How Do VFDs Work in Pumps?

Simple. As the load on the pump increases the current the pump draws also increases.

The current of the load (pump) is monitored by the VFD and an algorithm within the controller changes the frequency of the VFD based on that current.

"But I want to know how these VFD's senses the variation in pump load and changes the frequency/power input to the motor?"

Hmmm... you seem to have something backward here.

The VFD doesn't necessarily SENSE the variation in the pump load, the VFD technically is just what CONTROLS the variation of the pump load.

The VFD simply responds to a command signal given to it from something or someone that is "deciding" what the pump speed must be. What makes that decision is separate, often called a "Loop Controller". The loop controller takes in a process variable (PV) signal, as well as a setpoint (SP) from the user, then puts out a controlled variable (CV) output command that matches the desired condition. The VFD, attached to the motor, is simply the final actuator against that CV output command.

So for example let's say you want the pump to maintain a pressure in a piping system. You will have a pressure transducer that provides a 4-20mA signal proportional to pressure in the pipe. That goes into the Loop Controller as the PV, the value being measured. The end user (person, such as you) decides that the pressure that you want to maintain in the pipe is 15 PSI, the SP. So the transducer is calibrated to put out 12mA of current at 15 PSI. When the pipe pressure drops to 10PSI, the signal to the Loop Controller drops, so it tells the VFD to increase the speed with an output signal (CV), which the VFD uses to raise the frequency and increases the pressure. You need the Loop Controller in there because it must anticipate the rate of change, the magnitude of the response, and the how often it changes, otherwise the pump just constantly speeds up and slows down (called "hunting") without stabilizing.

An easy to understand example of a Loop Controller is if you have ever used the "Cruise Control" in your car. The Speed Setting is your SP, the speedometer has a transducer providing a feedback of the actual speed (PV), and the Loop Controller is acting (CV) upon a servo controller attached to your throttle mechanism to increase or decrease the engine power. In fact if you have ever rested your foot on the gas pedal and started going up a hill, you will feel the pedal get pulled down under you. That's the CV commanding the throttle servo. Same thing as telling the VFD to increase or decrease frequency (in fact if it is an electric car, it IS a VFD you are commanding).

In many cases, VFD manufacturers now build-in the Loop Controller as part of the VFD just for convenience and to save money for the user. But that isn't inherently part of the VFD, it's an added feature. Personally, I don't use that, I always use a separate Loop Controller. That way if the VFD dies, I can replace it with anything I can get quickly because the Loop Controller remains the same, and that's the complicated part of the system programming.

I read the data sheet on the link that you posted, and I see your confusion. This PARTICULAR system is custom designed to work with THEIR pump specifically. Here is what they say:

• Sensorless control is an innovative concept for circulating pumps.
  Pump performance and characteristic curves for ten different
  speeds are embedded in the memory of the speed controller during
  manufacture. This data includes power, pressure and flow
  across the flow range of the pump. During operation, the power
  and speed of the pump are monitored, enabling the controller to
  establish the hydraulic performance and position in the pumps
  head-flow characteristic.
  These measurements enable the pump to continuously identify the head and flow at any point in time, giving accurate pressure control
  without the need for external feedback signals. Patented software technology within the controller ensures trouble-free operation
  in all conditions.
  Incorporating the pump’s hydraulic data into the controller and removing sensors results in true integration of all components and
  removes the risk of sensor failure.

So in other words, this relates to their specific PACKAGE of their pump and VFD, in which empirical performance data of the pump is loaded into the VFD as a form of set point for a PID loop, using the pump power itself at the Control Variable feedback to the loop. Interesting concept, but not really something you could do on any pump with any drive, it relies upon that tested performance data and a "look-up table" of data points.

There are other drives out there that have the ability to be connected to a pump in which the user runs through a series of test points like this, and the drive learns this data. But again, YOU as the user must teach the drive this information by simulating it, which is not always possible or practical.

For the most part it is better to decide up front what your control variable will be (i.e. pressure, flow etc.) and use a separate transducer as the input into a PID control loop in the drive. That way if the drive dies, it can be replaced by any other drive that is readily available. In the example you provided, if their drive fails, it can ONLY be replaced by that EXACT same drive. That will likely be a problem in 5 years, the average length of time a product like that lasts in the marketplace before it is updated and changed.