Response;
OK, a Sub Drive 150 runs a 3 HP motor and usually has a 1.5 HP pump end. So you might call it a 3 X 1.5. The controller speeds up the motor from the standard 3450 RPM to about 4700 RPM, so they can use a 1.5 HP pump and get 3 HP performance. Increasing the speed of the pump is one problem with these type pumps, because every time you double the RPM, the pump wears out 4 times faster. So pumps will not last as long spinning 4700 RPM as they do when spinning the standard 3450 RPM.
The next problem with the Sub Drive or Mono Drive is the little switch. This system never sets the speed of the pump as needed. The little switch basically starts and stops the pump rapidly and continuously to match the flow being used. The switch makes it work much faster and eliminates some of the problems associated with "pressure transducers" that are used on nearly every other brand of variable speed pump. The problem is that this switch makes and brakes about 2,000,000 times per month. Nothing can survive two million changes of state per month. About three months of continuous use and the switch is toast. So if you only use your pump for an average of 4 hours per day, then every 18 months this switch, controller, or both are going to fail.
Another problem is the controller itself. I will bet your first controller was painted black. Then the replacement or "new" controller was painted white, supposedly to keep it cooler. These controllers produce and waste a lot of heat. The heat will destroy them. They are only rated to 50C ro 122 F temperature, and almost any well house will get this hot in the summer. Especially when the controller and it's fan are adding more heat inside the pump house. The lint and dirt that is drawn in by the cooling fan also increase the heat and shortens the life of the controller. Ever seen how much dirt collects in a computer sitting in your study for a couple of years? This is nothing compared to the dirt the controller picks up out in the garage or well house. Even the "new" white controllers are turning black, and making the wall they are mounted on turn black.
A better analogy of the difference between a so called "constant pressure pump", and a standard pump system using a CSV to produce "Constant Pressure" follows. When there are two or more ways to accomplish the same task, the simplest way is always the best way. (See; Occam's Razor)
NASA needed a pen for astronauts to take notes in space. They needed the pen to work in zero gravity or several G's. The pen needed to work in absolute vacuum or extremely high pressure. It needed to work in sub zero temperature or extremely hot temperature. It had to work right side up or upside down. Development of this pen cost millions of dollars, and the pens were so sophisticated that each pen cost a million dollars to produce. The Russians had the same requirements and they discovered that they got better performance at a fraction of the cost, using a #2 lead pencil. The variable speed pump is the NASA pen, and the CSV is the pencil. After all, we are just pumping water, a computer just makes it more expensive and less dependable?
If the "skin effect" from the variable speed controller has not caused a hole in the drop pipe, then your sediment issue is a whole other problem. We use a gravel pack in the well that is almost as small as sand. This filters out the dirt and doesn't let it enter the well. However, some sediment is so fine that is floats in the water and even a small gravel pack won't filter it out. Many times this can be pumped clean but, you have to pump the well hard, then let it recoup, and pump the well hard again. Sometimes this has to be done numerous times to get out the light sediment.
