When individuals look at a variable-frequency drive (VFD), all many see is a speed control device that is able to either vary the flow that a pump delivers, or establish a regulated pressure for a group of users; this thereby provides comfort to those using the liquids being delivered. VFDs can, however, provide a much higher value to pumping systems, by saving energy and optimizing performance.
By Tim Park, Sr. Applications Engineer, Danfoss Drives
One of the primary values that VFDs bring to the centrifugal pump is energy savings. VFDs have a positive load characteristic, see Figure 1. As the speed or flow of the pump goes up, the power consumption of the pump goes from a modest slope to an aggressive one. Slight reductions in speed can make a huge difference in power consumption. This works very nicely with the VFD’s inherent variable-speed capability.
If, however, there is not any excess capacity in the pump, slowing down or speeding up the flow is not viable. As the power loading (kW or HP) is increasing at a very sharp rate, a change of speed could cause the load to rise too sharply; this can be seen when the speed is cross rated for the motor moving upward.
Addressing Vibration Challenges
Often, the life of a pump is shortened due to mechanical and hydraulic issues. This is particularly true when a pump experiences vibration, as it can significantly impact the plumbing of the system. One way to mitigate the risk of vibration is by following diligent installation practices.
For example, pipe fill issues are a prevalent installation challenge. When the plumbing is initially pressurized, the inrush of fluid into an empty pipe can cause unnecessary shocks and vibration that will disappear as fluid levels reach more normal operating levels. Notably, the air in the pipes also disappear.
These vibration scenarios cause wear and tear on pipe fittings, connections, valves, and to a lesser degree, the normal runs of pipe. If there are combinations of horizontal and vertical runs of piping installed during installation, the wear and tear can be much more pronounced. Altogether, if not managed with correct installation processes, the risk of leaks and other secondary damage is quite real.
Many vibration risks are easy to spot by carefully listening to the flow of the fluid as the fixtures are energized. When valves, either manual or electromechanical (solenoids), are actuated, an operator can often hear a sound a banging sound that most call ‘water hammering’. This is generally a short-term, sharp noise that will happen only once and will stop right after flow is established or when fluid replaces the air that could well be in the pipe. It represents a shock to the mechanical system and if allowed to continue, will contribute to premature failure of mechanical parts.
Just as in air handling systems, some plumbing networks may also sustain vibration naturally at specific flow rates. This is sometimes thought of as a natural resonant frequency. Since VFDs create both the output voltage and frequency delivered to the motor (in proper proportion), once the characteristics of the resonance are understood, the VFD can become a very useful tool in minimizing or eliminating the condition. There are various terms for this, differing from manufacturer to manufacturer, but they all serve to dampen the vibration, see Figure 2.
To complicate matters, sometimes these natural resonant frequencies are not alone. There may be several, at distinctly different operating frequencies or speeds. Historically VFDs have come to offer three, five, or more frequencies or speed settings that will obliterate the effects of such resonance points. It is worth noting that other terms for this are: jump frequencies, critical frequencies, and prohibit or avoidance frequencies. Depending on the VFD design, various parameters are used to define the width of these resonance points and serve to minimize the vibration.
Acceleration and deceleration profiles can also have an impact on vibrations in a pump system. Typically, VFDs are configured to offer a fixed acceleration rate, typically linear. Many offer the ability to alter this linear characteristic to help control vibration and other performance anomalies. These are commonly referred to as S-Curve ramps. S-curves generally allow machinery to execute speed changes without causing major short-term disruptions and vibration.
The value that VFDs bring to the pumping world are significant. Although a significant amount has been written about energy savings, VFDs ability to minimize various vibration and other installation-specific performance challenges can also bring reliability and maintenance integrity to a good pump installation.
ABOUT THE AUTHOR
Tim Park is a Senior Applications Engineer at Danfoss Drives. He has spent over 40 years in the drives industry working for several manufacturers. His experience in applications, service and product management extends to AC & DC drives as well as electronic starters. He can be reached at email@example.com.