Here’s an example of the problem. If you study the standard EN 50598-2, a relatively new standard relating to the efficiency of VSDs and VSD systems, there is clearly flexibility allowed in the calculations/measurements and corresponding efficiency values that are published.
The point to note is that a VSD’s switching frequency can have a huge effect on the efficiency of the drive system. ABB uses 4 kHz as a default switching frequency and typically delivers its drives using this figure. However, other manufacturers choose to use 2 kHz. This lower switching frequency improves the figures that the manufacturer can publish for the VSD’s efficiency. However – and this is the bad news - the lower switching frequency has a negative impact on the overall electrical system efficiency.
This is because the standard does not consider the entire system variations and the interactions of other components such as transformers, motors and filters.
Going back to the automotive industry, most of us are aware that the way we drive affects the ‘actual’ miles per gallon that we can achieve. There is far more to consider such as fuel type, inflation level of tyres, roof bars and weight of passengers/luggage. The individual setup and use of the car clearly affects the efficiency.
And so it is with your VSD arrangement. Drives without chokes and filters seem to be more efficient, but in practice this is not the case. Furthermore, 12-pulse and 6-pulse drives seem to be more efficient than low harmonic drives because you do not have to include the transformer losses. However, this is not how it is in practice.
The message is clear: don’t believe everything that you read. Dig beneath the actual published efficiency figures for each component that is powering your pump system.
For more information, visit ABB’s dedicated system efficiency webpage
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