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VFD Articles Don't Tell The Whole Truth Controls for variable speed pumps that include a pressure transducer are designed to maintain a "constant pressure". When lifting water from wells or boosting pressure, a certain head or TDH (Total Dynamic Head) is required. TDH is the "constant pressure" needed. In the May issue of Water Well Journal 2003 there is a good article on variable speed pump curves. Using these same curves, you should shade in the tiny area that can be used for "constant pressure". With the 3656 pump curve we would pick a 100 HP pump because we need 700 GPM at a "constant pressure" of 145 PSI or 335' TDH. Drawing in a line at 335' shows that this pump can only be slowed from 3600 RPM to a minimum speed of 3348 RPM and still produce the TDH required. Slowed in RPM by 7% will reduce the 100 HP load to a minimum of 80 HP. Looking at a curve like this one might think it possible that this 100 HP could be slowed to a 10 HP load. This is definitely not the case as anything less than 3348 RPM or 80 HP will only pump water up 334' in a well that is 335' deep. The other curve used is an 18 GPM 1.5 HP pump model 18GS15. We would pick this pump when we need 18 GPM at 260' TDH. This is a pressure transducer setting of 50 PSI while pumping from a well and lifting 145'. This pump can only be slowed by 14% which could be 4,800 RPM to 4,128 RPM and still be producing 50 PSI "constant". 14% reduction in speed only reduces the horse power by 36%. This brings a 1.5 HP load down to about 1 HP. According to the affinity law the head that a pump produces drops off by the square of the speed. This causes small reductions in speed to make drastic decreases in the head produced. The following is simple way of figuring the minimum speed a pump can run and still produce the TDH required. Divide the TDH or "constant pressure" required by the shut off head of the pump at full speed. In the case of the 3656 pump that would be 335' divided by 390' which equals 86%. This means that the pump must produce 86% of the total head possible to maintain 335' TDH. Since the head is reduced by the square of the speed, what number squared equals this 86%. 93% squared or (.93 x .93) equals 86%. This means that the pump must be running a minimum of 93% of speed to produce the 335' TDH. Then 93% of 3600 RPM shows our minimum pump speed to be 3348 RPM. Motor speed is directly related to the frequency or hertz. 60 hertz is full speed and 30 hertz is ½ of full speed. This particular pump could only be ramped down from 60 to 55.8 hertz. Any slower (even 55.7 hertz) and this pump cannot produce enough head or TDH to buck the head on the system, and would be completely dead headed. When using variable speed curves, before you get all excited about the horse power characteristics, don't forget to draw in a line for TDH required. With such tiny changes in RPM possible, it can be just as efficient to use a standard full speed pump, with an ordinary magnetic starter, and simply let it work farther left on it's curve. If you have a variable speed pump you can easily try this yourself. Simply check the incoming amp draw at a low flow condition with the variable speed holding a "constant pressure". Removing a wire from the pressure transducer, or removing the transducer from the water line, will cause the pump to stay at full speed. The pressure will increase so be careful but, if you maintain the same flow rate using a gate or ball valve, the amperage will stay the same as when the pump was turning slower. With no savings in horse power, many other negative side effects created by variable frequency drives should all be weighed in. Harmonic vibration, transient voltage spikes, skin effect, electrical fluting of ball bearings, and many other problems exist with variable frequency drives. These are problems that you will never hear about from someone who sells drives. As pump installers we must usually learn these things the hard way. Having made countless free service calls to unhappy customers who have variable speed pumps we suggest that you choose your variable speed test subjects carefully. |
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