Francis.T.Griffith Bsc, MIET ABB Ltd, UK(free)
With the appearance of the Ecodesign Directive, attention to the achievement of system efficiencies has moved from products e.g pumps, motors etc, to systems made up from assemblies of products. An example of this is a Variable Speed Drive (VSD)/motor assembly controlling pump speed. The efficiency standard for a VSD/motor assembly is EN 50598-2, which defines the test methods used to determine the system efficiency of a VSD/motor. From information gained by using the test methods in EN 50598-2 the concept of an “Efficiency Island” arose. The “Efficiency Island” concept is shown to be a general principle for three phase motors and assists in the selection of a pump design; with the associated VSD/motor, such that the overall package (system) efficiency is the best achievable.
Efficiency Island Efficiency Contour Energy Efficiency Index EN 50598-2 Extended Product Approach Variable Speed Drive.
The Extended Product Approach of the ECODESIGN Directive considers driven equipment e.g pumps, and associated VSD/motor, as a combined package – ignoring individual component efficiencies, only looking at the overall efficiency, or, EEI (Energy Efficiency Index).
EN 50598-2 is the standard for assessing the VSD/motor package efficiency. One of the test methods defined in EN 50598-2, is the Input-Output method, where a VSD/motor system is tested on a dynamometer rig and real shaft torque/speed (output power) is compared with real electrical power (input power).
In the past, the majority of pumps, driven by a VSD/motor system, were confined to a speed range below synchronous speed – motor design speed defined by the mains supply frequency. The Input-Output method allows the full speed range, i.e sub-synchronous to super-synchronous, to be assessed and it is from the measurements obtained from this dynamometer testing, that efficiency contours (lines of equal efficiency) can be measured and displayed.
These contours always show an “Efficiency Island” with the best possible value, at, or above nominal speed.