Hello
Thanks for the email. I appreciate your position but I stand by my statement that VFD’s are a scam. VFD’s can only save energy on the rare occasion when you can decrease the pressure required. Anytime you reduce the speed of a pump to decrease flow while maintaining a set pressure, a VFD increases energy consumption per gallon produced.
A short example would be a 1 HP pump that can produce 10 GPM at BEP. That is 10 GPM per horsepower. Reducing the speed of this pump until it is only producing 1 GPM at the same pressure, that pump will still be pulling ½ of one horsepower. That is now only 2 GPM per horsepower, which is a 500% increase in energy consumption for varying the pump speed.
There are many more efficient ways of pumping water than using a VFD. Your example of a pump sized for a hundred year flood plus a safety factor is only for sewage or dewatering pumps. These pumps do not need to maintain a certain pressure. They only maintain a level in the sump, and just need to deliver enough pressure to pump water over an embankment. Although using multiple size pumps would still be more efficient, a VFD can vary the speed considerably because it is maintaining a level in the sump, not maintaining a set discharge pressure.
With a large pump for distribution of potable water, maintaining a constant discharge pressure greatly limits any reduction is speed, because head is lost by the square of the RPM. Again multiple pumps of different sizes staggered on can closely match the flow rate required more efficiently that reducing the speed of a single large pump. But even with a single large pump, a water tower or large hydro-pneumatic tank can keep the pump functioning at BEP all the time, while flow rates that vary from a little to a lot, are provided by the pressure tank. A VFD always uses more energy when compared to running a pump(s) at BEP. Everybody just sees the amps drop when reducing the RPM, and thinks the VFD is reducing energy consumption, even though the energy used per gallon increases dramatically.
Many people are also quick to say that “pinching the flow with a valve” “waste power and causes undue stress on the pump”. In reality the effect of “pinching with a valve” is counter intuitive. “Pinching with a valve” reduces the amp draw almost exactly the same as varying the speed. See the curves in the link provided by Grundfos pumps.
http://www.cyclestopvalves.com/pdf/pumpman_3.pdfIn the first of these curves it shows the VFD increasing the energy used per gallon by 1000%, and in the second curve shows a 400% increase in energy used. Both methods show an increase in energy used per gallon, but there is only a 12% to 15% difference in horsepower between varying the speed with a VFD and “pinching the flow with a valve”.
Also there is a lot more “undue stress” put on the pump/motor when using a VFD than when “pinching with a valve”.
With a VFD, harmonic content of the power increases the heat in the motor. Voltage spikes cause partial discharges that destroy the windings. Bearing currents greatly shorten the life of the motor. Destructive resonance vibrations happen at different critical frequencies along with many other problems.
When simply “pinching with a valve” none of these problems exists. The motor runs on smooth sinusoidal power, without harmonics, voltage spikes, or bearing currents. The pump is always at a constant speed, and never sees critical frequencies. The CSV always maintains enough flow for adequate cooling. About the only problem restricting with a valve can cause is radial deflection of the shaft and impeller. However, in over 20 years and hundreds of thousands of systems working, we have never seen this problem occur. Turbines and subs with bearings on both sides of the impeller are not susceptible to radial deflection. And end suction centrifugals with adequately size shafts and bearings don’t have a problem either.
The only real problem is that Cycle Stop Valves are a disruptive product. They make pumps last longer and use smaller pressure tanks. This greatly cuts the profits to pump and motor manufacturers, which is “disruptive” to the industry and is the real reason they say, “pinching with a valve waste energy and puts undue stress on the pump”.
Now even the pool pump manufacturers are getting into the VFD craze, which is ridiculous. Pool pumps need to produce a certain flow and pressure for the skimmers, filters, and vacuum to function properly. Reducing the pump speed even slightly prevents the pump from being able to provide these functions. The so called energy saving calculators show tremendous energy savings for running the pump 24/7, and reducing the pump speed about 75% of the time. However, at reduced speed the pump is doing nothing but wasting energy. A simple timer would have shut a standard pump off, which is the least energy that can be used. But the “energy savings calculator” won’t give you the option of turning the pump off, so it looks like a VFD is saving thousands of dollars in energy. That way you won’t mind paying $1600 for a variable speed pump instead of $300 for a standard pump. And you won’t mind replacing that variable speed pump every couple of years instead of having a standard pump that would last 20 years. That is until you check the electric bill and realize the VFD pump is actually increasing the energy consumption instead of saving energy.
Saying that VFD’s are a scam maybe harsh, but it is accurate. It is all about the Benjamin’s for the VFD, pump, and motor manufacturers. They will skew the truth and let you continue to believe misinformation in order to increase their cash flow. Your only defense is to educate yourself on the subject, and not just believe what the big manufacturers are telling you.
Thanks
Cary
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