Installing a variable speed drive on a pump motor is all about energy efficiency and pump longevity. These are good goals to have, but that doesn’t mean that a VFD will give you the payback you’re looking for in every application.
How do you determine if you need a variable speed drive?
There are two main questions you must ask yourself:
- Can I achieve a constant flow or pressure with a VFD?
- If so, how will a VFD affect pump efficiency and performance?
The first question is quite simple, really. If your pump is turning on often enough to achieve constant flow or pressure, then you should take a look at a VFD and the return-on-investment.
However, many pumps don’t experience the flow rates to justify the investment into a variable frequency drive. For example, residential lift stations move wastewater down the line, but due to the small quantity, a VFD is unlikely. However, as more sewer lines converge and lift stations get bigger, variable speed drives are much more common.
Air compressors and other pressure regulating pumps follow the same principle. In factories that rely heavily on air compressors, it’s typical for a fifth of the electricity bill to be from the compression pumps alone. In these cases, the variable frequency drive discussion is valid. However, most mechanic shops and construction professionals would hardly imagine investing in a VFD.
Just like any pump, it’s also important to understand efficiency. Answering the second question isn’t exactly hard, if you know what you’re doing. You really need to understand if achieving a constant pump speed – which is what a VFD will do – is going to improve pump efficiency enough to pay for the big investment. You also need to understand the Affinity Laws, and what operating a pump at a given speed will do to pump performance.
For example, if you change the speed of the pump, then the flow is also affected, along with head and power. Too much or too little speed can cause the pump to underperform or break apart. You may also find that running your pump at best efficiency point, which you should do, may not produce the efficiency gains you had envisioned.
On the other hand, simply having the ability to throttle your pumps may well be worth the investment just to control output. As always, everything depends on the application goals.
We’ll get into pump education a little later to discuss things like BEP and the Affinity Laws. For now we’ll warn you that before you get serious about a VFD for your pump, you really need to talk with an expert to understand if the investment will pay off, and how it will affect your pump. Look for more information in the coming weeks.
Let us know if you have any questions about pump control. We’re happy to help anyway we can!