley or similar guided along the mast, to follow with an adapted length the lift car upwards and downwards along the mast, suspended under the lift car. It should be understood that, as the lift car carries its own drive unit supply, the electrical power line must be extended and shortened as the lift car moves along the mast.

The lifts are, on the other hand, becoming evermore higher and it has proved to be the case that the weight of the electrical power line, for lift heights up to 400-500 m and in certain cases even higher, becomes so great that it influences the loading capacity of the lift. The extension of the electrical power line as the lift car moves along the mast creates also difficulties with housing the complete cable length, in particular, when the power cable is to supply powerful motors with the current they require. In particular, in the case in which lifts are to move between floors in very high buildings, the length, stiffness and deadweight of the electrical power line constitute problems, whereby the relatively heavy-duty electrical power line that accompanies the lift car becomes difficult to control and to carry. It should be understood that the limited possibilities of the rack and pinion lift to balance the potential energy of the lift car with a counterweight leads to a demand for not only powerful electric motors but also electrical power lines with a relatively large cross-sectional area in order to be able to supply the current required by the drive motors of the lift.

The greatest instantaneous force is required at the instant of starting, i.e. during the initial part of the transport cycle when the lift car is accelerated. The power requirement falls when the lift car has reached a constant speed. The potential energy of the lift car of a conventional lift provided with a counterweight is balanced by a counterweight during motion. Rack and pinion lifts normally lack this possibility and the potential energy must be continuously overcome by the power supply system, which, of course, places heavy demands on this system. The energy or force that is generated when the lift car moves with the said constant speed downwards and is braked (retarded) is normally caused to be dissipated as heat in separate resistances (braking resistances) or it is fed back into the mains power network by the use of what is known as "generative braking" and network return. In a comparison with a conventional lift provided with a counterweight, a rack and pinion lift produces in a corresponding manner when the lift car moves downwards considerably more energy than a conventional lift provided with a counterweight.

It is not unusual that rack and pinion lifts are used during the construction of buildings at locations that lack electrical power supply and the infrastructure of power stations, and this means that alternative sources of energy such as diesel-powered units are used as main power generators. It should be understood that it would be desirable from a number of points of view to be able to reduce not only the size but also the cost of the equipment used to generate power. It can also be mentioned in addition to this that the length of the mast that is used in the currently used type of rack and pinion lift is formed from a number of sections that can be stacked and mounted on each other, in order to be able to vary the length of the mast. A consequence of this is, of course, that also the electrical power line is given a length that is adapted such that it can accompany the lift car along the complete height of the mast. It can be mentioned in this context that rack and pinion lifts are normally intended to be used for non-permanent use within the building industry, i.e. the lift is demounted when the building construction has been completed.

It should be understood that the limited possibilities of the rack and pinion lifts to balance the potential energy of the lift car through its lack of a counterweight leads to a requirement for very powerful electric motors and associated electrical equipment such as electrical power lines with relatively large cross-sectional areas in order to be able to deliver the current required by the drive motors of the lift, in particular with respect to the instant of starting, or the "acceleration phase". When the lift car is driven to a certain level up the mast, its potential energy increases according to the equation: (Epo _t =mgh; where m = mass, h = height and g = acceleration due to gravity). In the absence of a counterweight, the potential energy of a lift car that has been lifted to a certain height has increased considerably, whereby the lift has been supplied with large amounts of energy through the power supply system. It should be realised that the energy that has been supplied is collected at the lift car as potential energy, when the lift car is in its elevated position.

In addition to the said high elevations of the lift, it is, of course, also a problem that the lift motors drive the lift at full power only during certain periods, while the power supply system of the lift must be dimensioned based on the highest most critical power that is normally required only during short periods of the operation, in particular during the instant of starting and also during motion of the lift upwards along the mast. The greatest instantaneous force is, however, required at the instant of starting, i.e. during the initial part of the transport cycle when the lift car is accelerated. The power requirement falls considerably when the lift car has reached a constant speed. Changes in potential energy of the lift car of a conventional lift provided with a counterweight are balanced during motion by a counterweight. Rack and pinion lifts normally lack this possibility and the potential energy must be continuously overcome by the power supply system, which, of course, places considerable demands on this system. Enormous amounts of energy are generated when a rack and pinion lift car moves downwards along the mast during braking (retardation). The potential energy that is in this way released by the lift car is normally converted to heat energy in separate resistances (braking resistances) or it is fed back into the mains power network by a process known as "regenerative braking". It should be realised that a rack and pinion lift produces a considerably greater amount of energy during its motion downwards than conventional lifts provided with counterweights produce, due to the absence of a counterweight. Previous attempts to equip rack and pinion lifts with counterweights have been less than successful, mainly as a result of complicated designs and the extra work that the counterweight arrangement introduces during the tasks of mounting and demounting the lift.

Since the power supply system and the associated electrical plant must be dimensioned to cope with the highest output power that is required during short periods, while the appearance of the standard load places considerably lower requirements for the capacity of the power plant, overload protection, the conductor system and other equipment in the consumer circuit will not be used fully with respect to the capacity of the equipment. As a consequence of this, the investment costs for the power supply system will be significant and more extensive than necessary and they will be inefficient from the point of view of costs. Examples are shown in Figures 1 , 1a and 2 of prior art technology whereby for reasons of clarity the same reference numbers are used as those shown in the subsequent Figures 3-6 but with an increase by addition the figure 100 in order to make clear that it is a case of the same or similar items that are described in the invention.

Figures 1 and 1a show how three-phase alternating current is fed to a transformer

116 in a main power generator 120 that is part of the public mains power network 115, in order to be transformed down to a suitable level of voltage. Through an AC bus 113 and a three-phase electrical power line 113', a three-phase electric motor 103, 104 supported by a first and a second lift car 101 , 102 is supplied with electrical energy. The lift cars 101 , 102 are rack and pinion cars and they can be driven along a mast 105 through the interaction between a cogged wheel 111 driven by the relevant electric motor 103, 104 and a cogged rod 112 arranged on the mast. The selected speed during raising and lowering is controlled through appropriate frequency conversion of the electric motor 103, 104. When a lift car 101 , 102 moves downwards along the mast 105 during braking, an inverted flow of AC alternating current is generated in the electric motor 103, 104. The inverted AC current flow during the "generator braking" is led back to the main power network 120.

Figure 2 shows an example in which a diesel-powered unit 125 intended to be used as a source of power is a component of the main power generator 120. The diesel-powered unit 125 is mechanically coupled to an AC power generator 126. The AC alternating current that is produced by the power generator 126 supplies the electric motors 103, 104 of the relevant lift car with AC alternating current through an electrical power line 113' and from this point the system is the same as that described in Figure 1. When one of the lift cars 101 , 102 moves downwards along the mast 105, an inverted flow of AC alternating current is generated in the motor.

A first principal aim of the present invention is to achieve a more efficient power supply system for rack and pinion lifts not least with the aim of reducing energy consumption and the rated power requirement for the generator system. A second aim is to achieve a system that facilitates the housing and the weight from the electrical power line that supplies the electric motors with power and extends from the unit at ground level to the lift car. A third aim of the invention is to achieve a power supply system for rack and pinion lifts that is simple to use together with a freely chosen type of main power generator, that can regain and store a considerable amount of energy through generative braking when the lift car moves downwards which can be used later, and in particular during the instant of starting, i.e. during the initial part of the transport cycle when the lift car is accelerated. A further aim of the invention is to achieve a method that allows a more efficient power supply of rack and pinion lifts and that the requirements placed on the generator system that is used can be reduced. These aims of the invention are achieved through a power supply system that demonstrates the distinctive features and characteristics that are stated in claim 1 , a method that demonstrates the distinctive features and characteristics that are stated in claim 12 and the use of a DC bus as power cable for the power supply of rack and pinion lifts as specified in claim 20.

Among the advantages of using a DC bus as a component of a power cable for the power supply of the drive machinery from the ground level can be mentioned that it makes it possible to transport power with lower resistance losses and dielectric losses than those experienced using alternating current at corresponding powers. The advantages become particularly large if the driving current in this case is constituted by a high-voltage DC current. A typical DC transmission cable includes conductors and an insulating layer. An AC voltage also gives rise to capacitance losses, which can be avoided with a DC direct current as the driving current. As a result of the use of a DC bus as power cable can be mentioned that the dimensions of the cable can be kept low, and this reduces not only the weight of the cable but also the problems of housing the complete length of the power cable when supplying powerful electric motors with driving current. A further advantage of using a DC bus architecture with a positive and a negative side is that it makes it possible to connect different types of electrical equipment in parallel directly to the power supply system in a freely chosen manner, such as an energy storage system, alternative power-generating main power networks, and a braking resistance directly to the mains power network.

The electrical drive motors 3, 4 of the lift cars 1, 2 may in one design be selected from a group consisting of frequency-controlled motors, AC current motors, and DC current motors.

The power-generating main network 12 may, in an alternative design, comprise a source of power 15, 25 selected from any one of the following: diesel engines, turbine engines, Stirling engines, Otto engines, fuel cells, solar cells, AC electrical networks, wind turbines, and combinations of these.

The control unit may, in an alternative design, be selected from any one of the following: an analogue unit, a programmable logic controller (PLC), and a computer.

An embodiment of the invention will be described below in more detail with reference to the attached drawings, of which:

Figure 1 shows schematically a block diagram of a prior art power supply system, connected to the mains power network, for rack and pinion lifts arranged in pairs that can be driven along a common mast, known as a "double car",

Figure 1a shows a view from the front and in greater detail of the drive unit that is a component of a rack and pinion lift of the type shown in Figure 1, Figure 2 shows schematically a block diagram of the power supply system according to Figure 1 but now in a design with a generator unit powered by a diesel engine, also known as a "generator system",

Figure 3 shows schematically a block diagram of a power supply system according to the invention that is connected to a mains power network for the power supply of rack and pinion lifts that are arranged in pairs and that can be driven along a common mast (known as a "double car"),

Figure 4 shows schematically a block diagram of the power supply system according to Figure 3 but now in a design with a generator unit powered by a diesel engine, also known as a "generator system",

Figure 5 shows in a schematic form a block diagram of a power supply system according to Figure 3, but with an energy storage system in the form of a supercondensor,

Figure 6 shows in a schematic form a block diagram of a power supply system according to Figure 3, but with an energy storage system in the form of a battery,

Figure 7 shows schematically in the form of a graph the energy requirement of a prior art rack and pinion lift at various stages A-F of a transport cycle, and

Figure 8 shows schematically in the form of a graph equivalent to Figure 7 the energy requirement during the use of a power supply system according to the present invention.

With reference to Figures 3-6, there is shown schematically a lift system comprising two rack and pinion lifts 1 and 2 for the transport of persons or goods, whereby each lift comprises a load carrier in the form of a lift car that can be driven, by means of an electric motor 3, 4 supported by this lift car and a transmission with a shaft that can be rotated and that interacts with a cogged wheel, along a track in the form of a mast 5 provided with a cogged rod (see Figure 1a). Each lift car 1 , 2 supports a converter 6, 7 for the conversion of DC to AC adapted for driving the electric motor. It is appropriate that the said relevant electric motor be of three-phase type, having, for example, a rated voltage of 380-500 V and a frequency of 50 or 60 Hz. The present power supply system is shown in Figures 3-5 incorporated as a component of the two prior art designs that are shown in Figures 1 , 1a and 2.

An energy storage system denoted generally by the reference number 10 is a component of the power supply system, which, with an energy store 1 1 located at ground level, can absorb and store energy that is produced through regenerative operation when one of the lift cars 1 , 2 is retarded during motion downwards along the mast 5, i.e. when the potential energy is reduced and converted to electrical energy in the electric motor. When one of the lift cars 1 , 2 moves upwards along the mast 5 through the influence of the power supply with the AC drive motor 3 or 4, respectively, that is supported by the lift car, the electric motor is supplied with the energy stored in the energy store 1 1 in order to achieve an increase in the potential energy of the lift car. In addition to stored energy in the energy store 11 , any further energy that is required is taken, either from a principal power network that generates power and that is denoted generally by the reference number 12, which may, for example, be constituted by an electrical power network located at ground level or from the diesel-powered unit and the associated power generator shown in Figure 2. Electrical power is supplied to the electric motors 3, 4 during the motion of the lift cars 1 , 2 upwards along the mast 5, whereby the potential energy of the lift cars increases, and electrical power is produced through regenerative braking of the electric motor during the motion of the lift cars 1 , 2 downwards along the mast, whereby the potential energy is reduced. Electrical energy that has been obtained through regenerative operation during the braking motion of the lift cars 1 , 2 downwards along the mast 5 is recycled and collected in the energy store 1 1 located at ground level. The term "ground level" is here used to denote generally the lowest level located along a track on which a lift car is normally located or the lower level from which electrical power is led up to the drive machinery of the lift car.

The energy storage system 10 is shown in Figures 3 and 4 surrounded by a dash- dot line and connected in parallel to a DC bus with a positive side 13 and a negative side 13'. The principal power network 12 that produces power is also connected to the positive and negative sides 13, 13' of the DC bus. The said principal power network 12 that produces power includes generally a mains power network 15 from which three-phase alternating current is fed to a transformer 16, to be transformed down to a suitable level of voltage. The alternating current is converted to a DC direct voltage by a converter 16 that is connected to the DC bus 13, 13' mentioned above. The three-phase AC drive motors 3, 4 of the relevant lift cars 1 , 2 are connected to the DC bus 13, 13' through converters 6, 7 and through a DC power cable or transmission cable 14 that extends between the said converters and the DC bus. It would be possible, as an alternative, that the DC power cable or transmission cable 14 be constituted by a current rail or similar attached to the mast 5 and running along it. As Figure 1 makes clear, it can be selected that the drive motors 3, 4 obtain current either from the energy storage system 10 or from the principal power network 12 that produces power, or as a combination of power from the said two power supplies.

The power to and from the relevant units that are connected to the DC bus 13, 13' are controlled and monitored by means of a control system 34, for example a programmable logic controller, a PLC, or a computer that is placed in connection with the relevant converters 6, 7, 17, 20 by channels 35, 36, 37, 38] in the form of, for example, a radio link 35 or wired connection. The power system described and shown here contains also a dynamic brake resistance 18 that can be connected by means of a switch 19 such that energy that has been produced by generative braking will be returned to the DC bus when one of the lifts 1 , 2 moves downwards along the mast 5 and its potential energy is reduced, and it can then be selected whether this energy is to be led either to the braking resistance in order to be dissipated as heat or to the energy storage system 10 to be stored and used later. It would be appropriate for monitoring and surveillance of the voltage levels of the DC bus that it should be possible to arrange to what is known as a "buck-boost" circuit or similar between the DC bus 13, 13' and the energy storage system 10.

The energy storage system 10 comprises a converter 20, a three-phase induction motor 21 and an energy storage 11 in the form of a flywheel 22. The induction motor 22 may be constituted by, for example, a traction motor, i.e. a three-phase synchronous motor with permanent magnets. When one of the lift cars 1 , 2 moves downwards through the influence of the associated electric motor 3, 4, energy is stored in the flywheel 22, and this takes place as a consequence of the electric motor 3, 4 that is supported by the lift car 1 , 2 being reversed and functioning in this case as a generator. The AC alternating current that is generated from the lift motor is thereby converted to DC direct current through the converter 6, 7, which direct current is led after passage through the DC bus to the energy storage system 10, which receives and stores the potential energy that has been received as kinetic energy in the flywheel 22 that is used in the energy store 1 1 , through the flywheel being accelerated by means of the motor 21. The kinetic energy in the flywheel 22 can, when power is needed, be converted subsequently to electrical energy, which can be used by one of the two lifts 1 and 2.

It should be realised that for lift applications of the type that are described here whereby rack and pinion lifts are arranged in pairs such that they can be driven along a common mast, known as "double-car" applications, the advantage is obtained that the regenerative braking energy (the collected potential energy at an elevated position) that is recycled when the drive motors of one lift 1 when it moves downwards can be stored in the energy storage system 10, to be used by the drive motors of the second lift car 2 when it moves upwards. In this way the lift cars in a double-car application of the type described here will function in approximately the same way as the counterweight in conventional lifts, where the collected and recycled potential energy from one lift car 1 is used during the acceleration and driving upwards along the mast of the second lift car 2, and vice versa. The potential energy levels of the two lift cars 1 , 2 can in this way be balanced through mutual energy transfer. This is very interesting, in particular during the acceleration phase, since it means that the main network 12 that produces power must supply only a limited part of the current that normally would be required during the critical acceleration phase of the lift car (see also Figures 7 and 8). The advantage that the main power unit 12 needs to deliver only a fraction of the energy that is normally required during the acceleration phase means that it is possible to reduce significantly the dimensions of the exertal power system. Figure 4 shows the principal power network 12 that produces power in an alternative design where a diesel-powered unit 25 is used as a source of power. The diesel-powered unit 25 is mechanically coupled to an AC power generator 26. The AC alternating current that is supplied by the power generator 26 is converted to DC direct current by means of a converter 27 and is led into the DC bus through the positive 13 side and the negative side 13'.

Figure 5 shows the energy storage system 10 in an alternative design comprising an energy store 1 1 in the form of a supercondensor 27 in which the potential energy that is obtained during the motion downwards of one of the lift cars 1 , 2 along the mast 5 and generative braking can be stored. In addition to the said supercondensor 27, a diode 28 and a charge switch 29 are present in a first branch whereby the branch is connected in parallel across the positive side 13 and the negative side 13' of the DC bus. Further, a second branch is present with a switch 30 that causes when closed the supercondensor 27 to be discharged. The diode 28 allows current to pass only in a direction that leads to charging of the supercondensor 27, whereby discharge cannot take place through the said first branch, which contains the diode 28. When the first branch is closed, the voltage of the supercondensor 27 increases such that it eventually exceeds the voltage across a condensor 31 that is component of the DC bus. Since the voltage across the supercondensor 27 is higher than the voltage across the condensor 31 of the DC bus, the supercondensor can be connected for the delivery of power to one of the drive motors 3, 4 of the lifts 1 , 2 through the relevant converter 6, 7, which takes place in practice through the second branch being closed by means of the switch 30.

Figure 6 shows the energy storage system 10 in an alternative design comprising an energy store 11 in the form of a battery 32 that is controlled by means of a switch 33 for the storage of energy and the delivery of the said energy in the form of a DC direct current.

With reference to Figure 7, there is shown schematically in the form of a graph the energy requirement for a rack and pinion lift at various stages A-F of a transport cycle whereby block A corresponds to the electricity consumed during acceleration of the lift car 1 , 2 to a predetermined speed in a direction of motion upwards along the mast 5. Block B corresponds to the power consumption when the lift car 1 , 2 increases its potential energy through moving at a constant speed upwards along the mast. Block C corresponds to the energy consumption during retardation and stop of the lift car 1 , 2. Block D represents the inverse power or the return of potential energy for storage during acceleration downwards of the lift car 1 , 2. Block E represents inverse energy consumption during motion at constant speed downwards and block F represents the inverse energy during retardation and stop of the lift car 1 , 2 during downwards motion. Figure 8 shows graphically the power consumption that can be achieved according to the principles of the present invention whereby the power consumption is illustrated as constant with time in the hatched block and is obtained through stored potential energy from regenerative motor operation being recycled as power that is superimposed on the power that is consumed in Figure 7. The graph is intended to give an example of how residual power that is recycled and stored in the energy storage system 10 that has been obtained during braking of the lift car during its motion downwards and that is stored as transferred potential energy in, for example as kinetic energy of the flywheel 22 of the energy storage system, can be returned at times during the transport cycle of the lifts 1 , 2 when the power that is required is at its greatest, for example, at the instant of starting when the lift car is accelerated. It should be furthermore understood that the collected energy in the power supply system can be regarded as a constant, as is stated by the general laws of thermodynamics, whereby the only energy that is consumed is the energy that is lost due to the appearance of mechanical and electrical losses.

The invention is not limited to that which has been described above and shown in the drawings: it can be changed and modified in several different ways within the scope of the innovative concept defined by the attached patent claims.