Typical VSD Applications There are three main types of variable speed loads: · Constant power - centre winders, machine tools · Constant torque - conveyors, agitators, crushers, some compressors  · Variable torque - fans, centrifugal pumps, some compressors Constant Power Power is constant, irrespective of speed. Torque however changes inversely with the speed. Constant Torque Irrespective of the speed, the torque remains constant. However, the power is directly proportional to the speed. Variable Torque As a general rule. Torque varies directly with the speed squared, whilst power varies directly with the speed cubed. It is the power varying with speed cubed that makes variable torque applications (fans and pumps) particularly attractive with a view to saving energy. Cost effective energy saving potential comes from pumping applications, air handling and compressor operation where load profiles / duty cycles are regularly below design specifications. The following graphs compare the energy consumed by a valve controlled pump and a VSD controlled pump. A damper controlled fan will show similar savings when controlled by a VSD. The actual savings can clearly be seen on the second graph above; the main reasons for the savings are as follows: · The throttling device is no longer necessary so these losses are removed · A fixed speed pump has only one efficiency curve, which drops steeply as the medium is throttled. In variable speed operation there is a family of efficiency curves; one curve for each discrete speed of operation. This serves to reduce the pump losses significantly. · Motor load is reduced so losses are also reduced in the motor. · "Hidden" savings will also contribute but are difficult to quantify: reduced mechanical stresses due to soft starting, elimination of water hammer, reduced hydraulic load on impellors etc. In summary, where a motor is required to operate for extended periods below full load, a VSD can provide cost effective energy savings and regulate more effectively the distribution of the medium being moved.