There are many brands of VFD's used to control variable speed pumps. A pressure sensor of some kind is needed to "sense" the pressure in your water line and send a signal to the VFD controller. A signal is needed for the VFD to know if the pump needs to speed up, slow down, or go to sleep. Speaking mostly of homeowner size, variable speed submersible pumps, (sometimes known as "constant pressure" pumps), there are two basic types of pressure sensors being tested presently.
One sensor uses a pressure transducer to measure the pressure in the system. A piston inside the transducer moves up and down as needed, which changes resistance and controls a 4 to 20 milliamp signal to the VFD controller. The controller is trying to maintain a set point or "constant pressure". Therefore, as the pressure increases, the amperage through the transducer increases, and the VFD decreases the frequency, which reduces the speed of the motor. Likewise, when the pressure decreases, the amperage through the transducer also decreases, and the VFD increases the frequency to speed up the motor.
Makers of the other kind of "sensor", like to point out that a transducer is very susceptible to freezing, clogging, and malfunctions. However, if the transducer gets cold enough to freeze, so will every other pipe or fitting in the system. Transducers usually fail one of two ways. They can be stuck in the low-pressure position, and the pump cannot shut off. In that case the pump could be destroyed from deadheading unless a pipe burst or a pressure relief valve pops open from the high pressure. A transducer can also fail in the high-pressure position. In this case the pump simply will not start when water is called for. Either way, the homeowner is out of water and must replace the equipment that failed.
One problem with a transducer is slow reaction speed. When a tap is opened and the pressure drops, it takes a little time for the transducer to change state, the VFD to receive the signal, the VFD controller to increase the frequency to the motor, and the motor to get the pump up to speed. A slight lull in pressure occurs, which can usually be overcome with water stored in a small pressure tank. When a large tap is closed or opened suddenly, like from an irrigation system, it takes a little time to get the motor to slow down or get up to speed. If the pump does not slow down fast enough, the little pressure tank cannot accept enough water, and the pressure increases until a pipe or fitting burst, or a pressure relief valve pops open. If the pump does not ramp up fast enough, the pressure goes to zero, and the bladder in the tank bounces. This can cause water hammer, as flow in the system hesitates, then starts up again.
Another brand of VFD uses a "switch" instead of a transducer, which the company calls a "pressure sensor". This switch also tries to maintain a set pressure. At the set pressure, the switch will make and break in a narrow bandwidth of about 1 PSI. In other words, when set at 50 PSI, the switch will break or open at 50.5 PSI, and make or close at 49.5 PSI. Only when there is enough water flowing in the house to use all the water the pump can produce, will the pressure stay below 49.5 PSI, and the pump remain running at full speed.
When there is less water being used than the pump can produce, which is usually the case, the pressure rises to 50.5 PSI, the switch opens and the pump goes to idle speed. Then because there is still water being used, the pressure quickly drops to 49.5 PSI, the switch closes, and the pump increases to full speed. Usually before the pump reaches full speed, the pressure has already increased to 50.5 PSI, the switch opens, and the pump drops to idle speed again. This opening and closing of the switch happens as much as 45 times per minute trying to maintain a constant pressure. The pump never really locks in at a particular speed. It simply goes from idle speed to full speed, as the switch continually opens and closes. When there is absolutely no water being used, the switch stays open. When the pump has run at idle speed for about 10 seconds, the pump ramps all the way down to 0 speed.
This switch helps make up for the slow reaction problem of the transducer. However, a switch opening and closing 45 times per minute is about 2,000,000 times per month of use. Mechanical devices like a switch are limited to how many times they can change state before they fail. A couple of million switches in 30 days of use is a lot of switching. This switch can fail in the open or closed position the same as a transducer, and causes the same problems of no pressure or too much pressure, depending on which way it fails.
The transducer has been around longer, controls the system with less movement, and therefore should, and has proven to, last somewhat longer. Slower reaction speeds, and failing in a closed or open position, are problems that can somewhat be addressed with larger pressure tanks and properly acting pressure relief valves.
The switch or "pressure sensor" as the company calls it, has not been on the market very long. In the short time it has been available, I have heard of several switch failures. Another problem I am hearing with this type system is from the torque of the motor going from idle to full speed 45 times per minute. The submersible pump, hanging on a long length of flexible drop pipe, torques a little to the left every time the switch closes. This continued torque action form the motor, can rub the insulation from the power wires down in the well. If the wires are tied up tight to the drop pipe, the drop pipe rubs off the insulation. If the wires are left with some slack in them, they slap against the casing to rub off the insulation. When torque arresters and cable spacers are used, they rub against the wire themselves. Either way, a short is created and the system fails. I have even heard of at least one case where the motor was touching the casing, and the torque action rubbed a hole in the side of the motor.
"Constant Pressure" has many benefits for your water system. "Constant Pressure" is really the only job that a VFD is trying to accomplish. The Cycle Stop Valve or CSV, was designed to mimic the "constant pressure" delivery of a VFD, without needing the computerized controls that create so many problems. The best technology is not always electronic. The best technology is one that has been reduced to the simplest form that will accomplish the job at hand. The CSV is newer technology and has been used as a reliable replacement for VFD's every day since 1993. The CSV reduces energy consumption of a pump as much as varying the speed with a VFD. The CSV works with a standard pressure switch. With the CSV, the pressure switch only closes when water is called for and the tank is empty. Then this switch stays in the closed position until the water is turned off and the tank is again full. Standard pressure switches are made to handle a few hundred switches per day. When used in conjunction with a CSV, standard pressure switches only change state a few times per day, and can last for many years or even decades without failure.