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Variable Speed Pumps Can Make a Fool Out of You! Variable Frequency Drives (VFD), Adjustable Speed Drives (ASD), Variable Speed Drives (VSD), are just a few of the names used to describe equipment that varies the speed of the pump to regulate the flow rate. Irrigation and pump equipment magazines are full of articles promoting the use of VFD's to save energy. Getting advice from someone who sells Variable Speed equipment can be letting the "fox guard the hen house". VFD salespeople are quick to point out that the affinity law reduces horsepower by the cube of the pump speed. What they fail to mention is that you will lose head by the square of the speed and this limits the minimum RPM of a pump when supplying a constant pressure. A 10% reduction in speed (3600 RPM to 3240 RPM) gives a 19% reduction in head (100 PSI to 81 PSI). If 90 PSI is our design point then this pump could only be slowed by about 5%. This is typical of variable speed systems and only reduces the horse power by 15%. (example 10 HP to 8.5 HP). Comparatively, horse power drops off the same or better when simply throttling the flow from a full speed pump. Most people do not understand this natural property of a centrifugal impeller pump. "Counter intuitive" is the word used by engineers who understand the basic principle of working a full speed pump left on it's curve. Throttling a pump back reduces the horse power which is opposite of what we think should happen. In other words choking a pump back makes it work easier not harder. This is true in any pumping application where a "constant pressure" is required. Regardless of the pressure needed being 15 PSI for a drip system, 50 PSI for a municipal system, 100 PSI for a golf course, or 350 PSI for a high rise building, the RPM of a correctly sized pump can be slowed very little and still produce the required pressure. Personally, the most I have ever been able to slow the RPM of a pump is 10% and still produce the pressure required. That is 3600 RPM to 3240 RPM or 60 hertz down to only 54 hertz. Any slower and it WILL NOT produce the constant pressure required. Anyone who tells you that a pump can be slowed more than about 10% does not understand that head drops off by the square of the speed. Any pump that can be slowed by more than 10% has been excessively oversized. Over sizing or derating of the pump motor is very common and usually mandatory when using VFD's. Over sizing a pump does not help save energy! If all of this is confusing to you, you are not alone. A VFD is a microprocessor based pulse width modulator with software and other components that makes it similar too and every bit as complicated as any computer. Adding the functions of a VFD and a pump together gives ample opportunity for misunderstanding. Many articles falsely give VFD the credit for saving energy. If leaks were repaired or a smaller pump was added to the system, it's not the VFD they threw in that is saving energy. How can you find an application expert who understands all of this? Find someone who understands that when pumping water at a constant pressure a VFD cannot save energy. Anyone who knows this will be able to give you honest answers to your pump control questions. With all of the articles on how VFD's supposedly save energy it is obvious that while many engineers think they understand VFD's, they do not understand how a VFD works with the natural characteristics of a centrifugal pump. Those who really understand pump systems embrace the idea of valve control over VFD's. Besides the high purchase cost and technical nature, VFD's usually add other complications to a pump control system. Transient voltage spikes, harmonics, vibration, carrier frequency complications, electrical fluting of ball bearings, and ambient temperature concerns are just a few of the drawbacks associated with a VFD. An expert on pumps who knows about these problems would use a VFD only on systems like escalators and conveyor belts and not when pumping cool water at a constant pressure. A more dependable solution to pump control would include a mechanical valve that throttles the pump discharge to match the usage. This type control can eliminate the problems associated with variable speed pumps. A common misconception is that energy is wasted across a valve. Some even go so far as to say "it's like driving a car with one foot on the brake and one on the gas". This is completely incorrect. If you do not understand that horse power naturally decreases as pressure in the pump case increases, then a VFD can make a fool out of you. |
| Cycle Stop Valves® is a registered trademark. All right reserved unless prior authorization is obtained. Cycle Stop Valves are patented: Patent number 5,988,984 and other patents pending. |
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