Overdimensioning of VSDS - What are the consequences?
How to dimension a variable speed drive system (VSDS) properly? Underdimensioning is a No Go but what about overdimensioning? Besides the initial investment cost, how does overdimensioning affect the technical side of the system? This post answers the most relevant questions related to overdimensioning of a drive system.
Issues linked to an overdimensioning of a drive system
Underdimensioning the system results in operating the equipment at higher loads than it was designed for. The consequences are higher stress possible thermal overload, accelerated aging or faster material wear, reduced reliability and in worst case a premature failure. Thus, underdimensioning shall always be avoided. ¹
¹ Specific short time overloading may be used e.g. for soft starter applicaitons. In this case it is time limited and done on purpose. It is not an underdimensioning as such.
But what about other extreme – an overdimesnioning of the equipment? Is it always on the safe side? Or can it have any drawbacks? How is the VFD performance affected? What about transformer and motor? What are the consequences for lifetime, losses or protection? Our article discusses many different aspects of the overdimensioning. Some facts might be clear, others may be a bit surprising.
Overdimensioning of a drive system may have its negative side as well
A common practice in power rating of a drive system is to take the maximum load demand, include some margin and determine the rated power of the motor on such base. Then the VFD is often specified to have a capability of 105%/110% or 120% of the motor rating. We talk about margin stacking…
A certain design margin is a good approach to increase robustness of the system and lifetime of the equipment. So the question is: what is a reasonable margin and when it starts to hurt? Is there actually an issue with overdimensioning of the system, apart from the higher investment cost for the equipment?
In fact, excessive overdimensioning has its dark side from technical point of view as well. Some points are to be expected while others are less obvious. Become the member of our premium subscribers and enhance your knowledge about electric drive systems. The best investment is the investment into your further education.
The full content is available for premium subscribers. Purchase our Premium Subscription and learn more about this subject and many other drive related topics!
Consider also publications to specific topics listed HERE.
3 Comments
Brian Beebee · July 16, 2022 at 6:40 pm
I don’t know that I agree about the under dimensioning argument. We under dimensioned a drive for a motor that is always lightly loaded (motor was oversized by over 200%). When we retrofit new drives, we installed a 350HP VFD for a 700HP motor.
admin · July 16, 2022 at 11:19 pm
Thanks a lot Brian! I appreciate your comment and totally agree with you. The term ‘underdimensioning’ in our blog post refers to too small sizing which is (mostly) not done intentionally but rather as an error. In your case you do it on purpose as you have the operational experience that the motor is oversized with relation to the real load demand.
(In your case one could argue whether it is underdimensioning or not… compared to the motor rating yes, compared to load demand not)
Brian Beebee · July 17, 2022 at 12:45 am
Copy.
Comments are closed.