How to choose a medium voltage VFD:
Design robustness
Every end user probably wishes that his VFD has a robust design and works as intended during the entire lifetime of at least 20 years. Unplanned shutdowns and outages are unwanted. But what is robust design and how can it be achieved?
There is no simple definition, but it is a set of design rules and design margins, past experience and continuous product improvements and some extras to achieve superior robustness. Robust design is manufacturer’s philosophy. You will not find the information in the technical datasheet. The internal design margins are typically manufacturer’s secret. Even if some would publish them most users could not start anything with it. The loadability of semiconductor is not just a value of current. It depends on switching frequency, temperature of cooling media etc. And it is not just about semiconductors (although these are of course the heart of VFD). In fact, there is plenty of other components inside the VFD. If you own a VFD simply check the part list and see how many components are inside. Is the cooling redundant or not? What happens if the cooling does not work? Will the VFD operate until some components “are cooked” or will the issue be recognized fast enough? Is the VFD self-protective? Is it capable to detect all major faults? VFD with robust design shall protect itself in every situation and turn into safe mode without any damages.
Example:
Liquid cooled drives with weak cooling design would suffer from leakages and consecutive trips. Such issue is not related to the VFD topology, semiconductor type, redundancies or so. Instead, it is a matter of sense for detail and proper design of piping joints.
Design margins in hardware components are not all. How does the VFD react to external disturbances? If the drive trips every time there is a voltage dip in the supply grid the user will not be very happy. Even if no components get damaged, the VFD is simply not robust enough to deal with such type of disturbance.
So how can you know if the particular VFD has a robust design? You can get a feeling when visiting the factory and seeing the design from inside. Do not check the main components only, but focus also on all the small parts around. Ask also about processes to ensure production quality. Do not miss the test procedures. How are the routine tests done? With rated conditions or with some higher values? What is the manufacturer’s history and product history? How much of the design and manufacturing is done in house and how much is outsourced? When was the product launched, how many units have been delivered so far and what is the feedback from users? For how long is the manufacturer in the business and how successful were the previous VFD models?
Such information will help you to make your own picture rather than relying just on marketing material and some figures that cannot be practically verified. Remember the quote of Sir Winston Churchill “the only statistics you can trust are the ones you have falsified yourself”. On the other hand questioning everything is also not the way forward.
Summary
Robust design is extremely important, but at the same time difficult to quantify and measure. It is a unique blend of hardware design, software features, well-proven protection concept, testing procedures and also manufacturer’s experience and quality processes.
