Several years ago, I sat through a round table discussion on Life Cycle Cost at a pump systems convention. The engineer orchestrating the discussion explained that they were trying to figure out how to make pumps last longer. In so doing, they had ask a couple of the largest pump manufacturers how to make pumps last longer. She said they were fully expecting the pump manufacturers to come back with suggestions like heavy ball bearings, stronger shafts, higher tensile strength materials, and oversized motors.
Instead they were shocked when the pump manufacturers came back saying that more elaborate controls were needed. According to the pump manufacturers, the best way to make pumps last longer was to better protect them from dry running, overloading, high or low voltage problems, current unbalance, and other such problems. In other words, pumps could be built with lower tensile strength materials, lower quality bearings, smaller footprints, lighter weight, even higher RPM's, and they would last longer if the controller would just shut them down before a failure occurred.
Since that time many so called "Smart" pump controllers have been introduced. Many of these controllers incorporate a Variable Frequency Drive (VFD or just Drive) to help sense and control parameters, as well as to reduce energy consumption. Many articles have been published proclaiming tremendous energy savings by varying the speed of the pump. Careful reading of these articles shows that even though the VFD is given full credit for the energy saved, it is usually the elimination of a dump valve, installation of a smaller pump, or lowering the pressure to the system that is responsible for energy savings, not the VFD.
As a matter of fact, anytime you reduce the speed of a centrifugal pump, there is more energy used per gallon, than when running at full RPM and BEP. A 3550 RPM, 1200 GPM, 100 HP pump uses .083 HP per gallon with a head of 231'. Maintaining the same head, this pump slowed to 3280 RPM, produces 100 GPM, and is only using 38 HP. However, that is .38 HP per gallon, which means the VFD is burning over 4.5 times more energy per gallon than when running at BEP and full speed. There is an average of only 7% difference in the energy burned by reducing the speed of the pump, as there would be across a control valve while the pump runs at full speed. When running the pump at full RPM, a VFD will cause more energy use than Across The Line controls. The VFD itself has "parasitic losses", and also runs the motor on power that is rich with harmonic content, causing a loss of motor efficiency. Therefore, in many cases, a VFD does not save energy but, actually "burns" energy.
Understanding that a VFD can actually increase energy consumption, leaves improvement of Life Cycle Cost to be determined solely by the life expectancy of the pump and equipment. On the incorrect pretense of saving energy and keeping the polar ice caps from melting, pump and motor companies can count on getting to sell you a new pump system much sooner. Getting you to purchase equipment on a regular and timely basis is the primary goal of most corporations. The fact that you must pay a premium price because it has a VFD incorporated is a bonus, as well as being what keeps the replacement cycle shorter. Life expectancy of equipment that has a long track record like magnetic starters and squirrel cage induction motors, can be predicted very closely. However, PREDICTING the life of newly designed equipment like VFD's, can lead to over exaggerations. Drive manufacturers may claim that the life expectancy for a VFD is about 17 years. In reality this has not been the case. Some who have been selling VFD's for many years are now saying that experience shows that the life of a VFD is less than 8 years. Others are saying that Drives for smaller horse power systems are not even averaging 5 years. Plugging in that Drives last 17 years makes Life Cycle Cost look inviting. When a Drive really only last an average of 5 years instead of the 17 years that was predicted, energy consumption becomes much less of a factor in the Life Cycle Cost, than the cost of replacing equipment.
Motor companies have had to change from 600 volt insulation to 2,000 volt insulation as standard, to better withstand the voltage spikes and rapid rate of voltage rise caused by Drive controls. Many are making insulated bearings or grounding the rotor, to limit electrical fluting ofball bearings caused by the naturally occurring EDM currents from VFD controls. Some even incorporate an additional squirrel cage fan to increase the cooling air flow to the motor when at low speeds. Motors being beefed up in this way is the only reason they can survive the abuses from a VFD. They still will not last as long as motors running on standard AC power. The real question for Life Cycle Cost is, how long do VFD's and the motors they control really last?
This link (http://www.powerqualityanddrives.com/payback_analysis_vfd/) talks about the life of a Drive being 8 years from their experience. This analysis shows the first Drive as being GIVEN to the user for free. In 8 years when the Drive fails, the end user must purchase the second Drive themselves. This shows a negative payout as soon as the end user spends money on a Drive. I submit that if you purchase the first Drive yourself, payout becomes negative immediately.
My experience has shown that a Drive lasting an average of even 5 years is overly optimistic. 5 years or 8, either way in most pump applications, the Drive will not last long enough to pay for itself. Especially because the difference between power consumption of a Drive verses a Valve is very slight in most cases.
As an example, I installed a couple of 10 HP centrifugal pumps with Drives as the primary control in 1992. These motors have been rewound, and the bearings and seals have been replaced several times. The Drives have been replaced twice in this time period, which totals 3 Drives per pump in 16 years. These pumps use almost exactly the same amount of power when running with the Drives as they do when controlled by Valves. So by comparison, there are no energy savings to justify the added maintenance of the motors and replacement cost of the Drives.
I have many other pump systems that are identical in every respect, except they do not use a VFD for control. These pump systems running Across The Line, have lasted 20+ years without any maintenance.
There are many good applications for Drives. However, there use on many centrifugal pump applications is overkill, overpriced, reduces dependability, and reduces the life of the system.
There is nothing more dependable, longer lasting, and efficient, than a standard squirrel cage induction motor, running on normal AC power, using across the line controls, and spinning a centrifugal impeller. Many of these type pumps have lasted 30+ years. It is also very common to find 30 or 40 year old pump control valves that are still working fine. It could very well be the energy used to mine, manufacture, transport, install, and maintain short lived VFD's and motors that is melting our polar ice caps, and increasing operating expense and Life Cycle Cost. Not to mention the associated down time and head aches associated with VFD controls.
Engineers should have stayed with their first instincts instead of asking pump manufacturers how to make pumps last longer. It is heavy duty bearings, stronger materials, and cooler running motors that make pumps last longer and decreases Life Cycle Cost. Smaller, lighter weight pumps and motors, with temperamental electronic controls, can increase Life Cycle Cost. Which after all is the most important consideration for many corporations, not saving money or energy.
Test will usually favor the tester. When big oil companies pay for studies and test, alternative fuels never advance. When pharmaceutical companies do studies and test, benefits of new drugs are advertised emphatically, while life threatening side effects get lost in the fine print. It is an oxymoron for a utilities company to suggest ways to save water and electricity, when selling water and electricity is how they make their money. When manufacturers of Variable Frequency Drives test for efficiency, they show 30% to 50% energy savings. In reality 3% to 5% energy savings with VFD's is more correct, and even that is overshadowed by the short life of the equipment.
The same is true of pump and motor manufacturers. When ask how to make pumps and motors last longer, manufacturers will make you think they are solving the problems and saving energy in the process. When behind closed doors, they use our naive ideals to build in "planned obsolescence" to their equipment, which makes purchasing of new pumps and equipment a predictable and repeatable necessity. Life Cycle Cost does have a lot to do with energy consumption but, it can easily be overshadowed by replacement cost of short lived equipment. Asking pump manufacturers how to increase the life of pumps and motors is not only letting the fox guard the henhouse but, is allowing the fox to design the henhouse.
A wolf in any other disguise is still a wolf. If you believe that manufacturers are dedicated to helping your pump system last longer or saving energy, and oil companies are earnestly trying to develop a "plug-in" car, then I have a great deal on some beach front property in Arizona, just for you.