Issue 2/2008
03/02/08
Efficiency gains in lift controls
Elevator drives
Jürg Wellstein1)
Conventional elevator control concepts dissipate braking energy unused and consume great amounts of electricity when in the standby mode. Both these drawbacks can be eliminated by changing to current inverters – and that means greater energy efficiency. A pilot plant was used to demonstrate this.
Category: Issue 2/2008
Posted by: Editor
Elevators are an integral component in multi-storey buildings. Whether used for passenger or freight conveyance, whether specially designed for installation on the façade or quite conventionally inside the building, elevators have today become a simple matter of course. Given the consistent employment of measures to reduce energy consumption in new construction and building rehabilitation, elevator technology has also come under scrutiny. Here, too, improvements in energy efficiency are the goals if power consumption in buildings is to be reduced. The happy fact is that the drive technologies used both in elevators and in industrial conveyor systems harbor high savings potentials without having to forego comfort or convenience.
Many elevators, taken together, add up to significant energy losses
The “Neptune” development project carried out by three companies in Bern – Telma AG2), Kanyo Electronic3) and Mefortis AG4) – with the support of the “Electricity” research program sponsored by the Swiss Federal Energy Office (BFE) and by the innoBE Society5) use two elevator market facts as the starting point.
About 6,000 new elevator drives are installed new in Switzerland each year; in the European Union that number comes to about 50,000. More than two-thirds of them incorporate 3-phase motors with frequency inverter control and velocity regulation. If one also takes into account elevator replacements in Switzerland and the EU, then one arrives at a total volume of more than 170,000 drive units. This represents a thoroughly promising foundation for a business plan.
Standby losses resulting from 24-hour operations
The second fact is the operating concept for such elevators. They have to be accelerated and stopped repeatedly. The braking energy required here accounts for between 30 and 40 % of overall energy consumption. This energy is converted into heat in the braking resistors and then dissipated to the atmosphere as prescribed by regulations. Moreover, elevators are normally kept in a state of operational readiness around the clock and standby power consumption can come to as much as 75 % of overall consumption. If both effects could be prevented by technological means, then considerable efficiency enhancements could be realized in buildings and in elevator technology.
Mario Bolla, Managing Director at Telma AG and project manager, explained these prospects for savings: “The basis for our development idea is to be found in the calculations made by Jürg Nipkow, S.A.F.E., in the course of a BFE project. He calculated a recuperation value of 30 % and assumes that the frequency inverter accounts for one-quarter of standby consumption. Within the research project, our company is responsible for industrialization, manufacturing and marketing the corresponding product in Switzerland.”
New technologies enable current inverter control
This analysis represents a challenge for experts in the field of electronics in that the focus is now on elevator controls. One fundamental finding is: A current inverter is to be used instead of the voltage inverter normally employed. This makes it possible to return power to the grid during braking and to draw far less energy when in the standby mode. The conventional solution for two-quadrant line rectifiers is a thyristor bridge. This requires large inductance components (line choke, DC choke), however. It is possible to make these components smaller with appropriate circuitry. Since no capacitors need to be charged, the inverter can be disconnected from the line supply when the lift is on standby; thus no energy flows.
The person who initiated this idea, Dr. Pál Kanyó, noted: “There are essentially two known types of inverter that are suitable for returning power to the AC grid and that store no energy when in a powerdown state: matrix inverters and current inverters. With our patented commutation process we have further refined the current inverter so that it achieves the goals named here.” Four-quadrant operation now makes return to the grid possible and the braking resistors can be eliminated. Moreover, synchronous motors can be operated like stepping motors; the car can be moved to the desired position without need for a pulse transducer. The current inverter technology is not fundamentally new and has been used successfully in other applications (such as railroads). Advances in semiconductors have now made it possible to develop a solution for elevator drives.
Development for a large market
Deemed to be a further prerequisite was to create a concept that can be implemented at industrial scale and at the same time is no more expensive than conventional voltage inverter technology. A prototype has been built in the meantime. It was first tested in the laboratory and then in a pilot plant which the Mefortis AG, involved in the project as one of the partners, intends to use. That company will also handle marketing in the EU. Alfred Meier of Mefortis AG: “We can draw upon more than 30 years of experience in elevatoring and have further refined the drive technology integrated into elevator construction in particular.”
Development work for the controls was initially limited to a power range of from 5 to 7.5 kW in this new project. These controls would be used with conventional elevator models carrying fi ve to ten passengers and thus would cover about two-thirds of all lifts now in place. The objective, however, is to later be able to serve the entire performance range. In addition to use in the various lift models, employment for conveyors and at construction cranes is also being considered. Given the loads to be handled there, such applications represent far greater potentials for energy recuperation. This is another important approach in the erection of energy-efficient buildings. A new company, Econodrives GmbH, was founded in March of 2006 to handle activities subsequent to the research project.
The project team is expecting, based on the system design and initial testing, to achieve energy reductions in the area of 30 to 40% when compared with conventional systems. The percentage values are far higher in the standby mode since the new principle exerts only a minimal energy load when the elevator is stopped. Taking Switzerland as a whole, it would be possible to reduce energy consumption by more than 75 GWh and that represents savings of more than 11 million Swiss francs (or about 10.75 million dollars) for energy costs.
1) Trade journalist SFJ
2) Telma AG, 3662 Seftigen, Switzerland, Mario Bolla
3) Kanyo Electronic, 3127 Mühlethurnen, Switzerland, Dr. Pál Kanyó
4) Mefortis AG, 3762 Erlenbach, Switzerland, Alfred Meier
5) „Electricity“ research program: www.electricityresearch. ch, Sector Manager Felix Frey, Program Manager Roland Brüniger
2/2008
