Issue 3/2006
05/02/06
Innovative traction drive concepts
Dipl.-Ing. Roland Hoppenstedt
One needs more than familiarity with individual components to formulate the ideal solution for today‘s drive assignments. To name just one example, Ziehl-Abegg has become more than just a manufacturer of motors with a long tradition in elevator technology. With the development of electronic regulator units designed specifically for elevator drives the firm has become a systems supplier with a steady eye on the lift as a whole.
Category: Issue 3/2006
Posted by: Editor
For almost ten years now the lift without a separate machine room has been a demanding and challenging topic for every elevator maker. In its role as a supplier of components to middle-market elevator manufacturers, Ziehl-Abegg has ever since that breakthrough worked on a concept for the entire lift. The ZETATOP concept is the result of this effort.
The chief difficulty faced by small and medium-sized companies was and is maintaining their position as a competitive alternative to major manufacturers. Simply purchasing the least expensive components will not suffice. The concept must be more flexible than a standard elevator but nonetheless almost equally economical. The assembly time will have to be minimized and the dependency on structural givens, such as access hatches or niches in the hoistway, will have to be eliminated.
A further demand being put forward is that not only the machine room be eliminated, but that the hoistway cross section be reduced. And finally the headroom and the pit are also to be made smaller.
In view of these trends it was impossible for drive manufacturers to rest on their laurels but instead had to set changes in motion.
Elevator concept without a machine room
The first step was the introduction of the ZETATOP concept with a cantilevered car, 1:1 reeving and the drive machinery in the headroom. With the highly unusual employment of a gearless drive for such low speeds in an elevator design, one which was particularly simple in its engineering, a lift became available that featured little assembly work, reasonable costs and great flexibility in application.
The cantilever design has enjoyed great success in hydraulic lifts and has made a major contribution to the wide acceptance of hydraulic elevators, with their low costs and ease of installation.
„Roomless“ lifts arrived on the scene with the intention of displacing hydraulic versions in particular. The reasons are to be found in environment compatibility, of course. Disposing of hydraulic fluid, its leaking into the ground, odors and the like are good reasons, as are energy consumption and operating economies. And so wouldn‘t it make sense to take an important engineering concept like the cantilever suspension for the car (or suspension on one side only) and just do away with the hydraulic fluid?
A further advantage is that an area is freed up in the hoistway, above the counterweight. This area is relatively wide and can accommodate the drive unit.
Cost pressures and shrinking hoistway dimensions, however, demand further steps toward size reduction and toward improved engineering integration of the drive into the system as a whole. There are two points of contact here:
1. Installing the motor inside the hoistway
2. Ropes between the motor and car or counterweight
A number of companies have attempted to patent concepts for installing the motor in the hoistway. And it nonetheless makes sense, and is possible, to select a design in which the motor rests on the car‘s guide rails and no particular construction work affecting the hoistway is necessary to attach the motor.
The ropes offer considerable potential for innovation.
New rope designs
A number of articles appearing recently in the press – and in Lift-Report – have dealt with ropes designed to handle smaller bending radii. Drs. Vogel and Scheunemann in particular have contributed to this discussion.
These ropes are the logical next step toward translating the long-term improvements in rope quality and service life into a significant advantage for users. What suspension means are to be used in any given lift might well be of little interest to the drive manufacturer, but there is nonetheless great potential here for further improving drives and matching them more closely to the lift as a whole.
One thing is quite apparent: The smaller the rope‘s permissible bending radius, the smaller the drive sheave can be. Applying a classical law of mechanics, “torque = force x lever arm“, a smaller drive sheave means that the motor will have to generate less torque. Drive output remains the same, since the rotation speed increases. But the torque is directly proportional to the volume of the active motor material. Thus the smaller drive sheave is a fundamental requirement for achieving smaller gearless drives in elevator applications.
We have already seen – in the concepts using belts or synthetic fiber ropes – the effects that the small drive sheave can have on the drive
These new suspension means involve a number of disadvantages, as well. The poor or limited availability to all elevator manufacturers is just one of them. More serious is that really everything about these suspension means is new: sockets, drive sheave grooves, the nature of rope guidance and in particular recognizing readiness for replacement.
That is why we are focusing our interest on the time-tested wire rope.
The cumulative experience derived through collaboration in approval testing for several wire ropes for small bending radii was very helpful. It is always necessary here to examine separately the way in which the replacement point is determined. Only a rope with unequivocal retirement criteria may be put into service. In the case of wire ropes which are in some way embedded in plastics, polyurethane or the like, there is hardly any wear at the outer strands and thus other ways must be found to identify wire breaks. This is time-consuming, requires additional testing equipment or in some cases is not possible at all, so that the suspension means will simply have to be replaced after a certain predetermined period of time.
The situation is much different for conventional wire ropes of a special design to handle small bending radii. The need for replacement can be recognized externally, without any additional aids. The ropes can even be calculated and selected using the normal and well-known rules for service life and traction given in EN 81-1.
After initial progress had been achieved, using an 8 mm rope and reaching a sheave diameter of 270 mm, the latest step in development has reached 240 mm using 6 or 6.5 mm ropes. Employing double wrapping makes possible a 210 mm drive sheave. Double wrapping cannot really make good sense in this application, however, since the loading on the motor shaft is doubled and an additional loop sheave requires considerably more space right at the drive.
Consequently the lift will preferably be built using the 6 or 6.5 mm rope and 2:1 reeving. Engineers should also consistently make use of this option for installing the smallest machinery configuration in the shaft. It would also be conceivable to once again suspend the car at its center rather than using the cantilever concept.
The drive motor
The new ZETATOP SM200B series of motors was introduced at the interlift 2005. The most important feature of these motors is consistent adaptation to the new rope and elevator concept.
It is not necessarily true that a smaller drive sheave will simplify everything for the motor maker. Using the smaller sheave reduces the amount of torque required and at the same time increases the speed. The loading on the shaft, i.e. the sum of all the loads acting on the drive sheaves, remains the same, however. In order for the bearings to absorb these forces ideally, one naturally attempts to position the bearing directly below the center of the drive sheave. But this is no longer possible at these small diameters.
In spite of this, the observer will recognize the familiar ZETATOP design. The drive sheave and brakes are located on opposite ends of the motor casing for ease of maintenance. The absolute rotary encoder is integrated into the brakes but is nonetheless easily accessible. The brakes are dual-circuit disk brakes with type examination as a safety component intended to prevent uncontrolled car movement. In the interest of further reducing the space required by the drive, the classical junction box has been eliminated. The shielded motor cable, in its standard 5 m length, exits the housing at the base through a threaded grommet and, with a favorable configuration, can be connected directly to the adjacent frequency inverter, such as the model ZETADYN 2SY.
The slender SM200.20B motor with width of less than 325 mm and overall length of less than 500 mm will also fit in hoistways with small dimensions where the payload is 630 or 675 kg, using 2:1 reeving. This is also true for the larger SM200.30B model, designed to handle 1000 kg. The SM200.30B is just 325 mm wide, total length less than 570 mm.
The design for the active parts of the motor is similar to that in the larger ZETATOP SM225 machines. These are permanent- magnet synchronous motors which, with the selection of the proper number of poles and magnet circuit, are optimized for a particularly comfortable and smooth ride. On the one hand the motor is able to execute even the slowest of speeds both precisely and uniformly while, on the other hand, the higher speed range further boosts the good efficiency.
The casing is able to accept rope tension in either the upward or downward direction. This opens many installation variations for the user.
Application range
Travel speeds of up to 1.6 m/s at maximum 30 m ascent height are standard for the new series.
It does not make sense to use drives and ropes such as this, optimized in this way for minimum installation space, in high-performance elevators. The objective of this concept continues to be to replace the classical hydraulic lift. Anyone who in the past had recommended a hydraulic elevator for customers can now offer genuine advantages at similar costs.
The traction lift using this new type of wire rope and a gearless drive such as the ZETATOP SM200B is persuasive, thanks to its ride comfort, speed, environment compatibility and economy.
Bibliography
[1] Scheunemann, Dr. W. “Development and employment of ropes for drives sheaves with D/d<40“, Lift-Report 1/2006, pp. 40-43.
[2] Vogel, Dr. W. “Suspension means for traction elevators“, Lift-Report 5/2003, pp. 19-28.
3/2006
