Thanks for the link to www.pumped101.com by Joe Evans. Joe has done a excellent job of explaining many things about pumps that are hard to understand. Joe does an excellent job of explaining how a VFD works with an AC induction motor. I was particularly impressed with Joe's candid remarks about the "potential problems" of VFD controls. His article "Variable Frequency 101" list many of these problems as does many other articles throughout the entire web page. There is a very good explanation of damage to ball bearings from electrical fluting caused by EDM currents. Details are given on harmonics being fed back into the power grid and how it effects other electrical devices in the area. He talks about motor insulation damage from thermal stress, and how a 480V motor receives spikes of 2,000V caused by, overshoot voltage, reflected voltage, and ringing. He discusses how partial discharges and corona discharges actually destroy the insulation on the windings. He mentions critical speeds or resonant frequencies and even has a short video of resonant frequencies destroying a glass, and makes the statement that multi stage pumps and pumps with long shafts are even more susceptible to resonant frequencies. He even uses one of my favorite sayings, "there is no such thing as a free lunch" when explaining the negative side effects of a VFD. He even goes on to say that fixed speed pumps running on standard AC voltage do not have any of these problems.
He explains why VFD's need pumps with steep performance curves because head is reduced by the square of the speed. There are examples of systems requiring constant pressure, constant flow, variable flow variable pressure, and discussions on other type pumps such as positive displacement pumps. He talks about pumping substances other than cool water such as hot water, sewage, and even gaseous materials.
I find it extremely rare and even brave, for someone in the pump industry to list and explain the problems associated with VFD controls. Most everyone else wants so badly for the VFD to be the "magic pill" that cures all pump related problems, that they neglect to mention the associated negative side effects. I only found a couple of things that I would even argue with Joe about. Even then my arguments would only be for one particular niche of the pumping industry, which is pumping fairly cool and fairly clean water at a constant pressure. Pumping any other substance other than cool, clean water, and in other way besides constant pressure, a VFD can save a tremendous amount of energy and can even be worth putting up with all the negative side effects. However, there are millions of applications that only pump cool, clean, water at a constant pressure. With this type of pumping application, a PCV can save as much energy as a VFD, and does so without any of the negative side effects of VFD control.
The first argument I have is with Joe's explanation of how pump control valves or (PCV's) work. Although he did a good job describing valves of yesterday, there have been as many advances in PCV's in recent years as there have been with VFD's. Modern PCV's such as Cycle Stop Valves are simpler, faster reacting, more dependable, and require less maintenance than old style PCV's. These valves are designed to protect the pump from overheating, cavitation, and cycling which extends the life expectancy of pump systems. The second thing I disagree with Joe about is energy savings with systems that require constant pressure. Since with constant pressure systems there is very little if any energy saved when comparing a VFD controlled pump to a valve controlled pump, it is impossible to save enough energy to pay off the added expense of the VFD. As VFD's, like all electronics, continue to become less expensive and maybe a bit longer lasting, this pay off may soon be easier to come by. However, because of all the negative side effects of a VFD that shorten the life of motors and pumps, the pump, motor, and/or drive must usually be replaced before a pay off is ever reached.
In his article "Constant Pressure Booster", Joe talks about how pumps with flat curves or unstable curves cannot be slowed very much and still produce a constant pressure. He talks about how pumps with steep curves can be slowed down to save energy at low flow rates and how pumps with flatter curves do not benefit much from VFD control. What I would like you to understand is that pumps with too steep a curve can use more energy than required at high flow rates to be able to be slowed enough to save some energy at low flow rates.
Below are two curves for comparison. The first curve is Joe's example of a pump that uses 83 HP to produce 550 GPM at 400' of head. Slowing this pump with a VFD, the flow is reduced to 75 GPM at 400' and only uses 18 HP. The next curve is one that I picked to work with a PCV. This pump only uses 68 HP to produce 550 GPM at 400' of head but, only drops to 30 HP at 75 GPM. With Joe's example curve, the VFD would be using 15 HP more at high flow than a constant speed pump using a PCV. The VFD controlled pump would then be using 12 HP less than a constant speed pump at low flow. However, the pump that only uses 68 HP at 550 GPM would only drop to 25 HP at 75 GPM when slowed to 1630 RPM with a VFD. Therefore, "there is no such thing as a free lunch" or, "you can't have your cake and eat it too".
Another example curve that Joe uses is in his "Variable Speed Puzzler". This curve shows a pump using 12.65 HP to deliver 245 GPM at 145'. When a PCV is used to reduce the flow to 100 GPM, the horse power drops to 8.5 HP compared to 6.8 HP when slowed with a VFD. The following shows both constant speed and variable speed on the same curve. As you can see there is only 1 HP difference between VFD and PCV at 100 GPM. How long will the pay off be for the VFD when there is only 1 HP of energy savings? Most likely the pay off will take longer than the VFD will last.
What we have talked about so far is the horse power requirements of the pump. We have not mentioned that the drive itself also uses power. As Joe describes in his articles, the VFD is a microprocessor based pulse width modulator which means it is a computer. As with any computer, it uses power of it's own even when the pump is not running. Different from ordinary computers though, the VFD has a lot of amperage running through it, depending on the horse power pump it is controlling. This amperage produces heat which must be cooled by a fan or in some cases an air conditioner, which causes the VFD to use even more power. Running on pulling DC voltage from a VFD, a motor can also be up to 5% less efficient than a motor that is running on true sinusoidal AC voltage. Add this loss of motor efficiency to the power used by the drive itself, and there may be absolutely no energy saved when using a VFD for constant pressure.
Another phenomenon of VFD that has just recently come to light and is not yet fully understood is stray or trash voltage. Harmonics fed back into the grid from a VFD cause dirty voltage to be emitted from power lines and related equipment probably as radio frequency interference or RFI. I am still not really sure how this dirty or trash voltage is affecting the environment but we can prove it is happening. Not only are radios, televisions, lights, and other equipment being affected but, it has been proven that dairy cattle produce less milk when a VFD is in the area. I have personally experienced problems with my cell phone not working when in the proximity of a VFD. Some reports have shown that cattle go from producing 90 pounds of milk per day to as little as 70 pounds per day when a VFD is installed in the area. We also know that when the VFD was removed that milk production went right back to 90 pounds per day. Many people are studying this phenomenon and some are even producing electronic filters to try and reduce the effect. Many things in this world are not fully understood, and we should heed any tell-tale signs of trouble. We cannot see or feel this stray voltage just as we cannot feel or see radioactivity or poison gas. If cows are losing milk production, we should use this information the same as a canary in a coal mine of long ago. I am not saying that this stray voltage is harmful to humans in anyway, just that it deserves more research. However, until it is fully understood, I for one would rather not have a VFD in my neighborhood.
Cycle Stop Valves, Inc.