VFD Saves Energy, Just Another "Green Lie"
A "Green Lie" is when companies, governments, or individuals use false information, or are "economical with the truth", to try and persuade the public into purchasing something they claim is good for the environment, when it really is not.
Countless articles preach that Variable Frequency Drives or VFD's will save energy and pay for themselves in just a few months or years. This is a "green lie". Just because the amps go down when the RPM of a pump is decreased, does not mean energy is being saved. In fact the opposite is true. A VFD causes more energy to be used per gallon when running at maximum RPM, and when the RPM is reduced.
A good example would be a 10 HP pump that can produce 100 GPM at 100 PSI. This would be 10 GPM per horse power without a VFD. At full speed and full flow, the energy used by a VFD itself, along with the VFD's sub standard voltage wave form, would cause an extra 5% in energy use. This means that the pump would only be producing 9.52 GPM per horse power, compared to 10 GPM per horse power for a pump without a VFD.
The Affinity Law states, "horse power is reduced by the cube of the speed." Many people use this to try and show energy is being saved, by reducing the RPM. This is another "green lie". The Affinity Law also states "head is reduced by the square of the speed". So reducing the speed of a pump by 50%, would also reduce the head produced by 75%. At this speed the pump would no longer be able to even lift water to the top of the well. Because the pump can no longer produce the head required, decreasing the horse power to 12.5% is not possible.
A maximum reduction in speed of 15% would be a more possible scenario. This would still be a reduction in head of 28%, but might be able to lift water to the top of the well and still produce the pressure required. Reducing the speed by 15% would reduce the horse power by 39%. If the example pump could be slowed by 15% and still deliver the 100 PSI required, it would be producing 10 GPM, and drawing 6.1 horse power. This would only be 1.66 GPM per horse power. That is 6 times or 600% more energy used per gallon when slowed with a VFD. Saving energy means reducing the energy cost per gallon of water pumped, not reducing the cost per hour to run the motor turning the pump. High flow or low, a VFD always causes more energy to be used per gallon produced.
It's another "green lie" to say that a VFD can spare the motor damage from full voltage high torque start ups. Surviving the voltage spikes, EDM currents, harmonics, and resonance frequency vibration of a VFD can override the benefits of any soft start.
Countless before and after "studies" have been published, "proving" that VFD's save energy. However, reading the fine print on these "studies", you can always find that a smaller pump was installed, dumping excess water was discontinued, or the pressure required was decreased. These are the real reasons for the documented energy savings. It's another "green lie" to give the newly installed VFD any credit for energy savings, because VFD's always increase energy used per gallon. These "green lies" have been used so frequently, that even people who are smart enough to know better, still cling to the propaganda that VFD's can save energy.
We all want to do our part to save energy and make the world a better place. Look past the glossy ads and the tricky web pages. Companies spend the most advertising dollars on products that are short lived, and make them the most money. Saving energy and money would be counter productive to company profits. Falling for "green lies" about VFD's can actually harm the environment, while taking a "lot of green" out of your wallet.