Wood chips Romanian style
05 June 2017
The third sawmill of the Austrian Schweighofer Group was commissioned in Reci (Romania). The key component of the fuel processing line there is a drum chipper made by BRUKS Klöckner (“BRUKS”).
What makes this chipper special is its custom drive system, consisting of a pair of smart low-voltage motors from WEG.
BRUKS makes drum chippers with rotor diameters up to 2.8m and up to 15 rows of knives, to meet a wide variety of requirements in the sawmill, cellulose and paper industries, chipboard plants, biomass power plants and pellet plants, as well as custom models for a wide variety of special applications.
BRUKS supplied the complete fuel processing line for the biomass power plant attached to the Schweighofer sawmill in Reci. The key component of the system is a type DH 850 x 1200 drum chipper with a total of 15 infeed rolls and drive power of 710kW. The chipper has a knife cutting circle diameter of 2m with two knife rows and produces 60,000kg of wood chips per hour, with a chip length of 65mm.
From logs to wood chips
The fuel processing line consists of several seamlessly meshed units. First a lateral conveyor receives the logs and passes them to a singulator, which feeds them into the chipper line. The line additionally receives short timber from a dispensing table. The logs reach the chipper over an infeed belt with integrated metal detection coil. Finally, the produced wood chips are picked up and carried away by an extraction and distribution claw conveyor.
Among other things, the chipper line is distinguished by its cleanliness: any dirt falling below the chipper belt or the lateral conveyor and singulator is caught by belts and transported to the material storage area. From there it is fed to thermal recycling.
The 10,000kg chipper rotor is driven by a WEG low-voltage motor with an output power of 710kW. The six-pole squirrel cage motor with frame size 450 J/H produces a nominal torque of 6,825Nm. It achieves an efficiency of 96.3 percent under full load. The rotor is made from copper bars, and the motor can be operated at ambient temperatures from -30 to +40°C.
A V-belt drive transfers power from the main motor to the drum chipper. The belt pulley is not mounted on the motor shaft, but instead in bearing blocks on each side, because the radial forces with this level of drive power are much too high for the motor shaft and motor bearings. The main motor is connected to the pulley shaft by an elastic pin coupling.
A six-pole WEG motor with a rated power of 55kW, which serves as an auxiliary starter motor, is coupled to the opposite end of the belt pulley. It is ramped up by a variable-speed drive unit operating in power control mode to slowly accelerate the entire system up to the rated speed of 994 rpm. The start-up ramp time is about two minutes. The main motor is only switched on when the rated speed is reached, avoiding high start-up currents. Due to the cold winters in Romania, the auxiliary motor is also designed for ambient temperatures down to -30°C.
In the other direction, this custom drive arrangement allows the chipper to be braked electronically after it is switched off, with the braking energy fed back into the power grid. The run-down time with this drive system, which is presently unique in the chipper market, is only about two minutes instead of approximately 11 minutes without braking.
In addition, there is a small gear motor on this motor axis, which is connected to the other end of the starter motor shaft through a solenoid clutch. In normal operation it is decoupled from the drive train. It is only connected in maintenance mode, so that the chipper rotor can be slowly rotated to the best position for changing the knives. On average, the knives must be changed every two shifts. Moving the 10,000kg motor by hand is virtually impossible, and it is not allowed for safety reasons.
Benefits for mechanical engineer and plant operator
BRUKS and WEG have worked together on a wide variety of projects over the course of many years. Plant operator Schweighofer also benefits. Thanks to the sturdiness and low maintenance needs of the motors, the drum chipper is very reliable in operation and costly downtime is reduced to a minimum. Furthermore, feeding the braking energy back into the grid and starting up the drum chipper with a variable-speed drive reduce energy consumption and operating costs.
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