Variable frequency drives (VFDs) are used to control the speed of fans, mills, conveyors and kilns in the cement industry. VFDs are also used to smoothly start large mill motors, synchronize and connect them across the line.
Four typical applications are:
- Induced draft fans.
- Cement kiln rotation.
- Crushers and roller mill drives.
- Slip power recovery drives.
The ID fan induces kiln airflow, which must be continuously varied to match the process requirements. Because cement production is a thermal and chemical process, both air volume and mass flow must be controlled. The process control system continuously monitors conditions such as inlet air temperature, kiln feed, cement composition and required fuel/air ratio. The system then directs the blower and flow control system to provide the optimum airflow.
Advantages of the drive system are:
Very High Reliability – The Dura-Bilt5i MV uses 3,300 Volt Insulated Gate Bipolar Transistors (IGBT) allowing a simpler, more reliable inverter design. Since mechanical flow devices are not used, process interruptions caused by mechanical failures are minimized.
Energy Savings – Elimination of the airflow losses through the dampers is usually the most compelling reason for applying a Dura-Bilt5i MV drive. The ID fan power can be several thousand horsepower and using a drive to vary airflow can result in energy savings of more than $100,000 per year, according to TMEIC.
Power System Friendly – The converter is a 24-pulse diode rectifier with a design exceeding the requirements of the IEEE 519-1992 standard for Total Harmonic Distortion (THD). This means that other equipment connected to the power system is not adversely affected by harmonic frequency disturbances.
Heat Pipe Cooling – The IGBTs in the three inverter legs are cooled with heat pipe technology, which maintains uniform working temperature, prolongs the semiconductor life, reduces fan noise and saves valuable floor space in the plant.