Not just speed control [variable speed AC drives]

Today, the use of variable speed A.C. drives in the cement industry is well established. The development of power semiconductors, microprocessors and memory circuits, together with the latest motor speed and torque control methods like the DTC (Direct Torque Control), enables creating more functionality inside variable speed drives. Advances in technological development have resulted in sophisticated frequency converter supply units with active front ends. Thus, the drive not only controls the speed but also carries out other tasks, unlike in the case of supply units with passive front ends. The cement process includes applications - like a kiln drive - in which the motors and frequency converters have to be dimensioned for high overloads. In these applications, it is ideal to use the frequency converter for other tasks than just for speed control. Also applications where regenerative braking is required, e.g. downhill conveyors, benefit from supply units with active front ends. This paper describes the possibility to use variable speed a.c. drives for conventional speed control, simultaneously using the remaining load capacity of the frequency converter for harmonic current reduction, power factor correction plus for other tasks. Consequently, substantial savings in installation costs and energy consumption are achieved thanks to lower harmonics level and a better power factor control. Further benefits given by an active front end are also discussed.