Monitoring pump performance is critical. Monitoring allows customers to observe slight deviations in the typical behavior of motors or to observe undercurrent that would not be detected with standard bimetallic relays. The monitoring is typically achieved with the VFD’s onboard I/O or a communication protocol such as Modbus, Profibus or Ethernet/IP. Slight changes in a motor’s typical power or current draw can help indicate a possible blockage, intrusion in the pump (such as a rag) or possibly indicate an impending bearing failure. This becomes more significant when considering a remote pump station or lift stations. Rather than send technicians on scheduled maintenance calls, monitoring can alert end users before a failure occurs.
Most pump systems are designed to meet peak operating needs. However, during normal operation, a system and the pumps within the system may only need to operate at a fraction of their full capacity or speed. As a general rule, 80 percent of the time the demand is less than 20 percent of the capacity of the system.
The affinity laws state that a pump’s power consumption changes to the cube of the speed. For example, when a pump’s speed is reduced by 20 percent, it equates to an energy usage of only 51.2 percent, or a saving of 48.8 percent.
(0.8)3 = 0.512 or 51.2 percent
Rather than mechanically limiting the flow or pressure in a system while running the motor at full speed, a VFD allows the speed to be reduced electrically to conserve energy usage.
The Case for Energy Efficiency: The Affinity Laws
The affinity laws can determine the system performance for centrifugal devices, including theoretical load requirements and potential energy savings. The first curve in Figure 1 shows that flow varies linearly with speed. If the speed decreases to 50 percent, flow decreases to 50 percent.
|Figure 2. The affinity laws|
The second curve demonstrates that pressure or head varies as the square of speed. If the speed is decreased by 50 percent, then the pressure or head will be only 25 percent from the second curve. The third curve shows the power required for a particular flow requirement. Energy varies as the cube of speed. If the speed is set to 50 percent, then power consumption is only 12.5. The potential for energy savings occurs anytime less than peak flow is required.
VFDs Save Energy, Increase Equipment Life
According to the Hydraulic Institute, pumps consume 5 percent of industry energy. Pumping systems account for nearly 20 percent of the world’s electrical energy usage and range from 20 to 25 percent of the energy usage in certain industrial plant operations. Maintenance and repair is a significant component of pumping system life-cycle costs. VFDs minimize wear and tear while also driving energy efficiency, which translates directly to the bottom line. This makes both processes and organizations more effective and efficient.
By matching power consumption to changing system requirements, VFDs are increasingly relied upon to provide steady, efficient power for pump applications. VFDs protect valuable motor and pump assets by minimizing power and consumption, reducing mechanical stress placed on the system, and monitoring the pumps’ health to predict maintenance needs.
VFDs are among the most technologically sophisticated methods of motor control and have benefited from years of evolution in motor starting technology. They continue to generate more economic and flexible motor control. Traditional across-the-line starters apply full voltage to motor terminals, which can sometimes generate high inrush currents, causing stress to mechanical equipment.
The latest generation of VFDs is more efficient, accurate and refined—ultimately leading to increased energy savings. New and sophisticated technologies are impacting efficiency, yielding more effectively and finely tuned VFDs. Specifically, advancements in capacitors, insulated gate bipolar transistor (IGBTs), heat management, processing power and measuring technology all enable the development of solutions to problems that were not previously available. Today’s drives control more than the motor or pump—they control themselves. Fundamentally, this means that drives are better able to deliver energy savings, improve equipment and process reliability, and protect valuable assets.