Press archive 2008

25 January 2008

EC external rotor motors in the dental practice

Fast, reliable amalgam separation on site.

In dental treatment, preparations used for tooth preservation and amalgam fillings are a part of everyday practice. This filling is based on a special mixture of carefully selected metals with mercury. The freshly prepared plastic mass solidifies after a short time. The mercury is securely contained and practically harmless. However, if one of these fillings has to be removed due wear after many years of use, the fine suspended particles have to be separated from the wastewater. Wastewater separators (amalgam separators) separate the heavy amalgam from the wastewater using centrifugal force. With their long service life, today's EC drives now allow compact separators to be developed for on-site disposal in the treatment instrument. This significantly reduces space requirements and maintenance costs.

Mercury is a poisonous heavy metal that must not be allowed to escape uncontrolled into the environment. On the other hand, even Paracelsus knew that the dose makes the poison. The more finely distributed a material is, the greater the surface area and thus the quantity of soluble material. In the dental practice, this means that amalgam is well tolerate in the compact filling, but the fine drill dust removed when replacing the filling has to collected. Because the density of amalgam is very different from that of water, Dürr Dental, a dental equipment specialist, uses separation by centrifuge. To reliably operate the centrifuge at high speeds, drive experts ebm-papst were already consulted in the development phase. The result was compact separators for reliable use in everyday practice.

Compact centrifuges-a particular challenge
Like all turbomachines, one thing is true of centrifuges: the smaller they are, the more difficult they are to build. Therefore, small compact devices require disproportionately greater design and manufacturing effort. The reason is the gap loss between the fixed and moving parts. In principle, the centrifuge works like a juicer that separates the juice from the pulp. The high speed of a rotor separates the higher-density solids from the lower-density fluids. Separators differ from centrifuges in that they can work in semi-continuous or fully continuous operation. This means high separation output (particularly related to the flow rate) with small dimensions.

Due to manufacturing tolerances and thermal expansion, a certain clearance must always be maintained to prevent the rotor from scraping against the housing while it turns. Attain high separation values without risking contact with the walls requires a substantial amount of knowledge and skill. In this case, the design engineers selected state-of-the-art plastics for the centrifuge's components. The materials are highly resistant to abrasive wear from the amalgam particles as well as commonly used cleaners and disinfectants. Using injection molding, it was possible to manufacture the parts in the optimum shape in terms of fluid dynamics at the lowest tolerances. This allows the shape of all parts to be optimally matched to each other. Narrow tolerances also mean minimal clearances between parts, meaning that the bearing of the drive has to be designed and built just as accurately as the separator unit.

The EC external rotor motor - drive and bearing in one
For high separation efficiency, the difference in density of the various materials and their centrifugal acceleration in the separator are critical. The higher the differences in density and the acceleration, the better the material will be separated. However, high centrifugal forces with a small rotor mean high speeds. Furthermore, despite the fact that the centrifuge is charged in "pulses", so to speak, during the flushing process, the speed must remain as constant as possible. Due to these design specifications, choosing an EC external rotor motor proved to be the ideal solution. Thanks to the large centrifugal masses on the rotor, the drive is relatively insensitive to load fluctuations. The internal stator offers ideal possibilities for a particularly low-tolerance bearing system. To meet the stringent requirements of centrifuge operation, the bearing seat for both precision ball bearings has a clamped design. The clearance between the two bearings guarantees optimal alignment of the drive shaft, which carries the rotor of the separator, while also providing a highly rigid bearing. This is a great advantage, particularly for centrifuges with potential imbalance due to the irregular supply of medium. The top position of the drive facilitates both the sealing of the drive shaft and access to the sludge trap of the centrifuge.

As an electronically commutated drive, the external rotor motor also provides the ability to adapt the motor characteristics to the various centrifuge models. Dürr Dental's Eberhardt Nonnenmacher says: "One of the reasons we chose ebm-papst is that they provide optimum support for the drive's control solution. We could depend on the skills and knowledge of their drive specialists to meet the special requirements for storage and reliability." Based on a tried-and-tested EC external rotor design, the drivers' properties were optimized for centrifuge insert. With programmable motor control, special motors are not required for each centrifuge model-the electronics permit the basic mechanics to be ideally adapted to the separator, within certain limits. The compact design and low operating noise level thus allow decentralized disposal of amalgam waste in the treatment chair without space problems or bothersome noise. Depending on the situation, various models are available. All share the uncomplicated, low-maintenance operation that significantly exceeds the legal requirements for separation efficiency (efficiency of the separator according to DIN EN ISO 11143).

Modern EC external rotor motors can be customized to the specific application. The earlier the stage at which the drive expertise of the motor specialist is integrated into development, the greater the advantages for efficiency, space requirements and cost-effectiveness. Many years of experience and customer-friendly "system support" give developers the ability to concentrate on their core competency. The time to market becomes shorter, and development and production costs are reduced, while cost-effectiveness is increased.

Advantages of EC drives
Long service life, very good speed control and dynamics at high output: electronically commutated direct current motors (also called EC or BLDC motors) fulfill these needs perfectly. The rotor bearing is the only component that is subject to mechanical wear; therefore, a service life of 40,000 hours is the rule rather than the exception. The drive lasts for the life of the unit, even when moved in continuous operation for years. Thanks to electronic control, these drives can be well regulated, and they have proven to have a high level of efficiency-typically, from 70 to even far beyond 80%. Electromagnetic compatibility plays an increasingly important role in many applications. Here, too, EC motors are the solution of choice. Thanks to integrated electronics, it is frequently possible to connect the drive directly to the device's internal bus. Furthermore, the safety functions in the engine control system, which can be linked with external data, allow the system to monitor itself. This is how localized intelligence relieves strain on the bus and all other control unit computer components. In their high-torque external rotor motor design, EC motors leave nothing to be desired, even in terms of start-up torque and synchronous running.

Saskia Tarantini
Phone: +49 7724 81-1342
Fax: +49 7724 81-51342

ebm-papst St. Georgen GmbH & Co. KG
Hermann-Papst-Straße 1
78112 St. Georgen