"Understanding that the power consumption is related to the work being done is not a difficult
concept."
I completely disagree with the above statement. If it were not a difficult concept to understand, a simple pump curve would be all that is needed to make the point. In this thread we have batted the math back and forth, studied pump curve after pump curve, and even demonstrated the idea with a working pump system. Apparently we are still arguing the point. Again, if you can read a pump curve, the test rig is simply proving the obvious. I don't agree that an amp meter to measure power on our test rig should cause a "big red flag". 167 watts or 1.2 amps difference is not much difference anyway you look at it. I guess if I am using a compass instead of a GPS, I cannot prove the sun rises in the east either, even though it is obvious.
I agree that each application should be evaluated to determine the best system to install. However, I believe that each application should be evaluated to see if it can be controlled efficiently by any other means, before a Drive is even considered. Adding a Drive to applications where the system could be easily controlled by other means, adds a multitude of other complications that must also be dealt with. Harmonics, as is the title to this thread, along with reflective waves, voltage spikes, resonance frequencies, bearing currents, and many other problems are never present, if you don't throw a Drive into an application that could easily be controlled by other means.
"Just because you built a system that can demonstrate that a valve is a good solution does not mean a valve is always a good solution."
Just because you can build a system that can demonstrate that a Drive is a good solution, does not mean that a Drive is always a good solution. However, most people have been brain washed to falsely believe that a Drive can always save energy. Therefore, they think that a Drive is the best solution, and that all the side effects that go with a Drive are worth it, because you are supposedly saving energy.
Hello Waross
"We sometimes have to modify existing equipment and do the best we can with equipment chosen by someone else."
This is what I deal with everyday. Someone who didn't know what they were doing, chose the wrong pump. Now the volume is too high, or the pump is cycling itself to death. As long as the pump has some drop in horse power, a valve can still be the best alternative. The existing and badly chosen pump may only drop from 30 HP to 22 HP when throttled with a valve. If the correct pump were chosen or a Drive were used, the 30 HP could drop to 15 HP at low flow rates. This small difference in power consumption still may not be worth changing out the pump, or the problems of added side effects when using a Drive.
"If throttling the pump output develops a fairly large pressure drop across the control valve is this an indication that the savings with a VFD may be more than shown with your test rig?"This is the concept that is hard to understand. It doesn't matter how much pressure differential you have across the valve, what matters is the Brake Horse Power characteristics of the pump. This is why I say that back pressure is a free byproduct of horse power. Even though the back pressure increases, the horse power decreases. This is also how you choose the best pump. Don't just look at horse power for maximum flow, also look at horse power required for smaller flow rates. The only reason back pressure is even considered, is to make sure the control valve and associated piping can handle the pressure, it has nothing to do with how much energy is supposedly "burned across a valve".
As I have explained many times, 1200 GPM at 100 HP is using much less energy per gallon, than when a Drive is used on the same pump to produce 100 GPM with 38 HP. So in this respect, you are also "burning energy with the Drive".
Now for picking a pump that works best with a Drive, you need a performance curve that is as steep as possible. Because head is reduced by the square of the speed, a steep performance curve gives you more possible reduction in speed while still producing the head required. You should temper this steep performance curve, with the Brake Horse Power curve as well. Because once you have reached the minimum speed that will produce the head required, further reduction in power come only from the pumps natural Brake Horse Power characteristics, the same as with valve control. You simply don't start using the pumps natural Brake Horse Power characteristics, until the Drive has slowed the pump down to the minimum possible speed, that will still produce the head required.
Greetings Crazytexacan, which I am also one of. Since you started using Drives 34 years ago, there have been new Drives introduced every few months. New Drives are supposed to solve the problems of the last Drives, which are only a few months old themselves. They do seem to be coming up with a few band aids for some problems but, unless you can change the laws of nature, most of the problems associated with Drives CANNOT be fixed. What I understand about the 18 pulse Drives, is that they do counteract much of the harmonics but, at the expense of decreased efficiency which causes increased power consumption. Kind of defeats the original purpose of the Drive. Also, the more pulses, the faster the rate of voltage rise, the more damage is done to the windings.
Down hole oil well submersibles, because of their very steep performance curve, can be reduced in speed more than other pumps, and still produce the head required. Also, oil well submersibles do not usually have to produce a constant outlet pressure as needed with such things as irrigation sprinklers. Maintaining a level in the well, instead of a constant outlet pressure, a Drive is able to further reduce the speed and horse power required. Of course efficiency is not as much of a concern as precise level control with these systems, as the fluid you are pumping is now worth more than $100 per barrel. I would think that the extra long wires to the motor could cause a problem. I am sure you must be using a really high voltage cable with extra shielding? You say a narrow range is 35 to 70 hertz, I say for most pump applications the range is even more narrow than that. 50 to 60 hertz is usually the range limit for a correctly sized pump in a constant pressure application.
"When I lose a drive I can usually run in Bypass mode..."
All the drives I installed ran in Bypass mode more than they ran in the Drive mode. Of course we used valves to control the flow when running on Bypass. This is how I discovered that the power consumption was the same with Valve control as with Drive control. This is also how I figured out that Valve control was much more reliable than Drive control. Then I should also mention that all the problems with harmonics, vibration, voltage spikes, electrical fluting of ball bearings, and other problems DO NOT even exist when running in Bypass with Valve control.
I know that many of you make your living by mitigating problems of Drive controls. I know you don't want to hear that Drives do not save energy, cause many side effects, and even cause health problems with animals and humans.
You can tell me that "Drives are hear to stay", and "I should just get used to it". I submit that I replace Drives with Valves everyday. You may not want to hear it but, the best way to solve the problems of Drive control, is to not cause the problems in the first place. Making me feel like I have stuck my head in a bee hive every time I bring it up, is not going to change this fact. People who use Drives on every application, and then try to mitigate the problems with the Drive, are just a few years behind the learning curve. There are good applications for Drives but, Drives are not good for many applications. Sorry for the long post but, it seems I am still having to prove the obvious.
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