The town of Alfeld an der Leine lies 60 km to the south of Hanover. It is where Amman Ima GmbH produces its asphalt mixing plants. Demand for this road surfacing material has risen continuously over the past few years, which means ever more ingenious manufacturing techniques are called for. In 1997, the Alfeld-based company set up a completely computer-controlled state-of-the-art plant for the Teerbau company in Münchehof, a town in the Harz region of Germany.
The stationary mixing plant at Münchehof is designed to mix 160 t/h. It produces rolled asphalt and bituminous mastic concrete from bitumen and mineral additives. Liquid bitumen is obtained directly from the refinery and stored there in heated storage tanks. The minerals, namely gravel, chippings and sand, are stored in bunkers and mixed in an electronically controlled metering plant in accordance with a recipe. Next, the mixture is fed into a drying drum where it is heated to 150 - 180 °C, thereby drying the mixture and removing its dust. The mixture is then sieved prior to being stored in hot mineral silos. During the mixing process, the mineral is weighed in accordance with the recipe and fed into the mixer. At this point, the precise quantity of bitumen is sprayed in as a fine mist and mixed together with the minerals - the asphalt is now ready.
Heating and drying
the minerals
Reliability is the Order of the Day
It is necessary to process a large number of measurement and control parameters in order to control and monitor asphalt production. There is a trend away from centralized data processing in the switch cabinet towards decentralized data processing in distribution boxes installed close to the source of the signals. For example, all measurement data recorded at Münchehof are conditioned by modular interface components before being transmitted to the central computer via INTERBUS.
Primary switched-mode compact power supply units CM 125 and MCR-PS-650 are used for providing the 24 V DC supply to the active subassemblies; after all, a highly advanced electronic control system can only function reliably for as long as it has a guaranteed supply of electrical power.
CM 125 compact power supply
units carry out their function
reliably in spite of the
harshest conditions
The Harshest Operating Conditions
It goes without saying that only tried-and-tested technology finds a place in Münchehof: extreme ambient conditions, dust, shocks and electromagnetic interference from motors starting up and switching operations place exacting demands on the electrotechnical components. Heavy dust levels require the subassemblies to be installed in distribution boxes offering protection class IP 65.
Temperatures in excess of 50°C are part and parcel of everyday service under conditions of direct sunlight. Primary switched-mode power supply units are just the thing here. In comparison to conventional power supply units, these units are lighter, smaller and considerably more frugal with energy. The units do not demonstrate any derating behavior at temperatures up to +60°C whilst their efficiency of approx. 90 % avoids any further heating of the distribution boxes.
In order to extend the plant serviceability, Ammann Ima decided to employ two compact power supply units at each distributor: one for the peripheral components and one for INTERBUS. 
Dipl.-Ing. Hartmut Henkel